Java 对象的创建与引用
一、使用 new 运算符创建对象
1.1 基本语法
类名 引用变量 = new 类名(构造参数);
// 示例
Student stu = new Student("张三", 20);
1.2 创建过程分解
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1.3 详细执行步骤
| 步骤 | 操作 | 说明 |
|---|---|---|
| ① | 加载类 | 如果 Student 类未加载，先加载到方法区 |
| ② | 分配内存 | 在堆中为新对象分配内存空间 |
| ③ | 默认初始化 | 属性赋默认值（0/null/false） |
| ④ | 显式初始化 | 执行属性声明时的赋值 |
| ⑤ | 构造方法 | 执行构造方法中的代码 |
| ⑥ | 返回地址 | 将对象内存地址赋给引用变量 |
1.4 代码示例：观察初始化顺序
public class Student {
// ② 显式初始化
private String name = "默认姓名";
private int age = 0;
// ③ 构造方法
public Student(String name, int age) {
System.out.println("构造前 name = " + this.name); // 输出：默认姓名
this.name = name;
this.age = age;
System.out.println("构造后 name = " + this.name); // 输出：张三
}
public static void main(String[] args) {
Student stu = new Student("张三", 20);
}
}
输出结果：
构造前 name = 默认姓名
构造后 name = 张三
二、堆内存与栈内存
2.1 JVM 内存结构概览
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• GC 负责回收
• 线程共享" vertex="1">
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2.2 栈内存 vs 堆内存 对比表
| 特性 | 栈内存 (Stack) | 堆内存 (Heap) |
|---|---|---|
| 存储内容 | 局部变量、方法参数、引用地址 | 对象实例、数组 |
| 生命周期 | 方法执行期间 | 直到无引用被 GC 回收 |
| 分配速度 | 快（指针移动） | 慢（需找空闲空间） |
| 空间大小 | 较小（默认 1MB） | 较大（可配置 GB 级） |
| 线程关系 | 线程私有 | 线程共享 |
| 内存管理 | 自动释放 | 垃圾回收器管理 |
| 溢出异常 | StackOverflowError | OutOfMemoryError |
2.3 代码示例：理解内存分配
public class MemoryDemo {
public static void main(String[] args) {
// 基本类型 → 值存在栈中
int num = 100;
// 引用类型 → 引用在栈中，对象在堆中
Student stu = new Student("张三", 20);
// 数组也是对象 → 堆中
int[] arr = new int[]{1, 2, 3};
// 调用方法
process(stu);
}
public static void process(Student s) {
// s 是新的引用，指向同一个对象
String name = s.getName(); // name 引用在栈中
}
}
内存图解：
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三、对象实体与对象引用
3.1 核心概念
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<mxCell id="13" value="" style="endArrow=blockThin;html=1;strokeWidth=5;strokeColor=#F59E0B;endFill=1;rounded=1;" edge="1" parent="1">
<mxGeometry width="50" height="50" relative="1" as="geometry">
<mxPoint x="340" y="229.5" as="sourcePoint" />
<mxPoint x="530" y="229.5" as="targetPoint" />
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</mxCell>
<mxCell id="14" value="栈内存" style="text;html=1;strokeColor=none;fillColor=#3B82F6;align=center;verticalAlign=middle;fontSize=16;fontStyle=1;fontColor=#FFFFFF;rounded=1;" vertex="1" parent="1">
<mxGeometry x="145" y="320" width="130" height="35" as="geometry" />
</mxCell>
<mxCell id="15" value="堆内存" style="text;html=1;strokeColor=none;fillColor=#22C55E;align=center;verticalAlign=middle;fontSize=16;fontStyle=1;fontColor=#FFFFFF;rounded=1;" vertex="1" parent="1">
<mxGeometry x="605" y="320" width="130" height="35" as="geometry" />
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<mxCell id="16" value="" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#EFF6FF;strokeColor=#93C5FD;strokeWidth=2;dashed=1;" vertex="1" parent="1">
<mxGeometry x="80" y="390" width="260" height="150" as="geometry" />
</mxCell>
<mxCell id="17" value="📦 存储" 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="390" width="70" height="28" as="geometry" />
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<mxCell id="18" value="对象的内存地址" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;fontSize=15;fontStyle=0;fontColor=#1E40AF;" vertex="1" parent="1">
<mxGeometry x="80" y="430" width="260" height="30" as="geometry" />
</mxCell>
<mxCell id="19" value="📐 大小" 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="470" width="70" height="28" as="geometry" />
</mxCell>
<mxCell id="20" value="固定（32/64位）" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;fontSize=15;fontStyle=0;fontColor=#1E40AF;" vertex="1" parent="1">
<mxGeometry x="80" y="505" width="260" height="30" as="geometry" />
</mxCell>
<mxCell id="21" value="" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#F0FDF4;strokeColor=#86EFAC;strokeWidth=2;dashed=1;" vertex="1" parent="1">
<mxGeometry x="530" y="390" width="280" height="150" as="geometry" />
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<mxCell id="22" value="📦 存储" 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="530" y="390" width="70" height="28" as="geometry" />
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<mxCell id="23" value="对象的实际数据" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;fontSize=15;fontStyle=0;fontColor=#166534;" vertex="1" parent="1">
<mxGeometry x="530" y="430" width="280" height="30" as="geometry" />
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<mxCell id="24" value="📐 大小" 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="530" y="470" width="70" height="28" as="geometry" />
</mxCell>
<mxCell id="25" value="取决于属性数量" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;fontSize=15;fontStyle=0;fontColor=#166534;" vertex="1" parent="1">
<mxGeometry x="530" y="505" width="280" height="30" as="geometry" />
</mxCell>
<mxCell id="26" value="指向" style="text;html=1;strokeColor=none;fillColor=#FEF3C7;align=center;verticalAlign=middle;fontSize=14;fontStyle=1;fontColor=#B45309;rounded=1;" vertex="1" parent="1">
<mxGeometry x="405" y="200" width="60" height="28" as="geometry" />
</mxCell>
</root>
</mxGraphModel>
</diagram>
</mxfile>
3.2 气球与绳子类比
<mxfile>
<diagram id="balloon-rope-analogy" name="气球与绳子类比">
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<root>
<mxCell id="0" />
<mxCell id="1" parent="0" />
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<mxGeometry height="50" width="920" x="40" y="20" as="geometry" />
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<mxCell id="4" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#EEF2FF;strokeColor=#A5B4FC;strokeWidth=2;" value="" vertex="1">
<mxGeometry height="80" width="880" x="60" y="90" as="geometry" />
</mxCell>
<mxCell id="5" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=16;fontStyle=1;fontColor=#4338CA;" value="🎈 气球 = 对象实体（堆内存中的实际数据）" vertex="1">
<mxGeometry height="30" width="400" x="80" y="100" as="geometry" />
</mxCell>
<mxCell id="6" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=16;fontStyle=1;fontColor=#4338CA;" value="🧵 绳子 = 对象引用（栈内存中的地址变量）" vertex="1">
<mxGeometry height="30" width="400" x="80" y="135" as="geometry" />
</mxCell>
<mxCell id="7" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#FEF2F2;strokeColor=#FCA5A5;strokeWidth=2;" value="" vertex="1">
<mxGeometry height="200" width="880" x="60" y="190" as="geometry" />
</mxCell>
<mxCell id="8" parent="1" style="text;html=1;strokeColor=none;fillColor=#EF4444;align=center;verticalAlign=middle;fontSize=18;fontStyle=1;fontColor=#FFFFFF;rounded=1;" value="规则 1：一条绳子可以不系气球" vertex="1">
<mxGeometry height="40" width="880" x="60" y="190" as="geometry" />
</mxCell>
<mxCell id="9" parent="1" style="text;html=1;strokeColor=none;fillColor=#FEE2E2;align=center;verticalAlign=middle;fontSize=16;fontStyle=1;fontColor=#991B1B;rounded=1;fontFamily=Consolas;" value="Student stu = null;" vertex="1">
<mxGeometry height="40" width="280" x="100" y="290" as="geometry" />
</mxCell>
<mxCell id="10" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;fontSize=40;fontStyle=0;" value="🧵" vertex="1">
<mxGeometry height="60" width="60" x="500" y="280" as="geometry" />
</mxCell>
<mxCell id="11" parent="1" style="text;html=1;strokeColor=none;fillColor=#DBEAFE;align=center;verticalAlign=middle;fontSize=16;fontStyle=1;fontColor=#1E40AF;rounded=1;fontFamily=Consolas;" value="stu" vertex="1">
<mxGeometry height="30" width="60" x="555" y="295" as="geometry" />
</mxCell>
<mxCell id="12" edge="1" parent="1" style="endArrow=blockThin;html=1;strokeWidth=4;strokeColor=#9CA3AF;endFill=1;dashed=1;" value="">
<mxGeometry height="50" relative="1" width="50" as="geometry">
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</mxCell>
<mxCell id="13" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;fontSize=36;fontStyle=0;" value="❌" vertex="1">
<mxGeometry height="60" width="60" x="730" y="280" as="geometry" />
</mxCell>
<mxCell id="14" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;fontSize=14;fontStyle=0;fontColor=#6B7280;" value="(没有气球)" vertex="1">
<mxGeometry height="30" width="100" x="790" y="295" as="geometry" />
</mxCell>
<mxCell id="15" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#FEF3C7;strokeColor=#FCD34D;strokeWidth=2;" value="" vertex="1">
<mxGeometry height="220" width="880" x="60" y="410" as="geometry" />
</mxCell>
<mxCell id="16" parent="1" style="text;html=1;strokeColor=none;fillColor=#F59E0B;align=center;verticalAlign=middle;fontSize=18;fontStyle=1;fontColor=#FFFFFF;rounded=1;" value="规则 2：一条绳子只能系一个气球" vertex="1">
<mxGeometry height="40" width="880" x="60" y="410" as="geometry" />
</mxCell>
<mxCell id="17" parent="1" style="text;html=1;strokeColor=none;fillColor=#FEF9C3;align=center;verticalAlign=middle;fontSize=14;fontStyle=1;fontColor=#92400E;rounded=1;fontFamily=Consolas;" value="Student stu = new Student("张三");" vertex="1">
<mxGeometry height="35" width="340" x="80" y="502.5" as="geometry" />
</mxCell>
<mxCell id="18" parent="1" style="text;html=1;strokeColor=none;fillColor=#FEF9C3;align=center;verticalAlign=middle;fontSize=14;fontStyle=1;fontColor=#92400E;rounded=1;fontFamily=Consolas;" value="stu = new Student("李四"); // 原对象可能被 GC" vertex="1">
<mxGeometry height="35" width="400" x="90" y="542.5" as="geometry" />
</mxCell>
<mxCell id="19" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;fontSize=40;fontStyle=0;" value="🧵" vertex="1">
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</mxCell>
<mxCell id="20" parent="1" style="text;html=1;strokeColor=none;fillColor=#DBEAFE;align=center;verticalAlign=middle;fontSize=16;fontStyle=1;fontColor=#1E40AF;rounded=1;fontFamily=Consolas;" value="stu" vertex="1">
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</mxCell>
<mxCell id="21" edge="1" parent="1" style="endArrow=blockThin;html=1;strokeWidth=4;strokeColor=#F59E0B;endFill=1;" value="">
<mxGeometry height="50" relative="1" width="50" as="geometry">
<mxPoint x="610" y="527.5" as="sourcePoint" />
<mxPoint x="700" y="527.5" as="targetPoint" />
</mxGeometry>
</mxCell>
<mxCell id="22" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;fontSize=40;fontStyle=0;" value="🎈" vertex="1">
<mxGeometry height="60" width="60" x="700" y="497.5" as="geometry" />
</mxCell>
<mxCell id="23" parent="1" style="text;html=1;strokeColor=none;fillColor=#DCFCE7;align=center;verticalAlign=middle;fontSize=16;fontStyle=1;fontColor=#166534;rounded=1;" value="李四" vertex="1">
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</mxCell>
<mxCell id="24" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;fontSize=32;fontStyle=0;opacity=50;" value="🎈" vertex="1">
<mxGeometry height="50" width="50" x="705" y="547.5" as="geometry" />
</mxCell>
<mxCell id="25" parent="1" style="text;html=1;strokeColor=none;fillColor=#FEE2E2;align=center;verticalAlign=middle;fontSize=13;fontStyle=2;fontColor=#991B1B;rounded=1;" value="张三 (无人引用，等待 GC)" vertex="1">
<mxGeometry height="28" width="180" x="755" y="557.5" as="geometry" />
</mxCell>
<mxCell id="26" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#DCFCE7;strokeColor=#86EFAC;strokeWidth=2;" value="" vertex="1">
<mxGeometry height="180" width="880" x="60" y="650" as="geometry" />
</mxCell>
<mxCell id="27" parent="1" style="text;html=1;strokeColor=none;fillColor=#22C55E;align=center;verticalAlign=middle;fontSize=18;fontStyle=1;fontColor=#FFFFFF;rounded=1;" value="规则 3：多条绳子可以系同一个气球" vertex="1">
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</mxCell>
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<mxGeometry height="30" width="320" x="80" y="705" as="geometry" />
</mxCell>
<mxCell id="29" parent="1" style="text;html=1;strokeColor=none;fillColor=#F0FDF4;align=center;verticalAlign=middle;fontSize=14;fontStyle=1;fontColor=#166534;rounded=1;fontFamily=Consolas;" value="Student s2 = s1;" vertex="1">
<mxGeometry height="30" width="180" x="84" y="745" as="geometry" />
</mxCell>
<mxCell id="30" parent="1" style="text;html=1;strokeColor=none;fillColor=#F0FDF4;align=center;verticalAlign=middle;fontSize=14;fontStyle=1;fontColor=#166534;rounded=1;fontFamily=Consolas;" value="Student s3 = s1;" vertex="1">
<mxGeometry height="30" width="180" x="83" y="785" as="geometry" />
</mxCell>
<mxCell id="31" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;fontSize=32;fontStyle=0;" value="🧵" vertex="1">
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</mxCell>
<mxCell id="32" parent="1" style="text;html=1;strokeColor=none;fillColor=#DBEAFE;align=center;verticalAlign=middle;fontSize=14;fontStyle=1;fontColor=#1E40AF;rounded=1;fontFamily=Consolas;" value="s1" 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=32;fontStyle=0;" value="🧵" vertex="1">
<mxGeometry height="50" width="50" x="500" y="735" as="geometry" />
</mxCell>
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<mxGeometry height="26" width="40" x="545" y="747" as="geometry" />
</mxCell>
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<mxGeometry height="50" width="50" x="500" y="780" as="geometry" />
</mxCell>
<mxCell id="36" parent="1" style="text;html=1;strokeColor=none;fillColor=#DBEAFE;align=center;verticalAlign=middle;fontSize=14;fontStyle=1;fontColor=#1E40AF;rounded=1;fontFamily=Consolas;" value="s3" vertex="1">
<mxGeometry height="26" width="40" x="545" y="792" as="geometry" />
</mxCell>
<mxCell id="37" edge="1" parent="1" style="endArrow=blockThin;html=1;strokeWidth=4;strokeColor=#22C55E;endFill=1;edgeStyle=orthogonalEdgeStyle;rounded=1;" value="">
<mxGeometry height="50" relative="1" width="50" as="geometry">
<Array as="points">
<mxPoint x="660" y="715" />
<mxPoint x="660" y="760" />
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<mxPoint x="590" y="715" as="sourcePoint" />
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</mxCell>
<mxCell id="38" edge="1" parent="1" style="endArrow=blockThin;html=1;strokeWidth=4;strokeColor=#22C55E;endFill=1;" value="">
<mxGeometry height="50" relative="1" width="50" as="geometry">
<mxPoint x="590" y="760" as="sourcePoint" />
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<mxGeometry height="50" relative="1" width="50" as="geometry">
<Array as="points">
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<mxPoint x="660" y="760" />
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<mxPoint x="590" y="805" as="sourcePoint" />
<mxPoint x="730" y="760" as="targetPoint" />
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</mxCell>
<mxCell id="40" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;fontSize=50;fontStyle=0;" value="🎈" vertex="1">
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</mxCell>
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3.3 代码验证
public class ReferenceDemo {
public static void main(String[] args) {
// 规则 1：绳子可以不系气球
Student stu1 = null;
// stu1.getName(); // ❌ NullPointerException
// 规则 2：绳子只能系一个气球
Student stu2 = new Student("张三", 20);
System.out.println(stu2.getName()); // 张三
stu2 = new Student("李四", 21); // stu2 改系新气球
System.out.println(stu2.getName()); // 李四
// "张三"对象无人引用，等待 GC 回收
// 规则 3：多条绳子系同一个气球
Student s1 = new Student("王五", 22);
Student s2 = s1; // s2 系上 s1 的气球
Student s3 = s1; // s3 也系上同一个气球
// 验证：修改一处，全部生效
s2.setAge(30);
System.out.println(s1.getAge()); // 30
System.out.println(s3.getAge()); // 30
}
}
四、引用相等 vs 对象相等
4.1 概念对比
<mxfile>
<diagram id="java-equality-comparison" name="Java相等性比较">
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<mxCell id="1" parent="0" />
<!-- 主容器背景 -->
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<!-- 左侧区域：引用相等 -->
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<!-- 左侧标题 -->
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<!-- 左侧说明文字 -->
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<!-- 左侧 s1 变量 -->
<mxCell id="6" value="s1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#fff2cc;strokeColor=#d6b656;fontStyle=1;fontSize=14;" vertex="1" parent="1">
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<!-- 左侧 s2 变量 -->
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<!-- 左侧问号 -->
<mxCell id="8" value="？" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=0;fontSize=16;fontStyle=1;fontColor=#e74c3c;" vertex="1" parent="1">
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<!-- 左侧 == 符号 -->
<mxCell id="9" value="==" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=0;fontSize=16;fontStyle=1;fontColor=#2980b9;" vertex="1" parent="1">
<mxGeometry x="200" y="275" width="60" height="30" as="geometry" />
</mxCell>
<!-- 左侧共享对象 -->
<mxCell id="10" value="同一对象<br><font style="font-size:11px;">0x1234</font>" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#d5e8d4;strokeColor=#82b366;fontStyle=1;fontSize=14;" vertex="1" parent="1">
<mxGeometry x="130" y="340" width="200" height="60" as="geometry" />
</mxCell>
<!-- s1 到对象的箭头 -->
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<mxGeometry relative="1" as="geometry" />
</mxCell>
<!-- s2 到对象的箭头 -->
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<!-- 右侧区域：对象相等 -->
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<!-- 右侧说明文字 -->
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</mxCell>
<!-- 右侧 s1 变量 -->
<mxCell id="16" value="s1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#fff2cc;strokeColor=#d6b656;fontStyle=1;fontSize=14;" vertex="1" parent="1">
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<!-- 右侧 s2 变量 -->
<mxCell id="17" value="s2" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#fff2cc;strokeColor=#d6b656;fontStyle=1;fontSize=14;" vertex="1" parent="1">
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</mxCell>
<!-- 右侧对象1 -->
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<!-- 右侧对象2 -->
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<mxGeometry x="640" y="300" width="90" height="60" as="geometry" />
</mxCell>
<!-- s1 到对象1的箭头 -->
<mxCell id="20" value="" style="endArrow=classic;html=1;rounded=0;strokeWidth=2;strokeColor=#666666;exitX=0.5;exitY=1;exitDx=0;exitDy=0;entryX=0.5;entryY=0;entryDx=0;entryDy=0;" edge="1" parent="1" source="16" target="18">
<mxGeometry relative="1" as="geometry" />
</mxCell>
<!-- s2 到对象2的箭头 -->
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<mxGeometry relative="1" as="geometry" />
</mxCell>
<!-- 右侧 == 符号 -->
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<!-- 右侧结果说明 -->
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</mxCell>
<!-- 分隔线 -->
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</mxCell>
</root>
</mxGraphModel>
</diagram>
</mxfile>
4.2 代码示例
public class EqualityDemo {
public static void main(String[] args) {
Student s1 = new Student("张三", 20); // 对象 A
Student s2 = s1; // 也指向对象 A
Student s3 = new Student("张三", 20); // 对象 B（内容相同）
// ====== 引用相等（==）：比较地址 ======
System.out.println(s1 == s2); // true (同一对象)
System.out.println(s1 == s3); // false (不同对象)
// ====== 对象相等（equals）：比较内容 ======
// 注意：需要 Student 类重写 equals 方法才能正确比较
System.out.println(s1.equals(s3)); // 取决于 equals 实现
}
}
内存示意图：
<mxfile>
<diagram id="stack-heap-memory" name="栈内存与堆内存">
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</mxCell>
<mxCell id="7" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#d5e8d4;strokeColor=#82b366;strokeWidth=2;" value="" vertex="1">
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</mxCell>
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</mxCell>
<mxCell id="26" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=0;fontSize=20;" value="💡" vertex="1">
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</mxCell>
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五、== 与 equals() 详解
5.1 == 运算符的行为
| 比较类型 | == 的作用 | 示例 |
|---|---|---|
| 基本类型 | 比较值 | 5 == 5 → true |
| 引用类型 | 比较地址 | s1 == s2 → 是否同一对象 |
// 基本类型用 == 比较值
int a = 100;
int b = 100;
System.out.println(a == b); // true（值相等）
// 引用类型用 == 比较地址
String str1 = new String("Java");
String str2 = new String("Java");
System.out.println(str1 == str2); // false（不同对象）
5.2 equals() 方法的行为
Object 类的默认实现（源码）
// java.lang.Object 类中的 equals 方法
public boolean equals(Object obj) {
return (this == obj); // 默认比较引用！
}
⚠️ 默认行为与 == 完全相同！ 需要子类重写才能比较内容。
String 类的重写实现（源码解析）
// java.lang.String 类中的 equals 方法
public boolean equals(Object anObject) {
// 1️⃣ 先比较引用，相同直接返回 true
if (this == anObject) {
return true;
}
// 2️⃣ 检查类型是否是 String
if (anObject instanceof String) {
String anotherString = (String) anObject;
int n = value.length;
// 3️⃣ 比较长度
if (n == anotherString.value.length) {
char v1[] = value;
char v2[] = anotherString.value;
int i = 0;
// 4️⃣ 逐字符比较
while (n-- != 0) {
if (v1[i] != v2[i])
return false;
i++;
}
return true;
}
}
return false;
}
流程图：
<mxfile>
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<span style="color:#c678dd;">public</span> <span style="color:#e5c07b;">boolean</span> <span style="color:#61afef;">equals</span>(<span style="color:#e5c07b;">Object</span> anObject) {
 <span style="color:#7f848e;">// 1. 引用比较：如果是同一个对象，直接返回 true</span>
 <span style="color:#c678dd;">if</span> (<span style="color:#e06c75;">this</span> == anObject) {
 <span style="color:#c678dd;">return</span> <span style="color:#d19a66;">true</span>;
 }
 
 <span style="color:#7f848e;">// 2. 类型检查：必须是 String 类型</span>
 <span style="color:#c678dd;">if</span> (anObject <span style="color:#c678dd;">instanceof</span> <span style="color:#e5c07b;">String</span>) {
 <span style="color:#e5c07b;">String</span> aString = (<span style="color:#e5c07b;">String</span>)anObject;
 
 <span style="color:#7f848e;">// 3. 编码/长度检查 & 4. 逐字符比较</span>
 <span style="color:#c678dd;">if</span> (coder() == aString.coder()) {
 <span style="color:#c678dd;">return</span> isLatin1() 
 ? StringLatin1.equals(value, aString.value)
 : StringUTF16.equals(value, aString.value);
 }
 }
 <span style="color:#7f848e;">// 类型不匹配或内容不同，返回 false</span>
 <span style="color:#c678dd;">return</span> <span style="color:#d19a66;">false</span>;
}
</pre>" vertex="1">
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5.3 自定义类重写 equals()
public class Student {
private String name;
private int age;
public Student(String name, int age) {
this.name = name;
this.age = age;
}
@Override
public boolean equals(Object obj) {
// 1. 引用相同
if (this == obj) return true;
// 2. null 检查 + 类型检查
if (obj == null || getClass() != obj.getClass()) return false;
// 3. 强制类型转换
Student student = (Student) obj;
// 4. 比较各属性
return age == student.age &&
Objects.equals(name, student.name);
}
// 重写 equals 必须重写 hashCode（后续笔记详解）
@Override
public int hashCode() {
return Objects.hash(name, age);
}
}
5.4 == vs equals() 对比总结表
| 比较方式 | 基本类型 | 引用类型（未重写） | 引用类型（已重写） |
|---|---|---|---|
== |
比较值 | 比较地址 | 比较地址 |
equals() |
❌ 不适用 | 比较地址（同==） | 比较内容 |
六、字符串的特殊情况
6.1 字符串常量池
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<mxCell id="10" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=15;fontColor=#F59E0B;fontFamily=Consolas;fontStyle=1;" value="String str3 = new String("Java");" vertex="1">
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<mxCell id="11" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=13;fontColor=#94A3B8;fontFamily=Consolas;" value="// 强制在堆中创建新对象" vertex="1">
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<mxCell id="12" parent="1" style="text;html=1;strokeColor=none;fillColor=#EF4444;align=center;verticalAlign=middle;fontSize=14;fontStyle=1;fontColor=#FFFFFF;rounded=1;" value="结果验证" vertex="1">
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<mxCell id="13" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=14;fontColor=#22C55E;fontFamily=Consolas;fontStyle=1;" value="str1 == str2 → true" vertex="1">
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<mxCell id="14" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=14;fontColor=#EF4444;fontFamily=Consolas;fontStyle=1;" value="str1 == str3 → false" vertex="1">
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<mxCell id="15" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=14;fontColor=#22C55E;fontFamily=Consolas;fontStyle=1;" value="str1.equals(str3) → true" vertex="1">
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• 存在则直接返回引用
• 不存在则创建并加入池
• 多个变量共享同一对象" vertex="1">
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• 即使常量池已有相同值
• 内部 value 仍指向常量池
• 浪费内存，应避免使用" vertex="1">
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6.2 代码验证
public class StringPoolDemo {
public static void main(String[] args) {
// 字面量创建（使用常量池）
String str1 = "Java";
String str2 = "Java";
// new 创建（强制新对象）
String str3 = new String("Java");
String str4 = new String("Java");
// ====== == 比较（地址）======
System.out.println("str1 == str2: " + (str1 == str2)); // true ✅ 常量池复用
System.out.println("str1 == str3: " + (str1 == str3)); // false ❌ 不同对象
System.out.println("str3 == str4: " + (str3 == str4)); // false ❌ 不同对象
// ====== equals 比较（内容）======
System.out.println("str1.equals(str3): " + str1.equals(str3)); // true ✅
System.out.println("str3.equals(str4): " + str3.equals(str4)); // true ✅
// ====== intern() 方法 ======
String str5 = str3.intern(); // 返回常量池中的引用
System.out.println("str1 == str5: " + (str1 == str5)); // true ✅
}
}
输出结果：
str1 == str2: true
str1 == str3: false
str3 == str4: false
str1.equals(str3): true
str3.equals(str4): true
str1 == str5: true
6.3 字符串创建方式对比
| 创建方式 | 代码示例 | 使用常量池 | 内存位置 |
|---|---|---|---|
| 字面量 | "Java" |
✅ 是 | 常量池 |
| new | new String("Java") |
❌ 否 | 堆内存 |
| intern() | str.intern() |
✅ 是 | 返回常量池引用 |
| 拼接（常量） | "Ja" + "va" |
✅ 是 | 编译期优化 |
| 拼接（变量） | str + "va" |
❌ 否 | 运行时新建 |
七、Integer 缓存池
7.1 缓存池机制
public class IntegerCacheDemo {
public static void main(String[] args) {
// -128 ~ 127 范围内使用缓存
Integer a = 100;
Integer b = 100;
System.out.println(a == b); // true ✅ 使用缓存
// 超出范围，创建新对象
Integer c = 200;
Integer d = 200;
System.out.println(c == d); // false ❌ 不同对象
// equals 比较内容
System.out.println(c.equals(d)); // true ✅
}
}
7.2 Integer.valueOf() 源码
// java.lang.Integer 类
public static Integer valueOf(int i) {
// 如果在缓存范围内，返回缓存对象
if (i >= IntegerCache.low && i <= IntegerCache.high)
return IntegerCache.cache[i + (-IntegerCache.low)];
// 否则创建新对象
return new Integer(i);
}
// 缓存池内部类
private static class IntegerCache {
static final int low = -128;
static final int high = 127; // 可通过 JVM 参数调整
static final Integer cache[];
static {
// 初始化缓存数组
cache = new Integer[(high - low) + 1];
int j = low;
for(int k = 0; k < cache.length; k++)
cache[k] = new Integer(j++);
}
}
流程图：
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7.3 各包装类缓存范围
| 包装类 | 缓存范围 | 说明 |
|---|---|---|
| Byte | -128 ~ 127 | 全部缓存 |
| Short | -128 ~ 127 | |
| Integer | -128 ~ 127 | high 可通过 JVM 参数调整 |
| Long | -128 ~ 127 | |
| Character | 0 ~ 127 | |
| Boolean | TRUE / FALSE | 只有两个值 |
| Float | ❌ 无缓存 | 浮点数范围太大 |
| Double | ❌ 无缓存 | 浮点数范围太大 |
八、最佳实践总结
8.1 比较规则速查表
| 场景 | 推荐方式 | 示例 |
|---|---|---|
| 基本类型比较 | <mark> |
int a </mark> int b |
| 对象内容比较 | equals() |
str1.equals(str2) |
| 对象是否同一个 | <mark> |
obj1 </mark> obj2 |
| null 安全比较 | Objects.equals() |
Objects.equals(a, b) |
8.2 常见陷阱
// ❌ 陷阱 1：用 == 比较字符串内容
String input = new String("admin");
if (input == "admin") { // false！
// 永远不会执行
}
// ✅ 正确写法
if (input.equals("admin")) {
// 正确执行
}
// ✅ 更安全的写法（避免 NullPointerException）
if ("admin".equals(input)) {
// 常量在前，避免 input 为 null 时报错
}
// ❌ 陷阱 2：用 == 比较包装类
Integer a = 200;
Integer b = 200;
if (a == b) { // false！超出缓存范围
// 可能不会执行
}
// ✅ 正确写法
if (a.equals(b)) {
// 正确执行
}
8.3 核心要点记忆图
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