变量与方法
一、变量详解
1.1 变量分类
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Java
public class VariableDemo {
// ═══════════════════════════════════════════════════════════════════
// 类变量（静态变量）：属于类，所有实例共享
// ═══════════════════════════════════════════════════════════════════
private static int classCount = 0; // 类变量
public static final String APP_NAME = "MyApp"; // 静态常量
// ═══════════════════════════════════════════════════════════════════
// 成员变量（实例变量）：属于对象，每个实例独立
// ═══════════════════════════════════════════════════════════════════
private String name; // 实例变量
private int age; // 实例变量
public void method(int param) { // param 是局部变量
// ═══════════════════════════════════════════════════════════════
// 局部变量：定义在方法内部
// ═══════════════════════════════════════════════════════════════
int localVar = 10; // 局部变量
String message; // 局部变量（未初始化，使用前必须赋值）
for (int i = 0; i < 10; i++) { // i 也是局部变量
// ...
}
// 代码块内的局部变量
{
int blockVar = 20; // 作用域仅限于这个代码块
}
// blockVar 在这里不可见
}
}
1.2 三种变量详细对比
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<mxCell id="44" parent="1" style="rounded=0;whiteSpace=wrap;html=1;fillColor=#e8eaf6;strokeColor=#9fa8da;strokeWidth=1;" value="" vertex="1">
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<mxCell id="50" parent="1" style="rounded=0;whiteSpace=wrap;html=1;fillColor=#e8f5e9;strokeColor=#a5d6a7;strokeWidth=1;" value="" vertex="1">
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1.3 变量存储位置图解
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1.4 为什么成员变量有默认值
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public class DefaultValueDemo {
// 成员变量有默认值
private int count; // 默认 0
private String name; // 默认 null
private boolean active; // 默认 false
private int[] array; // 默认 null
public void showDefaults() {
System.out.println("count: " + count); // 0
System.out.println("name: " + name); // null
System.out.println("active: " + active); // false
System.out.println("array: " + array); // null
}
public void localVariableDemo() {
int localVar;
// System.out.println(localVar); // ❌ 编译错误：变量未初始化
localVar = 10; // 必须先赋值
System.out.println(localVar); // ✅ 正确
}
}
1.5 静态变量的作用
Java
public class Counter {
// 静态变量：所有实例共享
private static int totalCount = 0;
// 实例变量：每个实例独立
private int instanceId;
public Counter() {
totalCount++;
instanceId = totalCount;
}
public static void main(String[] args) {
Counter c1 = new Counter();
Counter c2 = new Counter();
Counter c3 = new Counter();
System.out.println("c1.instanceId: " + c1.instanceId); // 1
System.out.println("c2.instanceId: " + c2.instanceId); // 2
System.out.println("c3.instanceId: " + c3.instanceId); // 3
// 静态变量通过类名访问
System.out.println("totalCount: " + Counter.totalCount); // 3
}
}
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二、方法详解
2.1 方法的分类
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<mxCell id="26" value="<font face="Courier New" color="#424242"><font color="#7b1fa2">int</font> <font color="#1565c0"><b>add</b></font>(<font color="#2e7d32">int a, int b</font>) { ... }</font>" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=0;fontSize=14;" vertex="1" parent="1">
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<!-- ==================== 第二部分：按是否静态分类 ==================== -->
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<!-- 第二部分标题 -->
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<mxGeometry x="50" y="385" width="800" height="45" as="geometry" />
</mxCell>
<!-- 表头 -->
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</mxCell>
<mxCell id="31" value="<font color="#ffffff"><b>类型</b></font>" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=0;fontSize=15;" vertex="1" parent="1">
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<mxCell id="33" value="<font color="#ffffff"><b>特点</b></font>" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=0;fontSize=15;" vertex="1" parent="1">
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<!-- 第1行：静态方法 -->
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<mxCell id="37" value="属于<b>类</b>，通过<font color="#1565c0"><b>类名</b></font>调用 &nbsp; <font face="Courier New" color="#757575">ClassName.method()</font>" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=0;fontSize=14;fontColor=#424242;" vertex="1" parent="1">
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<!-- 第2行：实例方法 -->
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<!-- 第3行：构造方法 -->
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<mxCell id="43" value="<font color="#c62828"><b>构造方法</b></font><br><font face="Courier New" color="#757575" style="font-size: 12px;">Constructor</font>" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=0;fontSize=14;" vertex="1" parent="1">
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</mxCell>
<mxCell id="45" value="创建对象时调用，<font color="#c62828"><b>无返回值</b></font> &nbsp; <font face="Courier New" color="#757575">new ClassName()</font>" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=0;fontSize=14;fontColor=#424242;" vertex="1" parent="1">
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</root>
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</diagram>
</mxfile>
public class MethodDemo {
private int count = 0;
// ═══════════════════════════════════════════════════════════════════
// 1. 无参无返回值
// ═══════════════════════════════════════════════════════════════════
public void printHello() {
System.out.println("Hello");
}
// ═══════════════════════════════════════════════════════════════════
// 2. 有参无返回值
// ═══════════════════════════════════════════════════════════════════
public void printMessage(String message) {
System.out.println(message);
}
// ═══════════════════════════════════════════════════════════════════
// 3. 无参有返回值
// ═══════════════════════════════════════════════════════════════════
public int getCount() {
return count;
}
// ═══════════════════════════════════════════════════════════════════
// 4. 有参有返回值
// ═══════════════════════════════════════════════════════════════════
public int add(int a, int b) {
return a + b;
}
// ═══════════════════════════════════════════════════════════════════
// 5. 静态方法
// ═══════════════════════════════════════════════════════════════════
public static int max(int a, int b) {
return a > b ? a : b;
}
}
2.2 静态方法 vs 实例方法
<mxfile>
<diagram id="static-vs-instance-method" name="静态方法vs实例方法">
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<mxCell id="4" value="对比项" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#1E293B;strokeColor=#1E293B;strokeWidth=2;fontColor=#FFFFFF;fontSize=14;fontStyle=1;" vertex="1" parent="1">
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</mxCell>
<mxCell id="7" value="关键字" style="rounded=0;whiteSpace=wrap;html=1;fillColor=#F8FAFC;strokeColor=#CBD5E1;strokeWidth=1;fontColor=#1E293B;fontSize=13;fontStyle=1;" vertex="1" parent="1">
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<mxCell id="9" value="无" style="rounded=0;whiteSpace=wrap;html=1;fillColor=#DCFCE7;strokeColor=#86EFAC;strokeWidth=1;fontColor=#166534;fontSize=13;" vertex="1" parent="1">
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<mxCell id="10" value="归属" style="rounded=0;whiteSpace=wrap;html=1;fillColor=#F8FAFC;strokeColor=#CBD5E1;strokeWidth=1;fontColor=#1E293B;fontSize=13;fontStyle=1;" vertex="1" parent="1">
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<mxCell id="11" value="属于类" style="rounded=0;whiteSpace=wrap;html=1;fillColor=#EDE9FE;strokeColor=#C4B5FD;strokeWidth=1;fontColor=#6D28D9;fontSize=13;fontStyle=1;" vertex="1" parent="1">
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<mxCell id="12" value="属于对象" style="rounded=0;whiteSpace=wrap;html=1;fillColor=#DCFCE7;strokeColor=#86EFAC;strokeWidth=1;fontColor=#166534;fontSize=13;fontStyle=1;" vertex="1" parent="1">
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</mxCell>
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<mxCell id="18" value="✅ 可以访问实例变量<br>✅ 可以访问静态变量" style="rounded=0;whiteSpace=wrap;html=1;fillColor=#DCFCE7;strokeColor=#86EFAC;strokeWidth=1;fontColor=#166534;fontSize=12;align=center;" vertex="1" parent="1">
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</mxCell>
<mxCell id="22" value="this / super" style="rounded=0;whiteSpace=wrap;html=1;fillColor=#F8FAFC;strokeColor=#CBD5E1;strokeWidth=1;fontColor=#1E293B;fontSize=13;fontStyle=1;fontFamily=Courier New;" vertex="1" parent="1">
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<mxCell id="23" value="❌ 不能使用" style="rounded=0;whiteSpace=wrap;html=1;fillColor=#EDE9FE;strokeColor=#C4B5FD;strokeWidth=1;fontColor=#6D28D9;fontSize=13;" vertex="1" parent="1">
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</mxCell>
<mxCell id="24" value="✅ 可以使用" style="rounded=0;whiteSpace=wrap;html=1;fillColor=#DCFCE7;strokeColor=#86EFAC;strokeWidth=1;fontColor=#166534;fontSize=13;" vertex="1" parent="1">
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</mxCell>
<mxCell id="25" value="典型场景" style="rounded=0;whiteSpace=wrap;html=1;fillColor=#F8FAFC;strokeColor=#CBD5E1;strokeWidth=1;fontColor=#1E293B;fontSize=13;fontStyle=1;" vertex="1" parent="1">
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</mxCell>
<mxCell id="26" value="工具方法、工厂方法" style="rounded=0;whiteSpace=wrap;html=1;fillColor=#EDE9FE;strokeColor=#C4B5FD;strokeWidth=1;fontColor=#6D28D9;fontSize=13;fontStyle=1;" vertex="1" parent="1">
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</mxCell>
<mxCell id="27" value="操作对象状态的方法" style="rounded=0;whiteSpace=wrap;html=1;fillColor=#DCFCE7;strokeColor=#86EFAC;strokeWidth=1;fontColor=#166534;fontSize=13;fontStyle=1;" vertex="1" parent="1">
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<mxCell id="28" value="💡 静态方法在类加载时就可用，无需创建对象；实例方法必须通过对象调用" style="text;html=1;strokeColor=none;fillColor=#FEF3C7;align=center;verticalAlign=middle;fontSize=12;fontStyle=1;fontColor=#92400E;rounded=1;" vertex="1" parent="1">
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</mxfile>
public class MethodAccessDemo {
private int instanceVar = 10; // 实例变量
private static int staticVar = 20; // 静态变量
// ═══════════════════════════════════════════════════════════════════
// 静态方法
// ═══════════════════════════════════════════════════════════════════
public static void staticMethod() {
System.out.println(staticVar); // ✅ 可以访问静态变量
// System.out.println(instanceVar); // ❌ 不能访问实例变量
// System.out.println(this.instanceVar); // ❌ 不能使用 this
anotherStaticMethod(); // ✅ 可以调用静态方法
// instanceMethod(); // ❌ 不能调用实例方法
}
public static void anotherStaticMethod() {
System.out.println("另一个静态方法");
}
// ═══════════════════════════════════════════════════════════════════
// 实例方法
// ═══════════════════════════════════════════════════════════════════
public void instanceMethod() {
System.out.println(instanceVar); // ✅ 可以访问实例变量
System.out.println(staticVar); // ✅ 可以访问静态变量
System.out.println(this.instanceVar); // ✅ 可以使用 this
staticMethod(); // ✅ 可以调用静态方法
anotherInstanceMethod(); // ✅ 可以调用实例方法
}
public void anotherInstanceMethod() {
System.out.println("另一个实例方法");
}
public static void main(String[] args) {
// 静态方法调用
MethodAccessDemo.staticMethod(); // 推荐：类名.方法名
staticMethod(); // 同一个类中可省略类名
// 实例方法调用
MethodAccessDemo demo = new MethodAccessDemo();
demo.instanceMethod(); // 必须通过对象调用
}
}
为什么静态方法不能访问非静态成员？
<mxfile>
<diagram id="static-method-limitation" name="静态方法不能访问非静态成员的原因">
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<root>
<mxCell id="0" />
<mxCell id="1" parent="0" />
<!-- 主容器 -->
<mxCell id="2" value="" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#fafafa;strokeColor=#5c6bc0;strokeWidth=3;shadow=1;" vertex="1" parent="1">
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<!-- 标题 -->
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</mxCell>
<!-- ==================== 时间线部分 ==================== -->
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<!-- 时间线标题 -->
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<!-- 时间线主轴 -->
<mxCell id="7" value="" style="endArrow=classic;html=1;strokeWidth=4;strokeColor=#2e7d32;flowAnimation=1;" edge="1" parent="1">
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<mxPoint x="80" y="185" as="sourcePoint" />
<mxPoint x="820" y="185" as="targetPoint" />
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</mxCell>
<!-- 阶段1：类加载 -->
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<mxCell id="9" value="<font color="#ffffff"><b>① 类加载</b></font>" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=0;fontSize=15;" vertex="1" parent="1">
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<!-- 阶段1说明 -->
<mxCell id="10" value="" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#bbdefb;strokeColor=#64b5f6;strokeWidth=1;" vertex="1" parent="1">
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</mxCell>
<!-- 连接线1 -->
<mxCell id="12" value="" style="endArrow=classic;html=1;strokeWidth=2;strokeColor=#1976d2;dashed=1;" edge="1" parent="1">
<mxGeometry width="50" height="50" relative="1" as="geometry">
<mxPoint x="160" y="185" as="sourcePoint" />
<mxPoint x="160" y="210" as="targetPoint" />
</mxGeometry>
</mxCell>
<!-- 阶段2：创建对象 -->
<mxCell id="13" value="" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#ff9800;strokeColor=#f57c00;strokeWidth=2;shadow=1;" vertex="1" parent="1">
<mxGeometry x="340" y="145" width="160" height="35" as="geometry" />
</mxCell>
<mxCell id="14" value="<font color="#ffffff"><b>② 创建对象</b></font>" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=0;fontSize=15;" vertex="1" parent="1">
<mxGeometry x="340" y="145" width="160" height="35" as="geometry" />
</mxCell>
<!-- 阶段2说明 -->
<mxCell id="15" value="" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#ffe0b2;strokeColor=#ffb74d;strokeWidth=1;" vertex="1" parent="1">
<mxGeometry x="325" y="210" width="190" height="85" as="geometry" />
</mxCell>
<mxCell id="16" value="<font color="#e65100"><b>实例成员就绪</b></font><br><font color="#424242">• 实例变量赋值</font><br><font color="#424242">• </font><font face="Courier New" color="#7b1fa2">this</font><font color="#424242"> 可用</font>" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=0;fontSize=13;" vertex="1" parent="1">
<mxGeometry x="325" y="210" width="190" height="85" as="geometry" />
</mxCell>
<!-- 连接线2 -->
<mxCell id="17" value="" style="endArrow=classic;html=1;strokeWidth=2;strokeColor=#ff9800;dashed=1;" edge="1" parent="1">
<mxGeometry width="50" height="50" relative="1" as="geometry">
<mxPoint x="420" y="185" as="sourcePoint" />
<mxPoint x="420" y="210" as="targetPoint" />
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</mxCell>
<!-- 阶段3：调用实例方法 -->
<mxCell id="18" value="" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#9c27b0;strokeColor=#7b1fa2;strokeWidth=2;shadow=1;" vertex="1" parent="1">
<mxGeometry x="600" y="145" width="180" height="35" as="geometry" />
</mxCell>
<mxCell id="19" value="<font color="#ffffff"><b>③ 调用实例方法</b></font>" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=0;fontSize=15;" vertex="1" parent="1">
<mxGeometry x="600" y="145" width="180" height="35" as="geometry" />
</mxCell>
<!-- 阶段3说明 -->
<mxCell id="20" value="" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#e1bee7;strokeColor=#ba68c8;strokeWidth=1;" vertex="1" parent="1">
<mxGeometry x="585" y="210" width="210" height="85" as="geometry" />
</mxCell>
<mxCell id="21" value="<font color="#7b1fa2"><b>所有成员可用</b></font><br><font color="#424242">• 静态成员 ✅</font><br><font color="#424242">• 实例成员 ✅</font>" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=0;fontSize=13;" vertex="1" parent="1">
<mxGeometry x="585" y="210" width="210" height="85" as="geometry" />
</mxCell>
<!-- 连接线3 -->
<mxCell id="22" value="" style="endArrow=classic;html=1;strokeWidth=2;strokeColor=#9c27b0;dashed=1;" edge="1" parent="1">
<mxGeometry width="50" height="50" relative="1" as="geometry">
<mxPoint x="690" y="185" as="sourcePoint" />
<mxPoint x="690" y="210" as="targetPoint" />
</mxGeometry>
</mxCell>
<!-- 箭头标签 -->
<mxCell id="23" value="<font color="#2e7d32" style="font-size: 12px;">时间</font>" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=0;" vertex="1" parent="1">
<mxGeometry x="790" y="165" width="40" height="20" as="geometry" />
</mxCell>
<!-- 时间线节点 -->
<mxCell id="24" value="" style="ellipse;whiteSpace=wrap;html=1;fillColor=#2e7d32;strokeColor=#1b5e20;strokeWidth=2;" vertex="1" parent="1">
<mxGeometry x="152" y="177" width="16" height="16" as="geometry" />
</mxCell>
<mxCell id="25" value="" style="ellipse;whiteSpace=wrap;html=1;fillColor=#2e7d32;strokeColor=#1b5e20;strokeWidth=2;" vertex="1" parent="1">
<mxGeometry x="412" y="177" width="16" height="16" as="geometry" />
</mxCell>
<mxCell id="26" value="" style="ellipse;whiteSpace=wrap;html=1;fillColor=#2e7d32;strokeColor=#1b5e20;strokeWidth=2;" vertex="1" parent="1">
<mxGeometry x="682" y="177" width="16" height="16" as="geometry" />
</mxCell>
<!-- ==================== 问题部分 ==================== -->
<mxCell id="27" value="" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#ffebee;strokeColor=#e53935;strokeWidth=2;shadow=1;" vertex="1" parent="1">
<mxGeometry x="45" y="385" width="810" height="330" as="geometry" />
</mxCell>
<!-- 问题标题 -->
<mxCell id="28" value="" style="rounded=0;whiteSpace=wrap;html=1;fillColor=#e53935;strokeColor=none;" vertex="1" parent="1">
<mxGeometry x="45" y="385" width="810" height="40" as="geometry" />
</mxCell>
<mxCell id="29" value="<font color="#ffffff" style="font-size: 16px;"><b>❓ 问题：为什么静态方法不能访问非静态成员？</b></font>" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=0;" vertex="1" parent="1">
<mxGeometry x="45" y="385" width="810" height="40" as="geometry" />
</mxCell>
<!-- 核心问题说明 -->
<mxCell id="30" value="" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#ffcdd2;strokeColor=#ef9a9a;strokeWidth=2;" vertex="1" parent="1">
<mxGeometry x="65" y="440" width="770" height="55" as="geometry" />
</mxCell>
<mxCell id="31" value="<font style="font-size: 15px;" color="#c62828">⚠️ <b>当调用静态方法时，可能还没有创建任何对象！</b></font>" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=0;" vertex="1" parent="1">
<mxGeometry x="65" y="440" width="770" height="55" as="geometry" />
</mxCell>
<!-- 代码示例 -->
<mxCell id="32" value="" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#263238;strokeColor=#37474f;strokeWidth=2;" vertex="1" parent="1">
<mxGeometry x="65" y="510" width="770" height="50" as="geometry" />
</mxCell>
<mxCell id="33" value="<font face="Courier New" color="#80cbc4" style="font-size: 14px;">ClassName.</font><font face="Courier New" color="#ffcc80" style="font-size: 14px;">staticMethod</font><font face="Courier New" color="#ffffff" style="font-size: 14px;">();</font><font color="#78909c" style="font-size: 13px;"> // 不需要对象就能调用</font>" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=0;" vertex="1" parent="1">
<mxGeometry x="65" y="510" width="770" height="50" as="geometry" />
</mxCell>
<!-- 此时说明标题 -->
<mxCell id="34" value="<font color="#c62828" style="font-size: 15px;"><b>此时的状态：</b></font>" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;whiteSpace=wrap;rounded=0;spacingLeft=10;" vertex="1" parent="1">
<mxGeometry x="65" y="570" width="150" height="30" as="geometry" />
</mxCell>
<!-- 三个状态卡片 -->
<!-- 静态变量已存在 -->
<mxCell id="35" value="" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#c8e6c9;strokeColor=#81c784;strokeWidth=2;shadow=1;" vertex="1" parent="1">
<mxGeometry x="65" y="605" width="240" height="90" as="geometry" />
</mxCell>
<mxCell id="36" value="<font color="#2e7d32" style="font-size: 22px;">✅</font>" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=0;" vertex="1" parent="1">
<mxGeometry x="65" y="610" width="240" height="30" as="geometry" />
</mxCell>
<mxCell id="37" value="<font color="#2e7d32"><b>静态变量已存在</b></font><br><font color="#424242" style="font-size: 12px;">类加载时就已初始化</font>" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=0;fontSize=14;" vertex="1" parent="1">
<mxGeometry x="65" y="640" width="240" height="50" as="geometry" />
</mxCell>
<!-- 实例变量不存在 -->
<mxCell id="38" value="" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#ffcdd2;strokeColor=#ef9a9a;strokeWidth=2;shadow=1;" vertex="1" parent="1">
<mxGeometry x="325" y="605" width="240" height="90" as="geometry" />
</mxCell>
<mxCell id="39" value="<font color="#c62828" style="font-size: 22px;">❌</font>" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=0;" vertex="1" parent="1">
<mxGeometry x="325" y="610" width="240" height="30" as="geometry" />
</mxCell>
<mxCell id="40" value="<font color="#c62828"><b>实例变量不存在</b></font><br><font color="#424242" style="font-size: 12px;">没有对象，哪来的实例变量？</font>" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=0;fontSize=14;" vertex="1" parent="1">
<mxGeometry x="325" y="640" width="240" height="50" as="geometry" />
</mxCell>
<!-- this 不存在 -->
<mxCell id="41" value="" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#ffcdd2;strokeColor=#ef9a9a;strokeWidth=2;shadow=1;" vertex="1" parent="1">
<mxGeometry x="585" y="605" width="240" height="90" as="geometry" />
</mxCell>
<mxCell id="42" value="<font color="#c62828" style="font-size: 22px;">❌</font>" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=0;" vertex="1" parent="1">
<mxGeometry x="585" y="610" width="240" height="30" as="geometry" />
</mxCell>
<mxCell id="43" value="<font face="Courier New" color="#c62828"><b>this</b></font><font color="#c62828"><b> 不存在</b></font><br><font color="#424242" style="font-size: 12px;">没有对象，this 指向谁？</font>" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=0;fontSize=14;" vertex="1" parent="1">
<mxGeometry x="585" y="640" width="240" height="50" as="geometry" />
</mxCell>
</root>
</mxGraphModel>
</diagram>
</mxfile>
2.3 方法重载（Overload）
<mxfile>
<diagram id="method-overloading" name="方法重载">
<mxGraphModel dx="1281" dy="581" grid="1" gridSize="10" guides="1" tooltips="1" connect="1" arrows="1" fold="1" page="1" pageScale="1" pageWidth="900" pageHeight="700" math="0" shadow="0">
<root>
<mxCell id="0" />
<mxCell id="1" parent="0" />
<mxCell id="2" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;fontSize=28;fontStyle=1;fontColor=#1E293B;" value="方法重载" vertex="1">
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<mxCell id="4" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#EDE9FE;strokeColor=#8B5CF6;strokeWidth=3;" value="" vertex="1">
<mxGeometry height="90" width="740" x="80" y="120" as="geometry" />
</mxCell>
<mxCell id="5" parent="1" style="text;html=1;strokeColor=none;fillColor=#8B5CF6;align=center;verticalAlign=middle;fontSize=16;fontStyle=1;fontColor=#FFFFFF;rounded=1;" value="定义" vertex="1">
<mxGeometry height="32" width="70" x="100" y="135" as="geometry" />
</mxCell>
<mxCell id="6" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=18;fontStyle=1;fontColor=#4C1D95;" value="同一个类中，方法名相同，参数列表不同" vertex="1">
<mxGeometry height="40" width="600" x="190" y="130" as="geometry" />
</mxCell>
<mxCell id="7" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#FFFFFF;strokeColor=#A78BFA;strokeWidth=2;fontColor=#6D28D9;fontSize=12;fontStyle=1;" value="同一个类" vertex="1">
<mxGeometry height="28" width="90" x="200" y="170" as="geometry" />
</mxCell>
<mxCell id="8" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;fontSize=18;fontStyle=1;fontColor=#8B5CF6;" value="+" vertex="1">
<mxGeometry height="28" width="30" x="295" y="170" as="geometry" />
</mxCell>
<mxCell id="9" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#FFFFFF;strokeColor=#A78BFA;strokeWidth=2;fontColor=#6D28D9;fontSize=12;fontStyle=1;" value="方法名相同" vertex="1">
<mxGeometry height="28" width="100" x="330" y="170" as="geometry" />
</mxCell>
<mxCell id="10" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;fontSize=18;fontStyle=1;fontColor=#8B5CF6;" value="+" vertex="1">
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</mxCell>
<mxCell id="11" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#FFFFFF;strokeColor=#A78BFA;strokeWidth=2;fontColor=#6D28D9;fontSize=12;fontStyle=1;" value="参数列表不同" vertex="1">
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<mxCell id="12" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#DCFCE7;strokeColor=#22C55E;strokeWidth=3;" value="" vertex="1">
<mxGeometry height="200" width="740" x="80" y="230" as="geometry" />
</mxCell>
<mxCell id="13" parent="1" style="text;html=1;strokeColor=none;fillColor=#22C55E;align=center;verticalAlign=middle;fontSize=16;fontStyle=1;fontColor=#FFFFFF;rounded=1;" value="参数列表不同指" vertex="1">
<mxGeometry height="32" width="140" x="100" y="248" as="geometry" />
</mxCell>
<mxCell id="14" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=12;fontColor=#166534;fontStyle=2;" value="✅ 有效的重载条件" vertex="1">
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</mxCell>
<mxCell id="15" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#FFFFFF;strokeColor=#86EFAC;strokeWidth=2;" value="" vertex="1">
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</mxCell>
<mxCell id="16" parent="1" style="ellipse;whiteSpace=wrap;html=1;fillColor=#22C55E;strokeColor=none;fontColor=#FFFFFF;fontSize=14;fontStyle=1;" value="1" vertex="1">
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</mxCell>
<mxCell id="17" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=15;fontStyle=1;fontColor=#166534;" value="参数个数不同" vertex="1">
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<mxCell id="18" parent="1" style="text;html=1;strokeColor=none;fillColor=#F0FDF4;align=left;verticalAlign=middle;fontSize=12;fontFamily=Courier New;fontColor=#15803D;rounded=1;spacingLeft=8;" value="add(int a)<br>add(int a, int b)" vertex="1">
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</mxCell>
<mxCell id="19" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#FFFFFF;strokeColor=#86EFAC;strokeWidth=2;" value="" vertex="1">
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</mxCell>
<mxCell id="20" parent="1" style="ellipse;whiteSpace=wrap;html=1;fillColor=#22C55E;strokeColor=none;fontColor=#FFFFFF;fontSize=14;fontStyle=1;" value="2" vertex="1">
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</mxCell>
<mxCell id="21" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=15;fontStyle=1;fontColor=#166534;" value="参数类型不同" vertex="1">
<mxGeometry height="30" width="150" x="395" y="310" as="geometry" />
</mxCell>
<mxCell id="22" parent="1" style="text;html=1;strokeColor=none;fillColor=#F0FDF4;align=left;verticalAlign=middle;fontSize=12;fontFamily=Courier New;fontColor=#15803D;rounded=1;spacingLeft=8;" value="add(int a)<br>add(double a)" vertex="1">
<mxGeometry height="55" width="190" x="355" y="350" as="geometry" />
</mxCell>
<mxCell id="23" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#FFFFFF;strokeColor=#86EFAC;strokeWidth=2;" value="" vertex="1">
<mxGeometry height="120" width="220" x="580" y="295" as="geometry" />
</mxCell>
<mxCell id="24" parent="1" style="ellipse;whiteSpace=wrap;html=1;fillColor=#22C55E;strokeColor=none;fontColor=#FFFFFF;fontSize=14;fontStyle=1;" value="3" vertex="1">
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</mxCell>
<mxCell id="25" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=15;fontStyle=1;fontColor=#166534;" value="参数顺序不同" vertex="1">
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</mxCell>
<mxCell id="26" parent="1" style="text;html=1;strokeColor=none;fillColor=#F0FDF4;align=left;verticalAlign=middle;fontSize=12;fontFamily=Courier New;fontColor=#15803D;rounded=1;spacingLeft=8;" value="add(int a, double b)<br>add(double a, int b)" vertex="1">
<mxGeometry height="55" width="190" x="595" y="350" as="geometry" />
</mxCell>
<mxCell id="27" parent="1" style="text;html=1;strokeColor=none;fillColor=#FEF3C7;align=center;verticalAlign=middle;fontSize=10;fontStyle=1;fontColor=#92400E;rounded=1;" value="⚠️ 不推荐" vertex="1">
<mxGeometry height="22" width="65" x="729" y="314" as="geometry" />
</mxCell>
<mxCell id="28" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#FEE2E2;strokeColor=#EF4444;strokeWidth=3;" value="" vertex="1">
<mxGeometry height="130" width="740" x="80" y="450" as="geometry" />
</mxCell>
<mxCell id="29" parent="1" style="text;html=1;strokeColor=none;fillColor=#EF4444;align=center;verticalAlign=middle;fontSize=16;fontStyle=1;fontColor=#FFFFFF;rounded=1;" value="⚠️ 注意事项" vertex="1">
<mxGeometry height="32" width="120" x="100" y="468" as="geometry" />
</mxCell>
<mxCell id="30" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=12;fontColor=#DC2626;fontStyle=2;" value="❌ 无效的重载条件" vertex="1">
<mxGeometry height="28" width="140" x="235" y="470" as="geometry" />
</mxCell>
<mxCell id="31" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#FFFFFF;strokeColor=#FCA5A5;strokeWidth=2;" value="" vertex="1">
<mxGeometry height="50" width="340" x="100" y="515" as="geometry" />
</mxCell>
<mxCell id="32" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;fontSize=20;" value="❌" vertex="1">
<mxGeometry height="30" width="30" x="110" y="525" as="geometry" />
</mxCell>
<mxCell id="33" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=15;fontStyle=1;fontColor=#DC2626;" value="与返回值类型无关！" vertex="1">
<mxGeometry height="30" width="180" x="145" y="525" as="geometry" />
</mxCell>
<mxCell id="34" parent="1" style="text;html=1;strokeColor=none;fillColor=#FEF2F2;align=center;verticalAlign=middle;fontSize=11;fontColor=#991B1B;rounded=1;" value="仅返回值不同不构成重载" vertex="1">
<mxGeometry height="24" width="120" x="310" y="528" as="geometry" />
</mxCell>
<mxCell id="35" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#FFFFFF;strokeColor=#FCA5A5;strokeWidth=2;" value="" vertex="1">
<mxGeometry height="50" width="340" x="460" y="515" as="geometry" />
</mxCell>
<mxCell id="36" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;fontSize=20;" value="❌" vertex="1">
<mxGeometry height="30" width="30" x="470" y="525" as="geometry" />
</mxCell>
<mxCell id="37" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=15;fontStyle=1;fontColor=#DC2626;" value="与参数名无关！" vertex="1">
<mxGeometry height="30" width="150" x="505" y="525" as="geometry" />
</mxCell>
<mxCell id="38" parent="1" style="text;html=1;strokeColor=none;fillColor=#FEF2F2;align=center;verticalAlign=middle;fontSize=11;fontColor=#991B1B;rounded=1;" value="仅参数名不同不构成重载" vertex="1">
<mxGeometry height="24" width="120" x="670" y="528" as="geometry" />
</mxCell>
<mxCell id="39" parent="1" style="text;html=1;strokeColor=none;fillColor=#DBEAFE;align=center;verticalAlign=middle;fontSize=12;fontStyle=1;fontColor=#1E40AF;rounded=1;" value="💡 编译器通过参数列表区分重载方法，调用时根据传入参数自动匹配" vertex="1">
<mxGeometry height="30" width="500" x="200" y="605" as="geometry" />
</mxCell>
</root>
</mxGraphModel>
</diagram>
</mxfile>
public class OverloadDemo {
// ═══════════════════════════════════════════════════════════════════
// ✅ 正确的重载
// ═══════════════════════════════════════════════════════════════════
// 参数个数不同
public int add(int a, int b) {
return a + b;
}
public int add(int a, int b, int c) {
return a + b + c;
}
// 参数类型不同
public double add(double a, double b) {
return a + b;
}
public String add(String a, String b) {
return a + b;
}
// ═══════════════════════════════════════════════════════════════════
// ❌ 错误的重载
// ═══════════════════════════════════════════════════════════════════
// 只有返回值不同，不构成重载
// public double add(int a, int b) { // ❌ 编译错误
// return a + b;
// }
// 只有参数名不同，不构成重载
// public int add(int x, int y) { // ❌ 编译错误
// return x + y;
// }
public static void main(String[] args) {
OverloadDemo demo = new OverloadDemo();
// 编译器根据参数类型和个数选择合适的方法
System.out.println(demo.add(1, 2)); // 调用 add(int, int)
System.out.println(demo.add(1, 2, 3)); // 调用 add(int, int, int)
System.out.println(demo.add(1.5, 2.5)); // 调用 add(double, double)
System.out.println(demo.add("Hello", "!")); // 调用 add(String, String)
}
}
重载的编译期绑定：
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</mxfile>
2.4 方法重写（Override）
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<mxCell id="17" value="✅ 返回值类型相同或是子类" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=13;fontColor=#166534;fontStyle=1;" vertex="1" parent="1">
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<mxCell id="25" value="// 父类<br>protected Object getValue() throws Exception" style="text;html=1;strokeColor=none;fillColor=#FFFFFF;align=left;verticalAlign=middle;fontSize=11;fontFamily=Courier New;fontColor=#166534;rounded=1;spacingLeft=8;" vertex="1" parent="1">
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<mxCell id="37" value="禁止重写" style="text;html=1;strokeColor=none;fillColor=#FEF2F2;align=center;verticalAlign=middle;fontSize=12;fontColor=#991B1B;rounded=1;" vertex="1" parent="1">
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class Animal {
protected String name;
public void eat() {
System.out.println("动物吃东西");
}
public Animal getAnimal() {
return new Animal();
}
// private 方法不能被重写
private void privateMethod() {
System.out.println("私有方法");
}
// final 方法不能被重写
public final void finalMethod() {
System.out.println("final方法");
}
// static 方法不是重写，是隐藏
public static void staticMethod() {
System.out.println("Animal的静态方法");
}
}
class Dog extends Animal {
// ═══════════════════════════════════════════════════════════════════
// ✅ 正确的重写
// ═══════════════════════════════════════════════════════════════════
@Override
public void eat() { // 方法名、参数列表相同
System.out.println("狗吃骨头");
}
// 协变返回类型：返回值可以是父类返回值的子类
@Override
public Dog getAnimal() { // 返回 Dog 是 Animal 的子类
return new Dog();
}
// ═══════════════════════════════════════════════════════════════════
// ❌ 错误的重写
// ═══════════════════════════════════════════════════════════════════
// 不能缩小访问权限
// @Override
// void eat() { } // ❌ default < protected
// 不能重写 final 方法
// @Override
// public void finalMethod() { } // ❌ 编译错误
// static 方法是隐藏，不是重写
public static void staticMethod() { // 没有 @Override
System.out.println("Dog的静态方法");
}
}
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<diagram id="overload-vs-override" name="重载vs重写对比">
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<mxCell id="4" value="对比项" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#374151;strokeColor=#1F2937;strokeWidth=2;fontColor=#FFFFFF;fontSize=15;fontStyle=1;" vertex="1" parent="1">
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<mxCell id="5" value="重载 Overload" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#8B5CF6;strokeColor=#7C3AED;strokeWidth=2;fontColor=#FFFFFF;fontSize=15;fontStyle=1;" vertex="1" parent="1">
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</mxCell>
<mxCell id="7" value="发生位置" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#F3F4F6;strokeColor=#D1D5DB;strokeWidth=1;fontColor=#374151;fontSize=14;fontStyle=1;" vertex="1" parent="1">
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</mxCell>
<mxCell id="8" value="同一个类中" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#EDE9FE;strokeColor=#C4B5FD;strokeWidth=1;fontColor=#5B21B6;fontSize=14;fontStyle=1;" vertex="1" parent="1">
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</mxCell>
<mxCell id="9" value="父子类之间" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#DCFCE7;strokeColor=#86EFAC;strokeWidth=1;fontColor=#166534;fontSize=14;fontStyle=1;" vertex="1" parent="1">
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</mxCell>
<mxCell id="10" value="方法名" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#F3F4F6;strokeColor=#D1D5DB;strokeWidth=1;fontColor=#374151;fontSize=14;fontStyle=1;" vertex="1" parent="1">
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<mxCell id="11" value="必须相同" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#EDE9FE;strokeColor=#C4B5FD;strokeWidth=1;fontColor=#5B21B6;fontSize=14;" vertex="1" parent="1">
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</mxCell>
<mxCell id="12" value="必须相同" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#DCFCE7;strokeColor=#86EFAC;strokeWidth=1;fontColor=#166534;fontSize=14;" vertex="1" parent="1">
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</mxCell>
<mxCell id="13" value="参数列表" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#F3F4F6;strokeColor=#D1D5DB;strokeWidth=1;fontColor=#374151;fontSize=14;fontStyle=1;" vertex="1" parent="1">
<mxGeometry x="80" y="295" width="120" height="50" as="geometry" />
</mxCell>
<mxCell id="14" value="⚡ 必须不同" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#DDD6FE;strokeColor=#A78BFA;strokeWidth=2;fontColor=#5B21B6;fontSize=14;fontStyle=1;" vertex="1" parent="1">
<mxGeometry x="210" y="295" width="290" height="50" as="geometry" />
</mxCell>
<mxCell id="15" value="⚡ 必须相同" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#BBF7D0;strokeColor=#4ADE80;strokeWidth=2;fontColor=#166534;fontSize=14;fontStyle=1;" vertex="1" parent="1">
<mxGeometry x="510" y="295" width="310" height="50" as="geometry" />
</mxCell>
<mxCell id="16" value="返回值" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#F3F4F6;strokeColor=#D1D5DB;strokeWidth=1;fontColor=#374151;fontSize=14;fontStyle=1;" vertex="1" parent="1">
<mxGeometry x="80" y="355" width="120" height="50" as="geometry" />
</mxCell>
<mxCell id="17" value="无要求" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#EDE9FE;strokeColor=#C4B5FD;strokeWidth=1;fontColor=#5B21B6;fontSize=14;" vertex="1" parent="1">
<mxGeometry x="210" y="355" width="290" height="50" as="geometry" />
</mxCell>
<mxCell id="18" value="相同或子类（协变返回）" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#DCFCE7;strokeColor=#86EFAC;strokeWidth=1;fontColor=#166534;fontSize=14;" vertex="1" parent="1">
<mxGeometry x="510" y="355" width="310" height="50" as="geometry" />
</mxCell>
<mxCell id="19" value="访问权限" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#F3F4F6;strokeColor=#D1D5DB;strokeWidth=1;fontColor=#374151;fontSize=14;fontStyle=1;" vertex="1" parent="1">
<mxGeometry x="80" y="415" width="120" height="50" as="geometry" />
</mxCell>
<mxCell id="20" value="无要求" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#EDE9FE;strokeColor=#C4B5FD;strokeWidth=1;fontColor=#5B21B6;fontSize=14;" vertex="1" parent="1">
<mxGeometry x="210" y="415" width="290" height="50" as="geometry" />
</mxCell>
<mxCell id="21" value="不能更小（≥ 父类）" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#DCFCE7;strokeColor=#86EFAC;strokeWidth=1;fontColor=#166534;fontSize=14;" vertex="1" parent="1">
<mxGeometry x="510" y="415" width="310" height="50" as="geometry" />
</mxCell>
<mxCell id="22" value="绑定时机" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#F3F4F6;strokeColor=#D1D5DB;strokeWidth=1;fontColor=#374151;fontSize=14;fontStyle=1;" vertex="1" parent="1">
<mxGeometry x="80" y="475" width="120" height="50" as="geometry" />
</mxCell>
<mxCell id="23" value="编译期（静态绑定）" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#EDE9FE;strokeColor=#C4B5FD;strokeWidth=1;fontColor=#5B21B6;fontSize=14;" vertex="1" parent="1">
<mxGeometry x="210" y="475" width="290" height="50" as="geometry" />
</mxCell>
<mxCell id="24" value="运行期（动态绑定）" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#DCFCE7;strokeColor=#86EFAC;strokeWidth=1;fontColor=#166534;fontSize=14;" vertex="1" parent="1">
<mxGeometry x="510" y="475" width="310" height="50" as="geometry" />
</mxCell>
<mxCell id="25" value="多态体现" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#F3F4F6;strokeColor=#D1D5DB;strokeWidth=1;fontColor=#374151;fontSize=14;fontStyle=1;" vertex="1" parent="1">
<mxGeometry x="80" y="535" width="120" height="50" as="geometry" />
</mxCell>
<mxCell id="26" value="编译时多态" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#DDD6FE;strokeColor=#A78BFA;strokeWidth=2;fontColor=#5B21B6;fontSize=14;fontStyle=1;" vertex="1" parent="1">
<mxGeometry x="210" y="535" width="290" height="50" as="geometry" />
</mxCell>
<mxCell id="27" value="运行时多态" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#BBF7D0;strokeColor=#4ADE80;strokeWidth=2;fontColor=#166534;fontSize=14;fontStyle=1;" vertex="1" parent="1">
<mxGeometry x="510" y="535" width="310" height="50" as="geometry" />
</mxCell>
<mxCell id="28" value="" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#FEF3C7;strokeColor=#F59E0B;strokeWidth=2;" vertex="1" parent="1">
<mxGeometry x="80" y="610" width="740" height="80" as="geometry" />
</mxCell>
<mxCell id="29" value="💡 核心区别" style="text;html=1;strokeColor=none;fillColor=#F59E0B;align=center;verticalAlign=middle;fontSize=14;fontStyle=1;fontColor=#FFFFFF;rounded=1;" vertex="1" parent="1">
<mxGeometry x="100" y="625" width="100" height="28" as="geometry" />
</mxCell>
<mxCell id="30" value="重载：同类中方法名同、参数不同 → 编译器根据参数选择方法" style="text;html=1;strokeColor=none;fillColor=#FFFFFF;align=center;verticalAlign=middle;fontSize=12;fontColor=#92400E;rounded=1;fontStyle=1;" vertex="1" parent="1">
<mxGeometry x="100" y="658" width="340" height="24" as="geometry" />
</mxCell>
<mxCell id="31" value="重写：父子类方法签名相同 → JVM运行时根据对象类型选择方法" style="text;html=1;strokeColor=none;fillColor=#FFFFFF;align=center;verticalAlign=middle;fontSize=12;fontColor=#92400E;rounded=1;fontStyle=1;" vertex="1" parent="1">
<mxGeometry x="460" y="658" width="345" height="24" as="geometry" />
</mxCell>
</root>
</mxGraphModel>
</diagram>
</mxfile>
2.5 可变参数（Varargs）
<mxfile>
<diagram id="varargs" name="可变参数">
<mxGraphModel dx="900" dy="680" grid="1" gridSize="10" guides="1" tooltips="1" connect="1" arrows="1" fold="1" page="1" pageScale="1" pageWidth="900" pageHeight="680" math="0" shadow="0">
<root>
<mxCell id="0" />
<mxCell id="1" parent="0" />
<!-- 主容器 -->
<mxCell id="2" value="" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#fafafa;strokeColor=#5c6bc0;strokeWidth=3;shadow=1;" vertex="1" parent="1">
<mxGeometry x="25" y="15" width="850" height="650" as="geometry" />
</mxCell>
<!-- 标题 -->
<mxCell id="3" value="可变参数" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=0;fontSize=28;fontStyle=1;fontColor=#3949ab;" vertex="1" parent="1">
<mxGeometry x="325" y="30" width="250" height="45" as="geometry" />
</mxCell>
<!-- ==================== 语法部分 ==================== -->
<mxCell id="4" value="" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#e3f2fd;strokeColor=#1976d2;strokeWidth=2;shadow=1;" vertex="1" parent="1">
<mxGeometry x="45" y="90" width="810" height="100" as="geometry" />
</mxCell>
<!-- 语法标题 -->
<mxCell id="5" value="" style="rounded=0;whiteSpace=wrap;html=1;fillColor=#1976d2;strokeColor=none;" vertex="1" parent="1">
<mxGeometry x="45" y="90" width="100" height="35" as="geometry" />
</mxCell>
<mxCell id="6" value="<font color="#ffffff"><b>📝 语法</b></font>" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=0;fontSize=14;" vertex="1" parent="1">
<mxGeometry x="45" y="90" width="100" height="35" as="geometry" />
</mxCell>
<!-- 语法代码 -->
<mxCell id="7" value="" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#263238;strokeColor=#37474f;strokeWidth=2;" vertex="1" parent="1">
<mxGeometry x="65" y="140" width="770" height="40" as="geometry" />
</mxCell>
<mxCell id="8" value="<font face="Courier New" color="#ce93d8" style="font-size: 16px;">类型</font><font face="Courier New" color="#ffcc80" style="font-size: 16px;"><b>...</b></font><font face="Courier New" color="#ffffff" style="font-size: 16px;"> 参数名</font>" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=0;" vertex="1" parent="1">
<mxGeometry x="65" y="140" width="770" height="40" as="geometry" />
</mxCell>
<!-- ==================== 本质部分 ==================== -->
<mxCell id="9" value="" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#fff3e0;strokeColor=#ff9800;strokeWidth=2;shadow=1;" vertex="1" parent="1">
<mxGeometry x="45" y="210" width="810" height="110" as="geometry" />
</mxCell>
<!-- 本质标题 -->
<mxCell id="10" value="" style="rounded=0;whiteSpace=wrap;html=1;fillColor=#ff9800;strokeColor=none;" vertex="1" parent="1">
<mxGeometry x="45" y="210" width="100" height="35" as="geometry" />
</mxCell>
<mxCell id="11" value="<font color="#ffffff"><b>⚙️ 本质</b></font>" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=0;fontSize=14;" vertex="1" parent="1">
<mxGeometry x="45" y="210" width="100" height="35" as="geometry" />
</mxCell>
<!-- 本质说明 -->
<mxCell id="12" value="<font color="#e65100" style="font-size: 16px;"><b>编译时转换为数组</b></font>" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=0;" vertex="1" parent="1">
<mxGeometry x="45" y="250" width="810" height="30" as="geometry" />
</mxCell>
<!-- 转换示意 -->
<mxCell id="13" value="" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#263238;strokeColor=#37474f;strokeWidth=1;" vertex="1" parent="1">
<mxGeometry x="100" y="280" width="200" height="30" as="geometry" />
</mxCell>
<mxCell id="14" value="<font face="Courier New" color="#80cbc4">int... nums</font>" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=0;fontSize=13;" vertex="1" parent="1">
<mxGeometry x="100" y="280" width="200" height="30" as="geometry" />
</mxCell>
<!-- 箭头 -->
<mxCell id="15" value="" style="endArrow=classic;html=1;strokeWidth=3;strokeColor=#ff9800;flowAnimation=1;" edge="1" parent="1">
<mxGeometry width="50" height="50" relative="1" as="geometry">
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<mxPoint x="420" y="295" as="targetPoint" />
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</mxCell>
<mxCell id="16" value="<font color="#e65100">编译</font>" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=0;fontSize=12;" vertex="1" parent="1">
<mxGeometry x="345" y="270" width="50" height="20" as="geometry" />
</mxCell>
<mxCell id="17" value="" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#263238;strokeColor=#37474f;strokeWidth=1;" vertex="1" parent="1">
<mxGeometry x="440" y="280" width="200" height="30" as="geometry" />
</mxCell>
<mxCell id="18" value="<font face="Courier New" color="#80cbc4">int[] nums</font>" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=0;fontSize=13;" vertex="1" parent="1">
<mxGeometry x="440" y="280" width="200" height="30" as="geometry" />
</mxCell>
<!-- 等号图标 -->
<mxCell id="19" value="<font color="#4caf50" style="font-size: 20px;"><b>≡</b></font>" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=0;" vertex="1" parent="1">
<mxGeometry x="660" y="275" width="40" height="40" as="geometry" />
</mxCell>
<!-- ==================== 规则部分 ==================== -->
<mxCell id="20" value="" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#e8f5e9;strokeColor=#4caf50;strokeWidth=2;shadow=1;" vertex="1" parent="1">
<mxGeometry x="45" y="340" width="810" height="305" as="geometry" />
</mxCell>
<!-- 规则标题 -->
<mxCell id="21" value="" style="rounded=0;whiteSpace=wrap;html=1;fillColor=#4caf50;strokeColor=none;" vertex="1" parent="1">
<mxGeometry x="45" y="340" width="100" height="35" as="geometry" />
</mxCell>
<mxCell id="22" value="<font color="#ffffff"><b>📋 规则</b></font>" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=0;fontSize=14;" vertex="1" parent="1">
<mxGeometry x="45" y="340" width="100" height="35" as="geometry" />
</mxCell>
<!-- 规则1 -->
<mxCell id="23" value="" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#ffffff;strokeColor=#81c784;strokeWidth=2;shadow=1;" vertex="1" parent="1">
<mxGeometry x="65" y="390" width="370" height="70" as="geometry" />
</mxCell>
<mxCell id="24" value="" style="ellipse;whiteSpace=wrap;html=1;fillColor=#2e7d32;strokeColor=none;" vertex="1" parent="1">
<mxGeometry x="80" y="405" width="40" height="40" as="geometry" />
</mxCell>
<mxCell id="25" value="<font color="#ffffff"><b>1</b></font>" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=0;fontSize=16;" vertex="1" parent="1">
<mxGeometry x="80" y="405" width="40" height="40" as="geometry" />
</mxCell>
<mxCell id="26" value="<font color="#2e7d32" style="font-size: 14px;"><b>一个方法最多只能有</b></font><br><font color="#2e7d32" style="font-size: 14px;"><b>一个可变参数</b></font>" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=0;" vertex="1" parent="1">
<mxGeometry x="130" y="390" width="290" height="70" as="geometry" />
</mxCell>
<!-- 规则2 -->
<mxCell id="27" value="" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#ffffff;strokeColor=#81c784;strokeWidth=2;shadow=1;" vertex="1" parent="1">
<mxGeometry x="465" y="390" width="370" height="70" as="geometry" />
</mxCell>
<mxCell id="28" value="" style="ellipse;whiteSpace=wrap;html=1;fillColor=#2e7d32;strokeColor=none;" vertex="1" parent="1">
<mxGeometry x="480" y="405" width="40" height="40" as="geometry" />
</mxCell>
<mxCell id="29" value="<font color="#ffffff"><b>2</b></font>" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=0;fontSize=16;" vertex="1" parent="1">
<mxGeometry x="480" y="405" width="40" height="40" as="geometry" />
</mxCell>
<mxCell id="30" value="<font color="#2e7d32" style="font-size: 14px;"><b>可变参数必须是</b></font><br><font color="#2e7d32" style="font-size: 14px;"><b>最后一个参数</b></font>" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=0;" vertex="1" parent="1">
<mxGeometry x="530" y="390" width="290" height="70" as="geometry" />
</mxCell>
<!-- 规则3 -->
<mxCell id="31" value="" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#ffffff;strokeColor=#81c784;strokeWidth=2;shadow=1;" vertex="1" parent="1">
<mxGeometry x="65" y="480" width="370" height="70" as="geometry" />
</mxCell>
<mxCell id="32" value="" style="ellipse;whiteSpace=wrap;html=1;fillColor=#2e7d32;strokeColor=none;" vertex="1" parent="1">
<mxGeometry x="80" y="495" width="40" height="40" as="geometry" />
</mxCell>
<mxCell id="33" value="<font color="#ffffff"><b>3</b></font>" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=0;fontSize=16;" vertex="1" parent="1">
<mxGeometry x="80" y="495" width="40" height="40" as="geometry" />
</mxCell>
<mxCell id="34" value="<font color="#2e7d32" style="font-size: 14px;"><b>可变参数可以接收</b></font><br><font color="#2e7d32" style="font-size: 14px;"><b>0 个或多个值</b></font>" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=0;" vertex="1" parent="1">
<mxGeometry x="130" y="480" width="290" height="70" as="geometry" />
</mxCell>
<!-- 规则4 -->
<mxCell id="35" value="" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#ffffff;strokeColor=#81c784;strokeWidth=2;shadow=1;" vertex="1" parent="1">
<mxGeometry x="465" y="480" width="370" height="70" as="geometry" />
</mxCell>
<mxCell id="36" value="" style="ellipse;whiteSpace=wrap;html=1;fillColor=#2e7d32;strokeColor=none;" vertex="1" parent="1">
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public class VarargsDemo {
// ═══════════════════════════════════════════════════════════════════
// 可变参数方法
// ═══════════════════════════════════════════════════════════════════
public int sum(int... numbers) {
// numbers 本质是 int[] 数组
int total = 0;
for (int num : numbers) {
total += num;
}
return total;
}
// 可变参数必须在最后
public void print(String prefix, String... messages) {
for (String msg : messages) {
System.out.println(prefix + msg);
}
}
// ❌ 错误：可变参数不是最后一个
// public void wrong(String... messages, String suffix) { }
// ❌ 错误：多个可变参数
// public void wrong(String... a, int... b) { }
public static void main(String[] args) {
VarargsDemo demo = new VarargsDemo();
// 可变参数的多种调用方式
System.out.println(demo.sum()); // 0 个参数，结果：0
System.out.println(demo.sum(1)); // 1 个参数，结果：1
System.out.println(demo.sum(1, 2, 3)); // 3 个参数，结果：6
System.out.println(demo.sum(1, 2, 3, 4, 5)); // 5 个参数，结果：15
// 也可以直接传入数组
int[] nums = {1, 2, 3, 4, 5};
System.out.println(demo.sum(nums)); // 传入数组，结果：15
}
}
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2.6 参数传递机制
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<!-- ==================== 引用类型区域 ==================== -->
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<!-- 引用类型标题 -->
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<!-- 原始引用 ref -->
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<!-- 传递地址副本箭头 -->
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<!-- 副本引用 copy -->
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<!-- 从 ref 指向对象的箭头 -->
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<!-- 从 copy 指向对象的箭头 -->
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<!-- 指向同一对象的标注 -->
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<!-- 堆内存对象 -->
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<!-- 结论说明区域 -->
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<!-- 关键区别说明 -->
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public class ParameterPassDemo {
public static void main(String[] args) {
// ═══════════════════════════════════════════════════════════════
// 1. 基本类型传递
// ═══════════════════════════════════════════════════════════════
int num = 10;
changeValue(num);
System.out.println("num = " + num); // 还是 10，未改变
// ═══════════════════════════════════════════════════════════════
// 2. 引用类型传递 - 修改对象内容
// ═══════════════════════════════════════════════════════════════
int[] arr = {1, 2, 3};
changeArray(arr);
System.out.println("arr[0] = " + arr[0]); // 变成 100
// ═══════════════════════════════════════════════════════════════
// 3. 引用类型传递 - 试图改变引用指向
// ═══════════════════════════════════════════════════════════════
int[] arr2 = {1, 2, 3};
changeReference(arr2);
System.out.println("arr2[0] = " + arr2[0]); // 还是 1，引用未改变
// ═══════════════════════════════════════════════════════════════
// 4. String 的特殊性
// ═══════════════════════════════════════════════════════════════
String str = "Hello";
changeString(str);
System.out.println("str = " + str); // 还是 "Hello"
}
// 基本类型：修改的是副本
public static void changeValue(int n) {
n = 100; // 只修改了副本
}
// 引用类型：可以通过引用修改对象内容
public static void changeArray(int[] array) {
array[0] = 100; // 修改了原对象的内容
}
// 引用类型：不能改变原引用的指向
public static void changeReference(int[] array) {
array = new int[]{100, 200, 300}; // 只改变了副本的指向
}
// String 是不可变的
public static void changeString(String s) {
s = "World"; // 创建了新对象，原引用不变
}
}
<mxfile>
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2.7 递归方法
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<mxCell id="16" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=0;fontSize=18;" value="<font color="#ffffff"><b>1</b></font>" vertex="1">
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<mxCell id="31" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#ffffff;strokeColor=#4caf50;strokeWidth=2;" value="<font face="Courier New" color="#2e7d32"><b>f(n-2)</b></font>" vertex="1">
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<mxCell id="35" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#f44336;strokeColor=#d32f2f;strokeWidth=2;" value="<font face="Courier New" color="#ffffff"><b>f(0)</b></font>" vertex="1">
<mxGeometry height="40" width="80" x="685" y="350" as="geometry" />
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<mxCell id="38" parent="1" style="rounded=0;whiteSpace=wrap;html=1;fillColor=#ff9800;strokeColor=none;" value="" vertex="1">
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<mxCell id="39" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=0;fontSize=14;" value="<font color="#ffffff"><b>⚠️ 注意事项</b></font>" vertex="1">
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public class RecursionDemo {
// ═══════════════════════════════════════════════════════════════════
// 1. 阶乘：n! = n × (n-1)!
// ═══════════════════════════════════════════════════════════════════
public static long factorial(int n) {
// 递归出口
if (n <= 1) {
return 1;
}
// 递归体
return n * factorial(n - 1);
}
// ═══════════════════════════════════════════════════════════════════
// 2. 斐波那契数列：F(n) = F(n-1) + F(n-2)
// ═══════════════════════════════════════════════════════════════════
public static long fibonacci(int n) {
// 递归出口
if (n <= 1) {
return n;
}
// 递归体（效率较低，有重复计算）
return fibonacci(n - 1) + fibonacci(n - 2);
}
// 优化版：使用记忆化
public static long fibonacciOptimized(int n, long[] memo) {
if (n <= 1) {
return n;
}
if (memo[n] != 0) {
return memo[n]; // 已计算过，直接返回
}
memo[n] = fibonacciOptimized(n - 1, memo) + fibonacciOptimized(n - 2, memo);
return memo[n];
}
// ═══════════════════════════════════════════════════════════════════
// 3. 二分查找
// ═══════════════════════════════════════════════════════════════════
public static int binarySearch(int[] arr, int target, int left, int right) {
// 递归出口
if (left > right) {
return -1; // 未找到
}
int mid = left + (right - left) / 2;
if (arr[mid] == target) {
return mid; // 找到了
} else if (arr[mid] > target) {
return binarySearch(arr, target, left, mid - 1);
} else {
return binarySearch(arr, target, mid + 1, right);
}
}
public static void main(String[] args) {
System.out.println("5! = " + factorial(5)); // 120
System.out.println("fib(10) = " + fibonacci(10)); // 55
int[] arr = {1, 3, 5, 7, 9, 11, 13};
System.out.println("index of 7: " + binarySearch(arr, 7, 0, arr.length - 1)); // 3
}
}
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<mxCell id="19" value="main()" style="text;html=1;strokeColor=none;fillColor=#22C55E;align=center;verticalAlign=middle;fontSize=13;fontStyle=1;fontColor=#FFFFFF;rounded=1;fontFamily=Courier New;" vertex="1" parent="1">
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</mxCell>
<mxCell id="26" value="返回 1" style="text;html=1;strokeColor=none;fillColor=#EF4444;align=center;verticalAlign=middle;fontSize=14;fontStyle=1;fontColor=#FFFFFF;rounded=1;" vertex="1" parent="1">
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<mxCell id="27" value="基准情况触发" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;fontSize=11;fontColor=#DC2626;" vertex="1" parent="1">
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<mxCell id="29" value="2 × 1 = 2" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;fontSize=14;fontStyle=1;fontColor=#92400E;fontFamily=Courier New;" vertex="1" parent="1">
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<mxCell id="30" value="返回 2" style="text;html=1;strokeColor=none;fillColor=#F59E0B;align=center;verticalAlign=middle;fontSize=13;fontStyle=1;fontColor=#FFFFFF;rounded=1;" vertex="1" parent="1">
<mxGeometry x="550" y="283" width="80" height="24" as="geometry" />
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</mxCell>
<mxCell id="32" value="3 × 2 = 6" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;fontSize=14;fontStyle=1;fontColor=#1E40AF;fontFamily=Courier New;" vertex="1" parent="1">
<mxGeometry x="520" y="340" width="140" height="24" as="geometry" />
</mxCell>
<mxCell id="33" value="返回 6" style="text;html=1;strokeColor=none;fillColor=#3B82F6;align=center;verticalAlign=middle;fontSize=13;fontStyle=1;fontColor=#FFFFFF;rounded=1;" vertex="1" parent="1">
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<mxCell id="34" value="" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#EDE9FE;strokeColor=#8B5CF6;strokeWidth=2;" vertex="1" parent="1">
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</mxCell>
<mxCell id="35" value="4 × 6 = 24" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;fontSize=14;fontStyle=1;fontColor=#5B21B6;fontFamily=Courier New;" vertex="1" parent="1">
<mxGeometry x="520" y="425" width="140" height="24" as="geometry" />
</mxCell>
<mxCell id="36" value="返回 24" style="text;html=1;strokeColor=none;fillColor=#8B5CF6;align=center;verticalAlign=middle;fontSize=13;fontStyle=1;fontColor=#FFFFFF;rounded=1;" vertex="1" parent="1">
<mxGeometry x="545" y="453" width="90" height="24" as="geometry" />
</mxCell>
<mxCell id="37" value="" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#F0FDF4;strokeColor=#22C55E;strokeWidth=2;" vertex="1" parent="1">
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</mxCell>
<mxCell id="38" value="结果: 24" style="text;html=1;strokeColor=none;fillColor=#22C55E;align=center;verticalAlign=middle;fontSize=14;fontStyle=1;fontColor=#FFFFFF;rounded=1;" vertex="1" parent="1">
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<mxCell id="39" value="" style="endArrow=blockThin;html=1;strokeWidth=3;strokeColor=#EF4444;endFill=1;exitX=1;exitY=0.5;exitDx=0;exitDy=0;entryX=0;entryY=0.5;entryDx=0;entryDy=0;" edge="1" parent="1" source="6" target="25">
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<mxPoint x="590" y="325" as="sourcePoint" />
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</mxCell>
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<mxGeometry width="50" height="50" relative="1" as="geometry">
<mxPoint x="590" y="410" as="sourcePoint" />
<mxPoint x="590" y="425" as="targetPoint" />
</mxGeometry>
</mxCell>
<mxCell id="43" value="" style="endArrow=blockThin;html=1;strokeWidth=2;strokeColor=#8B5CF6;endFill=1;exitX=0.5;exitY=1;exitDx=0;exitDy=0;entryX=0.5;entryY=0;entryDx=0;entryDy=0;" edge="1" parent="1" source="34" target="37">
<mxGeometry width="50" height="50" relative="1" as="geometry">
<mxPoint x="590" y="495" as="sourcePoint" />
<mxPoint x="590" y="510" as="targetPoint" />
</mxGeometry>
</mxCell>
<mxCell id="44" value="⬆" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;fontSize=24;fontStyle=1;fontColor=#64748B;" vertex="1" parent="1">
<mxGeometry x="10" y="300" width="40" height="40" as="geometry" />
</mxCell>
<mxCell id="45" value="栈增长方向" style="text;html=1;strokeColor=none;fillColor=#F1F5F9;align=center;verticalAlign=middle;fontSize=11;fontColor=#64748B;rounded=1;" vertex="1" parent="1">
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</mxCell>
<mxCell id="46" value="" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#F8FAFC;strokeColor=#94A3B8;strokeWidth=2;" vertex="1" parent="1">
<mxGeometry x="740" y="160" width="230" height="200" as="geometry" />
</mxCell>
<mxCell id="47" value="💡 递归要点" style="text;html=1;strokeColor=none;fillColor=#475569;align=center;verticalAlign=middle;fontSize=14;fontStyle=1;fontColor=#FFFFFF;rounded=1;" vertex="1" parent="1">
<mxGeometry x="760" y="175" width="110" height="28" as="geometry" />
</mxCell>
<mxCell id="48" value="1. 基准条件" style="text;html=1;strokeColor=none;fillColor=#EF4444;align=left;verticalAlign=middle;fontSize=12;fontStyle=1;fontColor=#FFFFFF;rounded=1;spacingLeft=8;" vertex="1" parent="1">
<mxGeometry x="760" y="215" width="100" height="22" as="geometry" />
</mxCell>
<mxCell id="49" value="n=1 时直接返回" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=11;fontColor=#64748B;spacingLeft=10;" vertex="1" parent="1">
<mxGeometry x="760" y="240" width="190" height="18" as="geometry" />
</mxCell>
<mxCell id="50" value="2. 递归条件" style="text;html=1;strokeColor=none;fillColor=#3B82F6;align=left;verticalAlign=middle;fontSize=12;fontStyle=1;fontColor=#FFFFFF;rounded=1;spacingLeft=8;" vertex="1" parent="1">
<mxGeometry x="760" y="265" width="100" height="22" as="geometry" />
</mxCell>
<mxCell id="51" value="n × factorial(n-1)" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=11;fontColor=#64748B;spacingLeft=10;fontFamily=Courier New;" vertex="1" parent="1">
<mxGeometry x="760" y="290" width="190" height="18" as="geometry" />
</mxCell>
<mxCell id="52" value="3. 栈帧管理" style="text;html=1;strokeColor=none;fillColor=#22C55E;align=left;verticalAlign=middle;fontSize=12;fontStyle=1;fontColor=#FFFFFF;rounded=1;spacingLeft=8;" vertex="1" parent="1">
<mxGeometry x="760" y="315" width="100" height="22" as="geometry" />
</mxCell>
<mxCell id="53" value="每次调用创建新栈帧" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=11;fontColor=#64748B;spacingLeft=10;" vertex="1" parent="1">
<mxGeometry x="760" y="340" width="190" height="18" as="geometry" />
</mxCell>
<mxCell id="54" value="" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#FEF3C7;strokeColor=#F59E0B;strokeWidth=2;" vertex="1" parent="1">
<mxGeometry x="740" y="380" width="230" height="175" as="geometry" />
</mxCell>
<mxCell id="55" value="📊 执行流程" style="text;html=1;strokeColor=none;fillColor=#F59E0B;align=center;verticalAlign=middle;fontSize=14;fontStyle=1;fontColor=#FFFFFF;rounded=1;" vertex="1" parent="1">
<mxGeometry x="760" y="395" width="110" height="28" as="geometry" />
</mxCell>
<mxCell id="56" value="factorial(4)" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=11;fontColor=#92400E;fontFamily=Courier New;spacingLeft=5;" vertex="1" parent="1">
<mxGeometry x="760" y="430" width="90" height="18" as="geometry" />
</mxCell>
<mxCell id="57" value="→ factorial(3)" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=11;fontColor=#92400E;fontFamily=Courier New;spacingLeft=5;" vertex="1" parent="1">
<mxGeometry x="770" y="450" width="100" height="18" as="geometry" />
</mxCell>
<mxCell id="58" value="→ factorial(2)" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=11;fontColor=#92400E;fontFamily=Courier New;spacingLeft=5;" vertex="1" parent="1">
<mxGeometry x="785" y="470" width="100" height="18" as="geometry" />
</mxCell>
<mxCell id="59" value="→ factorial(1)" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=11;fontColor=#92400E;fontFamily=Courier New;spacingLeft=5;" vertex="1" parent="1">
<mxGeometry x="800" y="490" width="100" height="18" as="geometry" />
</mxCell>
<mxCell id="60" value="← 1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=11;fontColor=#22C55E;fontFamily=Courier New;fontStyle=1;spacingLeft=5;" vertex="1" parent="1">
<mxGeometry x="900" y="490" width="50" height="18" as="geometry" />
</mxCell>
<mxCell id="61" value="← 2" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=11;fontColor=#22C55E;fontFamily=Courier New;fontStyle=1;spacingLeft=5;" vertex="1" parent="1">
<mxGeometry x="885" y="470" width="50" height="18" as="geometry" />
</mxCell>
<mxCell id="62" value="← 6" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=11;fontColor=#22C55E;fontFamily=Courier New;fontStyle=1;spacingLeft=5;" vertex="1" parent="1">
<mxGeometry x="870" y="450" width="50" height="18" as="geometry" />
</mxCell>
<mxCell id="63" value="← 24" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=11;fontColor=#22C55E;fontFamily=Courier New;fontStyle=1;spacingLeft=5;" vertex="1" parent="1">
<mxGeometry x="850" y="430" width="50" height="18" as="geometry" />
</mxCell>
<mxCell id="64" value="最终结果: 24" style="text;html=1;strokeColor=none;fillColor=#22C55E;align=center;verticalAlign=middle;fontSize=13;fontStyle=1;fontColor=#FFFFFF;rounded=1;" vertex="1" parent="1">
<mxGeometry x="790" y="520" width="130" height="26" as="geometry" />
</mxCell>
<mxCell id="65" value="" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#DBEAFE;strokeColor=#3B82F6;strokeWidth=2;" vertex="1" parent="1">
<mxGeometry x="60" y="580" width="910" height="70" as="geometry" />
</mxCell>
<mxCell id="66" value="⚠️ 注意事项" style="text;html=1;strokeColor=none;fillColor=#3B82F6;align=center;verticalAlign=middle;fontSize=13;fontStyle=1;fontColor=#FFFFFF;rounded=1;" vertex="1" parent="1">
<mxGeometry x="80" y="595" width="110" height="24" as="geometry" />
</mxCell>
<mxCell id="67" value="• 每次递归调用都会创建新的栈帧，占用栈空间" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=12;fontColor=#1E40AF;spacingLeft=10;" vertex="1" parent="1">
<mxGeometry x="200" y="590" width="340" height="24" as="geometry" />
</mxCell>
<mxCell id="68" value="• 递归过深会导致 StackOverflowError" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=12;fontColor=#1E40AF;spacingLeft=10;" vertex="1" parent="1">
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三、构造方法
3.1 构造方法基础
<mxfile>
<diagram id="constructor-method" name="构造方法">
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<root>
<mxCell id="0" />
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<mxCell id="11" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=0;fontSize=14;" value="<font color="#ffffff"><b>✨ 特点</b></font>" vertex="1">
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<mxGeometry height="55" width="380" x="65" y="245" as="geometry" />
</mxCell>
<mxCell id="13" parent="1" style="ellipse;whiteSpace=wrap;html=1;fillColor=#1976d2;strokeColor=none;" value="" vertex="1">
<mxGeometry height="35" width="35" x="80" y="255" as="geometry" />
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<mxCell id="14" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=0;fontSize=14;" value="<font color="#ffffff"><b>1</b></font>" vertex="1">
<mxGeometry height="35" width="35" x="80" y="255" as="geometry" />
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<mxCell id="17" parent="1" style="ellipse;whiteSpace=wrap;html=1;fillColor=#1976d2;strokeColor=none;" value="" vertex="1">
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</mxCell>
<mxCell id="18" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=0;fontSize=14;" value="<font color="#ffffff"><b>2</b></font>" vertex="1">
<mxGeometry height="35" width="35" x="480" y="255" as="geometry" />
</mxCell>
<mxCell id="19" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=0;" value="<font color="#1565c0" style="font-size: 14px;"><b>没有返回值类型</b></font><br><font color="#1976d2" style="font-size: 12px;">（连 void 都没有）</font>" vertex="1">
<mxGeometry height="55" width="305" x="520" y="245" as="geometry" />
</mxCell>
<mxCell id="20" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#ffffff;strokeColor=#42a5f5;strokeWidth=2;shadow=1;" value="" vertex="1">
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</mxCell>
<mxCell id="21" parent="1" style="ellipse;whiteSpace=wrap;html=1;fillColor=#1976d2;strokeColor=none;" value="" vertex="1">
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</mxCell>
<mxCell id="25" parent="1" style="ellipse;whiteSpace=wrap;html=1;fillColor=#1976d2;strokeColor=none;" value="" vertex="1">
<mxGeometry height="35" width="35" x="480" y="320" as="geometry" />
</mxCell>
<mxCell id="26" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=0;fontSize=14;" value="<font color="#ffffff"><b>4</b></font>" vertex="1">
<mxGeometry height="35" width="35" x="480" y="320" as="geometry" />
</mxCell>
<mxCell id="27" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=0;" value="<font color="#1565c0" style="font-size: 14px;">创建对象时<b>自动调用</b></font><br><font color="#1976d2" style="font-size: 11px;">（不能手动调用）</font>" vertex="1">
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<mxCell id="29" parent="1" style="ellipse;whiteSpace=wrap;html=1;fillColor=#1976d2;strokeColor=none;" value="" vertex="1">
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<mxCell id="30" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=0;fontSize=14;" value="<font color="#ffffff"><b>5</b></font>" vertex="1">
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<mxCell id="32" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#e8eaf6;strokeColor=#5c6bc0;strokeWidth=2;" value="" vertex="1">
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<mxCell id="35" parent="1" style="rounded=0;whiteSpace=wrap;html=1;fillColor=#ff9800;strokeColor=none;" value="" vertex="1">
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public class Person {
private String name;
private int age;
// ═══════════════════════════════════════════════════════════════════
// 无参构造方法
// ═══════════════════════════════════════════════════════════════════
public Person() {
this.name = "Unknown";
this.age = 0;
System.out.println("无参构造方法被调用");
}
// ═══════════════════════════════════════════════════════════════════
// 有参构造方法
// ═══════════════════════════════════════════════════════════════════
public Person(String name) {
this.name = name;
this.age = 0;
System.out.println("一个参数的构造方法被调用");
}
// ═══════════════════════════════════════════════════════════════════
// 全参构造方法
// ═══════════════════════════════════════════════════════════════════
public Person(String name, int age) {
this.name = name;
this.age = age;
System.out.println("两个参数的构造方法被调用");
}
// ═══════════════════════════════════════════════════════════════════
// 构造方法调用另一个构造方法（this()）
// ═══════════════════════════════════════════════════════════════════
public Person(String name, int age, String ignored) {
this(name, age); // 调用两个参数的构造方法，必须在第一行
// 其他初始化代码...
}
public static void main(String[] args) {
Person p1 = new Person(); // 无参构造
Person p2 = new Person("Tom"); // 一个参数
Person p3 = new Person("Jerry", 20); // 两个参数
}
}
3.2 构造方法与继承
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class Parent {
private String name;
static {
System.out.println("1. 父类静态代码块");
}
{
System.out.println("3. 父类实例代码块");
}
public Parent() {
System.out.println("4. 父类无参构造方法");
}
public Parent(String name) {
this.name = name;
System.out.println("4. 父类有参构造方法");
}
}
class Child extends Parent {
private int age;
static {
System.out.println("2. 子类静态代码块");
}
{
System.out.println("5. 子类实例代码块");
}
public Child() {
// 隐式调用 super();
System.out.println("6. 子类无参构造方法");
}
public Child(String name, int age) {
super(name); // 显式调用父类有参构造方法
this.age = age;
System.out.println("6. 子类有参构造方法");
}
}
// 运行 new Child() 输出：
// 1. 父类静态代码块
// 2. 子类静态代码块
// 3. 父类实例代码块
// 4. 父类无参构造方法
// 5. 子类实例代码块
// 6. 子类无参构造方法
四、this 与 super 关键字
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<mxCell id="33" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=0;" value="<font face="Courier New" color="#e65100" style="font-size: 14px;"><b>super(参数)</b></font><br><font color="#f57c00" style="font-size: 12px;">调用父类构造方法</font>" vertex="1">
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<mxCell id="46" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#fce4ec;strokeColor=#f48fb1;strokeWidth=2;shadow=1;" value="" vertex="1">
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<mxCell id="47" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=0;" value="<font color="#c2185b" style="font-size: 14px;">⚠️ <b>注意</b>：this() 和 super() <b>不能同时出现</b>在同一个构造方法中<br>（因为它们都必须在第一行）</font>" vertex="1">
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<mxCell id="48" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#e3f2fd;strokeColor=#64b5f6;strokeWidth=1;" value="" vertex="1">
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class Animal {
protected String name = "Animal";
public void eat() {
System.out.println("Animal eating");
}
}
class Dog extends Animal {
private String name = "Dog"; // 同名成员变量
public void eat() {
System.out.println("Dog eating");
}
public void demo() {
// ═══════════════════════════════════════════════════════════════
// this 的使用
// ═══════════════════════════════════════════════════════════════
System.out.println(this.name); // "Dog" - 当前类的 name
this.eat(); // "Dog eating" - 当前类的方法
// ═══════════════════════════════════════════════════════════════
// super 的使用
// ═══════════════════════════════════════════════════════════════
System.out.println(super.name); // "Animal" - 父类的 name
super.eat(); // "Animal eating" - 父类的方法
}
public void showThis() {
// this 可以作为参数传递，表示当前对象
printObject(this);
}
public void printObject(Dog d) {
System.out.println(d.name);
}
}
五、方法调用的内存过程
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</mxCell>
<mxCell id="31" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;fontSize=28;fontStyle=1;fontColor=#6366F1;" value="⬆" vertex="1">
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</mxCell>
<mxCell id="32" parent="1" style="text;html=1;strokeColor=none;fillColor=#EEF2FF;align=center;verticalAlign=middle;fontSize=12;fontColor=#6366F1;rounded=1;fontStyle=1;" value="栈顶" vertex="1">
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</mxCell>
<mxCell id="33" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;fontSize=28;fontStyle=1;fontColor=#6366F1;" value="⬇" vertex="1">
<mxGeometry height="40" width="40" x="820" y="400" as="geometry" />
</mxCell>
<mxCell id="34" parent="1" style="text;html=1;strokeColor=none;fillColor=#EEF2FF;align=center;verticalAlign=middle;fontSize=12;fontColor=#6366F1;rounded=1;fontStyle=1;" value="栈底" vertex="1">
<mxGeometry height="24" width="50" x="815" y="440" as="geometry" />
</mxCell>
<mxCell id="35" parent="1" style="ellipse;whiteSpace=wrap;html=1;fillColor=#3B82F6;strokeColor=#1D4ED8;fontColor=#FFFFFF;fontSize=14;fontStyle=1;" value="1" vertex="1">
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<mxCell id="36" parent="1" style="ellipse;whiteSpace=wrap;html=1;fillColor=#F59E0B;strokeColor=#D97706;fontColor=#FFFFFF;fontSize=14;fontStyle=1;" value="2" vertex="1">
<mxGeometry height="28" width="28" x="120" y="310" as="geometry" />
</mxCell>
<mxCell id="37" parent="1" style="ellipse;whiteSpace=wrap;html=1;fillColor=#8B5CF6;strokeColor=#7C3AED;fontColor=#FFFFFF;fontSize=14;fontStyle=1;" value="3" vertex="1">
<mxGeometry height="28" width="28" x="120" y="420" as="geometry" />
</mxCell>
<mxCell id="38" parent="1" style="ellipse;whiteSpace=wrap;html=1;fillColor=#22C55E;strokeColor=#16A34A;fontColor=#FFFFFF;fontSize=14;fontStyle=1;" value="4" vertex="1">
<mxGeometry height="28" width="28" x="120" y="515" as="geometry" />
</mxCell>
<mxCell id="39" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#F1F5F9;strokeColor=#94A3B8;strokeWidth=2;" value="" vertex="1">
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<mxCell id="40" parent="1" style="text;html=1;strokeColor=none;fillColor=#475569;align=center;verticalAlign=middle;fontSize=13;fontStyle=1;fontColor=#FFFFFF;rounded=1;" value="💡 要点" vertex="1">
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</mxCell>
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六、常见面试题
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// 经典面试题：下面代码输出什么？
public class Test {
public static void main(String[] args) {
String str = "hello";
change(str);
System.out.println(str); // 输出：hello
StringBuilder sb = new StringBuilder("hello");
change(sb);
System.out.println(sb); // 输出：helloworld
}
public static void change(String s) {
s = "world"; // 新对象，不影响原引用
}
public static void change(StringBuilder sb) {
sb.append("world"); // 修改原对象内容
}
}
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