Java 构造方法详解
一、构造方法概述
1.1 什么是构造方法
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1.2 基本语法
[访问修饰符] 类名(参数列表) {
// 初始化代码
}
public class Student {
private String name;
private int age;
// 构造方法
public Student(String name, int age) {
this.name = name;
this.age = age;
}
}
// 使用
Student stu = new Student("张三", 20);
1.3 构造方法 vs 普通方法
| 对比项 | 构造方法 | 普通方法 |
|---|---|---|
| 命名 | 必须与类名相同 | 任意合法标识符 |
| 返回值 | 无（连 void 都没有） | 必须声明返回类型 |
| 调用时机 | new 时自动调用 | 手动调用 |
| 调用次数 | 每个对象只调用一次 | 可多次调用 |
| 作用 | 初始化对象 | 执行功能 |
| 继承 | 不能被继承 | 可以被继承 |
| 重写 | 不能重写 | 可以重写 |
二、默认构造方法
2.1 编译器自动生成
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</mxCell>
<mxCell id="33" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;fontSize=12;fontColor=#92400E;fontStyle=1;" value="一旦定义了任何构造方法
默认构造就【不再】生成" vertex="1">
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</mxCell>
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<mxGeometry height="20" width="140" x="575" y="415" as="geometry" />
</mxCell>
</root>
</mxGraphModel>
</diagram>
</mxfile>
代码示例
// 源代码（没有写构造方法）
public class Product {
private String name;
private double price;
}
// 编译后等价于（编译器自动添加）
public class Product {
private String name;
private double price;
// 编译器添加的默认构造方法
public Product() {
// 空实现
}
}
// 可以正常使用无参构造
Product p = new Product(); // ✅ 正确
2.2 查看字节码验证
# 编译
javac Product.java
# 查看字节码
javap -c Product.class
输出结果：
public class Product {
public Product(); // ← 编译器生成的默认构造方法
Code:
0: aload_0
1: invokespecial #1 // Method java/lang/Object."<init>":()V
4: return
}
2.3 默认构造方法的特点
| 特点 | 说明 |
|---|---|
| 访问修饰符 | 与类的访问修饰符相同 |
| 参数 | 无参数 |
| 方法体 | 空（仅调用父类无参构造） |
| 生成条件 | 类中没有任何显式构造方法 |
// 类是 public，默认构造也是 public
public class A {
}
// 类是默认访问权限，默认构造也是默认权限
class B {
}
三、手动定义构造方法
3.1 ⚠️ 重要规则
<mxfile>
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<mxCell id="5" value="📝 手动定义了【任何】构造方法" 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="50" y="95" width="320" height="35" as="geometry" />
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<mxCell id="6" value="" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#1E293B;strokeColor=#475569;strokeWidth=2;" vertex="1" parent="1">
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</mxCell>
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<mxGeometry x="85" y="152" width="250" height="18" as="geometry" />
</mxCell>
<mxCell id="8" value=" // 有参构造" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=11;fontColor=#6B7280;fontFamily=Consolas;fontStyle=2;" vertex="1" parent="1">
<mxGeometry x="85" y="172" width="250" height="18" as="geometry" />
</mxCell>
<mxCell id="9" value=" public Person(String n) {" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=11;fontColor=#FBBF24;fontFamily=Consolas;fontStyle=1;" vertex="1" parent="1">
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<mxCell id="10" value=" this.name = n;" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=11;fontColor=#FBBF24;fontFamily=Consolas;fontStyle=1;" vertex="1" parent="1">
<mxGeometry x="85" y="212" width="250" height="18" as="geometry" />
</mxCell>
<mxCell id="11" value=" }" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=11;fontColor=#FBBF24;fontFamily=Consolas;fontStyle=1;" vertex="1" parent="1">
<mxGeometry x="85" y="232" width="250" height="18" as="geometry" />
</mxCell>
<mxCell id="12" value="}" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=11;fontColor=#E2E8F0;fontFamily=Consolas;" vertex="1" parent="1">
<mxGeometry x="85" y="250" width="250" height="18" as="geometry" />
</mxCell>
<mxCell id="13" value="" style="shape=flexArrow;endArrow=classic;html=1;strokeWidth=2;fillColor=#DC2626;strokeColor=#991B1B;width=25;endSize=10;" edge="1" parent="1">
<mxGeometry relative="1" as="geometry">
<mxPoint x="380" y="185" as="sourcePoint" />
<mxPoint x="480" y="185" as="targetPoint" />
</mxGeometry>
</mxCell>
<mxCell id="14" value="导致" style="text;html=1;strokeColor=none;fillColor=#FEE2E2;align=center;verticalAlign=middle;fontSize=13;fontColor=#991B1B;rounded=1;fontStyle=1;" vertex="1" parent="1">
<mxGeometry x="400" y="155" width="50" height="24" as="geometry" />
</mxCell>
<mxCell id="15" value="" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#FEE2E2;strokeColor=#EF4444;strokeWidth=3;shadow=1;" vertex="1" parent="1">
<mxGeometry x="480" y="95" width="330" height="180" as="geometry" />
</mxCell>
<mxCell id="16" value="🚫 编译器【不再】提供默认构造" style="text;html=1;strokeColor=none;fillColor=#EF4444;align=center;verticalAlign=middle;fontSize=13;fontStyle=1;fontColor=#FFFFFF;rounded=1;" vertex="1" parent="1">
<mxGeometry x="480" y="95" width="330" height="35" as="geometry" />
</mxCell>
<mxCell id="17" value="" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#1E293B;strokeColor=#475569;strokeWidth=2;" vertex="1" parent="1">
<mxGeometry x="500" y="145" width="290" height="115" as="geometry" />
</mxCell>
<mxCell id="18" value="// 尝试使用无参构造" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=11;fontColor=#6B7280;fontFamily=Consolas;fontStyle=2;" vertex="1" parent="1">
<mxGeometry x="515" y="152" width="260" height="18" as="geometry" />
</mxCell>
<mxCell id="19" value="Person p = new Person();" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=11;fontColor=#F87171;fontFamily=Consolas;textDecoration=line-through;" vertex="1" parent="1">
<mxGeometry x="515" y="175" width="260" height="18" as="geometry" />
</mxCell>
<mxCell id="20" value="// ❌ 编译错误！" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=11;fontColor=#EF4444;fontFamily=Consolas;fontStyle=1;" vertex="1" parent="1">
<mxGeometry x="515" y="198" width="260" height="18" as="geometry" />
</mxCell>
<mxCell id="21" value="// 必须使用有参构造" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=11;fontColor=#6B7280;fontFamily=Consolas;fontStyle=2;" vertex="1" parent="1">
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</mxCell>
<mxCell id="22" value="Person p = new Person("Tom");" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=11;fontColor=#4ADE80;fontFamily=Consolas;fontStyle=1;" vertex="1" parent="1">
<mxGeometry x="515" y="241" width="260" height="18" as="geometry" />
</mxCell>
<mxCell id="23" value="✅" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=14;" vertex="1" parent="1">
<mxGeometry x="755" y="238" width="30" height="24" as="geometry" />
</mxCell>
<mxCell id="24" value="" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#FEE2E2;strokeColor=#DC2626;strokeWidth=4;shadow=1;dashed=1;dashPattern=8 8;" vertex="1" parent="1">
<mxGeometry x="50" y="295" width="760" height="245" as="geometry" />
</mxCell>
<mxCell id="25" value="❌ 常见错误：定义了有参构造后，无参构造消失了" style="text;html=1;strokeColor=none;fillColor=#DC2626;align=center;verticalAlign=middle;fontSize=16;fontStyle=1;fontColor=#FFFFFF;rounded=1;" vertex="1" parent="1">
<mxGeometry x="50" y="295" width="760" height="40" as="geometry" />
</mxCell>
<mxCell id="26" value="" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#FFFFFF;strokeColor=#F87171;strokeWidth=2;" vertex="1" parent="1">
<mxGeometry x="70" y="350" width="340" height="170" as="geometry" />
</mxCell>
<mxCell id="27" value="😰 错误示例" style="text;html=1;strokeColor=none;fillColor=#EF4444;align=center;verticalAlign=middle;fontSize=13;fontStyle=1;fontColor=#FFFFFF;rounded=1;" vertex="1" parent="1">
<mxGeometry x="70" y="350" width="100" height="28" as="geometry" />
</mxCell>
<mxCell id="28" value="" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#1E293B;strokeColor=#475569;strokeWidth=2;" vertex="1" parent="1">
<mxGeometry x="85" y="390" width="310" height="115" as="geometry" />
</mxCell>
<mxCell id="29" value="class Student {" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=11;fontColor=#E2E8F0;fontFamily=Consolas;" vertex="1" parent="1">
<mxGeometry x="100" y="395" width="280" height="16" as="geometry" />
</mxCell>
<mxCell id="30" value=" Student(String name) {...}" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=11;fontColor=#FBBF24;fontFamily=Consolas;" vertex="1" parent="1">
<mxGeometry x="100" y="413" width="280" height="16" as="geometry" />
</mxCell>
<mxCell id="31" value="}" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=11;fontColor=#E2E8F0;fontFamily=Consolas;" vertex="1" parent="1">
<mxGeometry x="100" y="431" width="280" height="16" as="geometry" />
</mxCell>
<mxCell id="32" value="// 调用" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=11;fontColor=#6B7280;fontFamily=Consolas;fontStyle=2;" vertex="1" parent="1">
<mxGeometry x="100" y="452" width="280" height="16" as="geometry" />
</mxCell>
<mxCell id="33" value="Student s = new Student();" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=11;fontColor=#F87171;fontFamily=Consolas;textDecoration=line-through;" vertex="1" parent="1">
<mxGeometry x="100" y="470" width="200" height="16" as="geometry" />
</mxCell>
<mxCell id="34" value="// 报错!" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=11;fontColor=#EF4444;fontFamily=Consolas;fontStyle=1;" vertex="1" parent="1">
<mxGeometry x="300" y="470" width="80" height="16" as="geometry" />
</mxCell>
<mxCell id="35" value="" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#FFFFFF;strokeColor=#22C55E;strokeWidth=2;" vertex="1" parent="1">
<mxGeometry x="450" y="350" width="340" height="170" as="geometry" />
</mxCell>
<mxCell id="36" value="😊 正确做法" 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="450" y="350" width="100" height="28" as="geometry" />
</mxCell>
<mxCell id="37" value="" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#1E293B;strokeColor=#475569;strokeWidth=2;" vertex="1" parent="1">
<mxGeometry x="465" y="390" width="310" height="115" as="geometry" />
</mxCell>
<mxCell id="38" value="class Student {" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=11;fontColor=#E2E8F0;fontFamily=Consolas;" vertex="1" parent="1">
<mxGeometry x="480" y="395" width="280" height="16" as="geometry" />
</mxCell>
<mxCell id="39" value=" Student() {}" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=11;fontColor=#4ADE80;fontFamily=Consolas;fontStyle=1;" vertex="1" parent="1">
<mxGeometry x="480" y="413" width="200" height="16" as="geometry" />
</mxCell>
<mxCell id="40" value="// 手动添加!" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=10;fontColor=#22C55E;fontFamily=Consolas;fontStyle=2;" vertex="1" parent="1">
<mxGeometry x="670" y="413" width="90" height="16" as="geometry" />
</mxCell>
<mxCell id="41" value=" Student(String name) {...}" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=11;fontColor=#FBBF24;fontFamily=Consolas;" vertex="1" parent="1">
<mxGeometry x="480" y="431" width="280" height="16" as="geometry" />
</mxCell>
<mxCell id="42" value="}" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=11;fontColor=#E2E8F0;fontFamily=Consolas;" vertex="1" parent="1">
<mxGeometry x="480" y="449" width="280" height="16" as="geometry" />
</mxCell>
<mxCell id="43" value="// 调用" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=11;fontColor=#6B7280;fontFamily=Consolas;fontStyle=2;" vertex="1" parent="1">
<mxGeometry x="480" y="470" width="280" height="16" as="geometry" />
</mxCell>
<mxCell id="44" value="Student s = new Student();" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=11;fontColor=#4ADE80;fontFamily=Consolas;fontStyle=1;" vertex="1" parent="1">
<mxGeometry x="480" y="488" width="200" height="16" as="geometry" />
</mxCell>
<mxCell id="45" value="// 成功!" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=11;fontColor=#22C55E;fontFamily=Consolas;fontStyle=1;" vertex="1" parent="1">
<mxGeometry x="680" y="488" width="80" height="16" as="geometry" />
</mxCell>
<mxCell id="46" value="→" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;fontSize=36;fontColor=#22C55E;fontStyle=1;" vertex="1" parent="1">
<mxGeometry x="405" y="420" width="50" height="50" as="geometry" />
</mxCell>
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</mxGraphModel>
</diagram>
</mxfile>
3.2 错误示例
public class Product {
private String name;
// 手动定义了有参构造
public Product(String name) {
this.name = name;
}
}
// 使用
public class Main {
public static void main(String[] args) {
Product p1 = new Product("手机"); // ✅ 正确
Product p2 = new Product(); // ❌ 编译错误！
}
}
编译错误信息：
Error: java: 无法将类 Product 中的构造器 Product 应用到给定类型
需要: java.lang.String
找到: 没有参数
原因: 实际参数列表和形式参数列表长度不同
3.3 解决方案：显式定义无参构造
public class Product {
private String name;
// ✅ 显式定义无参构造
public Product() {
this.name = "未命名商品";
}
// 有参构造
public Product(String name) {
this.name = name;
}
}
// 现在都可以使用
Product p1 = new Product(); // ✅ 调用无参构造
Product p2 = new Product("手机"); // ✅ 调用有参构造
3.4 流程图：默认构造的生成逻辑
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显式构造方法？" vertex="1">
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<mxCell id="8" parent="1" style="text;html=1;strokeColor=none;fillColor=#DCFCE7;align=center;verticalAlign=middle;fontSize=14;fontColor=#166534;rounded=1;fontStyle=1;" value="NO" vertex="1">
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<mxPoint x="700" y="235" />
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<mxPoint x="560" y="235" as="sourcePoint" />
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<mxCell id="11" parent="1" style="text;html=1;strokeColor=none;fillColor=#22C55E;align=center;verticalAlign=middle;fontSize=16;fontColor=#FFFFFF;rounded=1;fontStyle=1;" value="没有" vertex="1">
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</mxCell>
<mxCell id="15" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#DCFCE7;strokeColor=#22C55E;strokeWidth=3;shadow=1;" value="" vertex="1">
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<mxCell id="17" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#1E293B;strokeColor=#475569;strokeWidth=2;" value="" vertex="1">
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<mxGeometry height="20" width="230" x="85" y="440" as="geometry" />
</mxCell>
<mxCell id="20" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=12;fontColor=#4ADE80;fontFamily=Consolas;fontStyle=1;" value=" super();" vertex="1">
<mxGeometry height="20" width="230" x="85" y="462" as="geometry" />
</mxCell>
<mxCell id="21" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=12;fontColor=#4ADE80;fontFamily=Consolas;fontStyle=1;" value="}" vertex="1">
<mxGeometry height="20" width="230" x="85" y="484" as="geometry" />
</mxCell>
<mxCell id="22" parent="1" style="text;html=1;strokeColor=none;fillColor=#BBF7D0;align=center;verticalAlign=middle;fontSize=12;fontColor=#166534;rounded=1;fontStyle=1;" value="💡 可以使用 new 类名() 创建对象" vertex="1">
<mxGeometry height="30" width="260" x="70" y="525" as="geometry" />
</mxCell>
<mxCell id="23" parent="1" style="text;html=1;strokeColor=none;fillColor=#BBF7D0;align=center;verticalAlign=middle;fontSize=12;fontColor=#166534;rounded=1;" value="📌 访问修饰符与类一致" vertex="1">
<mxGeometry height="30" width="260" x="70" y="565" as="geometry" />
</mxCell>
<mxCell id="24" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#FEE2E2;strokeColor=#EF4444;strokeWidth=3;shadow=1;" value="" vertex="1">
<mxGeometry height="255" width="300" x="550" y="355" as="geometry" />
</mxCell>
<mxCell id="25" parent="1" style="text;html=1;strokeColor=none;fillColor=#EF4444;align=center;verticalAlign=middle;fontSize=14;fontStyle=1;fontColor=#FFFFFF;rounded=1;" value="🚫 不生成默认构造方法" vertex="1">
<mxGeometry height="40" width="300" x="550" y="355" as="geometry" />
</mxCell>
<mxCell id="26" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#1E293B;strokeColor=#475569;strokeWidth=2;" value="" vertex="1">
<mxGeometry height="100" width="260" x="570" y="410" as="geometry" />
</mxCell>
<mxCell id="27" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=11;fontColor=#6B7280;fontFamily=Consolas;fontStyle=2;" value="// 只能使用已定义的构造方法" vertex="1">
<mxGeometry height="18" width="230" x="585" y="418" as="geometry" />
</mxCell>
<mxCell id="28" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=12;fontColor=#FBBF24;fontFamily=Consolas;fontStyle=1;" value="public 类名(参数) {" vertex="1">
<mxGeometry height="20" width="230" x="585" y="440" as="geometry" />
</mxCell>
<mxCell id="29" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=12;fontColor=#94A3B8;fontFamily=Consolas;" value=" // 初始化代码" vertex="1">
<mxGeometry height="20" width="230" x="585" y="462" as="geometry" />
</mxCell>
<mxCell id="30" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=12;fontColor=#FBBF24;fontFamily=Consolas;fontStyle=1;" value="}" vertex="1">
<mxGeometry height="20" width="230" x="585" y="484" as="geometry" />
</mxCell>
<mxCell id="31" parent="1" style="text;html=1;strokeColor=none;fillColor=#FECACA;align=center;verticalAlign=middle;fontSize=12;fontColor=#991B1B;rounded=1;fontStyle=1;" value="⚠️ new 类名() 将编译失败" vertex="1">
<mxGeometry height="30" width="260" x="570" y="525" as="geometry" />
</mxCell>
<mxCell id="32" parent="1" style="text;html=1;strokeColor=none;fillColor=#FECACA;align=center;verticalAlign=middle;fontSize=12;fontColor=#991B1B;rounded=1;" value="📌 需要无参构造时必须手动添加" vertex="1">
<mxGeometry height="30" width="260" x="570" y="565" as="geometry" />
</mxCell>
<mxCell id="33" parent="1" style="shape=curlyBracket;whiteSpace=wrap;html=1;rounded=1;flipH=1;labelPosition=right;verticalLabelPosition=middle;align=left;verticalAlign=middle;strokeWidth=2;strokeColor=#6366F1;size=0.5;" value="" vertex="1">
<mxGeometry height="225" width="30" x="360" y="370" as="geometry" />
</mxCell>
<mxCell id="34" parent="1" style="shape=curlyBracket;whiteSpace=wrap;html=1;rounded=1;labelPosition=right;verticalLabelPosition=middle;align=left;verticalAlign=middle;strokeWidth=2;strokeColor=#6366F1;size=0.5;" value="" vertex="1">
<mxGeometry height="225" width="30" x="510" y="370" as="geometry" />
</mxCell>
<mxCell id="35" parent="1" style="text;html=1;strokeColor=none;fillColor=#E0E7FF;align=center;verticalAlign=middle;fontSize=13;fontColor=#4338CA;rounded=1;fontStyle=1;" value="编译器
决策" vertex="1">
<mxGeometry height="50" width="100" x="400" y="460" as="geometry" />
</mxCell>
</root>
</mxGraphModel>
</diagram>
</mxfile>
四、构造方法重载
4.1 什么是构造方法重载
<mxfile>
<diagram id="constructor-overload" name="构造方法重载">
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<mxCell id="0" />
<mxCell id="1" parent="0" />
<mxCell id="2" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#F8FAFC;strokeColor=#64748B;strokeWidth=2;" value="" vertex="1">
<mxGeometry height="550" width="860" x="20" y="20" as="geometry" />
</mxCell>
<mxCell id="3" parent="1" style="text;html=1;strokeColor=none;fillColor=#8B5CF6;align=center;verticalAlign=middle;fontSize=24;fontStyle=1;fontColor=#FFFFFF;rounded=1;" value="🔄 构造方法重载" vertex="1">
<mxGeometry height="55" width="860" x="20" y="20" as="geometry" />
</mxCell>
<mxCell id="4" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#F3E8FF;strokeColor=#A855F7;strokeWidth=2;shadow=1;" value="" vertex="1">
<mxGeometry height="60" width="820" x="40" y="90" as="geometry" />
</mxCell>
<mxCell id="5" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;fontSize=16;fontColor=#6B21A8;fontStyle=1;" value="💡 同一个类中可以定义多个构造方法，只要它们的【参数列表不同】" vertex="1">
<mxGeometry height="60" width="820" x="40" y="90" as="geometry" />
</mxCell>
<mxCell id="6" parent="1" style="text;html=1;strokeColor=none;fillColor=#8B5CF6;align=center;verticalAlign=middle;fontSize=16;fontStyle=1;fontColor=#FFFFFF;rounded=1;" value="参数列表不同包括" vertex="1">
<mxGeometry height="35" width="200" x="350" y="165" as="geometry" />
</mxCell>
<mxCell id="7" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#DBEAFE;strokeColor=#3B82F6;strokeWidth=3;shadow=1;" value="" vertex="1">
<mxGeometry height="200" width="260" x="40" y="215" as="geometry" />
</mxCell>
<mxCell id="8" parent="1" style="text;html=1;strokeColor=none;fillColor=#3B82F6;align=center;verticalAlign=middle;fontSize=14;fontStyle=1;fontColor=#FFFFFF;rounded=1;" value="1️⃣ 参数个数不同" vertex="1">
<mxGeometry height="35" width="260" x="40" y="215" as="geometry" />
</mxCell>
<mxCell id="9" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#1E293B;strokeColor=#475569;strokeWidth=2;" value="" vertex="1">
<mxGeometry height="145" width="230" x="55" y="260" as="geometry" />
</mxCell>
<mxCell id="10" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=11;fontColor=#6B7280;fontFamily=Consolas;fontStyle=2;" value="// 无参" vertex="1">
<mxGeometry height="16" width="210" x="65" y="268" as="geometry" />
</mxCell>
<mxCell id="11" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=11;fontColor=#60A5FA;fontFamily=Consolas;fontStyle=1;" value="Person() {}" vertex="1">
<mxGeometry height="16" width="210" x="65" y="286" as="geometry" />
</mxCell>
<mxCell id="12" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=11;fontColor=#6B7280;fontFamily=Consolas;fontStyle=2;" value="// 1个参数" vertex="1">
<mxGeometry height="16" width="210" x="65" y="308" as="geometry" />
</mxCell>
<mxCell id="13" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=11;fontColor=#60A5FA;fontFamily=Consolas;fontStyle=1;" value="Person(String name) {}" vertex="1">
<mxGeometry height="16" width="210" x="65" y="326" as="geometry" />
</mxCell>
<mxCell id="14" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=11;fontColor=#6B7280;fontFamily=Consolas;fontStyle=2;" value="// 2个参数" vertex="1">
<mxGeometry height="16" width="210" x="65" y="348" as="geometry" />
</mxCell>
<mxCell id="15" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=11;fontColor=#60A5FA;fontFamily=Consolas;fontStyle=1;" value="Person(String name, int age) {}" vertex="1">
<mxGeometry height="16" width="210" x="65" y="366" as="geometry" />
</mxCell>
<mxCell id="16" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;fontSize=20;" value="✅" vertex="1">
<mxGeometry height="24" width="30" x="250" y="378" as="geometry" />
</mxCell>
<mxCell id="17" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#D1FAE5;strokeColor=#10B981;strokeWidth=3;shadow=1;" value="" vertex="1">
<mxGeometry height="200" width="260" x="320" y="215" as="geometry" />
</mxCell>
<mxCell id="18" parent="1" style="text;html=1;strokeColor=none;fillColor=#10B981;align=center;verticalAlign=middle;fontSize=14;fontStyle=1;fontColor=#FFFFFF;rounded=1;" value="2️⃣ 参数类型不同" vertex="1">
<mxGeometry height="35" width="260" x="320" y="215" as="geometry" />
</mxCell>
<mxCell id="19" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#1E293B;strokeColor=#475569;strokeWidth=2;" value="" vertex="1">
<mxGeometry height="145" width="230" x="335" y="260" as="geometry" />
</mxCell>
<mxCell id="20" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=11;fontColor=#6B7280;fontFamily=Consolas;fontStyle=2;" value="// String 类型" vertex="1">
<mxGeometry height="16" width="210" x="345" y="268" as="geometry" />
</mxCell>
<mxCell id="21" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=11;fontColor=#4ADE80;fontFamily=Consolas;fontStyle=1;" value="Person(String name) {}" vertex="1">
<mxGeometry height="16" width="210" x="345" y="286" as="geometry" />
</mxCell>
<mxCell id="22" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=11;fontColor=#6B7280;fontFamily=Consolas;fontStyle=2;" value="// int 类型" vertex="1">
<mxGeometry height="16" width="210" x="345" y="308" as="geometry" />
</mxCell>
<mxCell id="23" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=11;fontColor=#4ADE80;fontFamily=Consolas;fontStyle=1;" value="Person(int age) {}" vertex="1">
<mxGeometry height="16" width="210" x="345" y="326" as="geometry" />
</mxCell>
<mxCell id="24" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=11;fontColor=#6B7280;fontFamily=Consolas;fontStyle=2;" value="// double 类型" vertex="1">
<mxGeometry height="16" width="210" x="345" y="348" as="geometry" />
</mxCell>
<mxCell id="25" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=11;fontColor=#4ADE80;fontFamily=Consolas;fontStyle=1;" value="Person(double salary) {}" vertex="1">
<mxGeometry height="16" width="210" x="345" y="366" as="geometry" />
</mxCell>
<mxCell id="26" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;fontSize=20;" value="✅" vertex="1">
<mxGeometry height="24" width="30" x="530" y="378" as="geometry" />
</mxCell>
<mxCell id="27" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#FEF3C7;strokeColor=#F59E0B;strokeWidth=3;shadow=1;" value="" vertex="1">
<mxGeometry height="200" width="260" x="600" y="215" as="geometry" />
</mxCell>
<mxCell id="28" parent="1" style="text;html=1;strokeColor=none;fillColor=#F59E0B;align=center;verticalAlign=middle;fontSize=14;fontStyle=1;fontColor=#FFFFFF;rounded=1;" value="3️⃣ 参数顺序不同" vertex="1">
<mxGeometry height="35" width="260" x="600" y="215" as="geometry" />
</mxCell>
<mxCell id="29" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#1E293B;strokeColor=#475569;strokeWidth=2;" value="" vertex="1">
<mxGeometry height="105" width="230" x="615" y="260" as="geometry" />
</mxCell>
<mxCell id="30" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=11;fontColor=#6B7280;fontFamily=Consolas;fontStyle=2;" value="// String, int 顺序" vertex="1">
<mxGeometry height="16" width="210" x="625" y="268" as="geometry" />
</mxCell>
<mxCell id="31" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=11;fontColor=#FBBF24;fontFamily=Consolas;fontStyle=1;" value="Person(String s, int i) {}" vertex="1">
<mxGeometry height="16" width="210" x="625" y="286" as="geometry" />
</mxCell>
<mxCell id="32" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=11;fontColor=#6B7280;fontFamily=Consolas;fontStyle=2;" value="// int, String 顺序" vertex="1">
<mxGeometry height="16" width="210" x="625" y="308" as="geometry" />
</mxCell>
<mxCell id="33" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=11;fontColor=#FBBF24;fontFamily=Consolas;fontStyle=1;" value="Person(int i, String s) {}" vertex="1">
<mxGeometry height="16" width="210" x="625" y="326" as="geometry" />
</mxCell>
<mxCell id="34" parent="1" style="text;html=1;strokeColor=none;fillColor=#FEE2E2;align=center;verticalAlign=middle;fontSize=12;fontColor=#991B1B;rounded=1;fontStyle=1;" value="⚠️ 不推荐依赖此方式" vertex="1">
<mxGeometry height="30" width="230" x="615" y="375" as="geometry" />
</mxCell>
<mxCell id="35" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#1E293B;strokeColor=#8B5CF6;strokeWidth=3;shadow=1;" value="" vertex="1">
<mxGeometry height="120" width="820" x="40" y="430" as="geometry" />
</mxCell>
<mxCell id="36" parent="1" style="text;html=1;strokeColor=none;fillColor=#8B5CF6;align=center;verticalAlign=middle;fontSize=14;fontStyle=1;fontColor=#FFFFFF;rounded=1;" value="📝 完整示例" vertex="1">
<mxGeometry height="30" width="120" x="40" y="430" as="geometry" />
</mxCell>
<mxCell id="37" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=12;fontColor=#E2E8F0;fontFamily=Consolas;" value="public class Person {" vertex="1">
<mxGeometry height="18" width="200" x="60" y="468" as="geometry" />
</mxCell>
<mxCell id="38" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=12;fontColor=#60A5FA;fontFamily=Consolas;fontStyle=1;" value="public Person() { }" vertex="1">
<mxGeometry height="18" width="200" x="80" y="488" as="geometry" />
</mxCell>
<mxCell id="39" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=11;fontColor=#6B7280;fontFamily=Consolas;fontStyle=2;" value="// 无参构造" vertex="1">
<mxGeometry height="18" width="100" x="260" y="488" as="geometry" />
</mxCell>
<mxCell id="40" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=12;fontColor=#4ADE80;fontFamily=Consolas;fontStyle=1;" value="public Person(String name) { this.name = name; }" vertex="1">
<mxGeometry height="18" width="400" x="370" y="468" as="geometry" />
</mxCell>
<mxCell id="41" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=11;fontColor=#6B7280;fontFamily=Consolas;fontStyle=2;" value="// 1参构造" vertex="1">
<mxGeometry height="18" width="90" x="760" y="468" as="geometry" />
</mxCell>
<mxCell id="42" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=12;fontColor=#FBBF24;fontFamily=Consolas;fontStyle=1;" value="public Person(String name, int age) { this.name = name; this.age = age; }" vertex="1">
<mxGeometry height="18" width="480" x="370" y="488" as="geometry" />
</mxCell>
<mxCell id="43" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=11;fontColor=#6B7280;fontFamily=Consolas;fontStyle=2;" value="// 2参构造" vertex="1">
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<mxCell id="44" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=12;fontColor=#E2E8F0;fontFamily=Consolas;" value="}" vertex="1">
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4.2 代码示例
public class Student {
private String name;
private int age;
private String school;
// 构造方法 1：无参
public Student() {
this.name = "未知";
this.age = 0;
this.school = "未知";
}
// 构造方法 2：一个参数
public Student(String name) {
this.name = name;
this.age = 0;
this.school = "未知";
}
// 构造方法 3：两个参数
public Student(String name, int age) {
this.name = name;
this.age = age;
this.school = "未知";
}
// 构造方法 4：三个参数（全参构造）
public Student(String name, int age, String school) {
this.name = name;
this.age = age;
this.school = school;
}
}
// 多种创建方式
Student s1 = new Student(); // 调用构造1
Student s2 = new Student("张三"); // 调用构造2
Student s3 = new Student("李四", 20); // 调用构造3
Student s4 = new Student("王五", 21, "清华大学"); // 调用构造4
4.3 使用 this() 调用其他构造方法
public class Student {
private String name;
private int age;
private String school;
// 全参构造（主构造方法）
public Student(String name, int age, String school) {
this.name = name;
this.age = age;
this.school = school;
System.out.println("创建学生：" + name);
}
// 无参构造 → 调用全参构造
public Student() {
this("未知", 0, "未知"); // ✅ 调用全参构造
}
// 一参构造 → 调用全参构造
public Student(String name) {
this(name, 0, "未知"); // ✅ 调用全参构造
}
// 两参构造 → 调用全参构造
public Student(String name, int age) {
this(name, age, "未知"); // ✅ 调用全参构造
}
}
调用链示意图：
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<mxCell id="0" />
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<mxGeometry height="380" width="280" x="40" y="90" as="geometry" />
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</mxCell>
<mxCell id="6" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#DBEAFE;strokeColor=#3B82F6;strokeWidth=3;shadow=1;" value="" vertex="1">
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</mxCell>
<mxCell id="8" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;fontSize=12;fontColor=#1E40AF;fontStyle=1;" value="无参构造" vertex="1">
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4.4 this() 使用规则
| 规则 | 说明 |
|---|---|
| 必须在第一行 | this() 必须是构造方法的第一条语句 |
| 只能调用一次 | 一个构造方法中只能有一个 this() |
| 不能循环调用 | A 调 B，B 不能再调 A |
| 不能和 super() 共存 | 二者都要求在第一行 |
// ❌ 错误示例
public Student() {
System.out.println("准备创建"); // 错误！this() 必须在第一行
this("默认");
}
// ❌ 错误示例：循环调用
public Student() {
this("默认"); // 调用一参构造
}
public Student(String name) {
this(); // 又调用无参构造 → 死循环！编译器报错
}
五、构造方法的特点总结
5.1 五大特点一览表
| 序号 | 特点 | 说明 | 示例 |
|---|---|---|---|
| 1️⃣ | 方法名与类名相同 | 严格一致，包括大小写 | class Dog { Dog() {} } |
| 2️⃣ | 没有返回值类型 | 连 void 都不写 | public Student() {} |
| 3️⃣ | 自动调用 | new 时自动执行 | new Student() |
| 4️⃣ | 可以重载 | 多个构造，参数不同 | 见上文示例 |
| 5️⃣ | 不能被修饰 | 不能用 static/final/abstract | — |
5.2 特点详解图
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5.3 易混淆点：构造方法 vs 同名普通方法
public class Demo {
// ✅ 这是构造方法（无返回类型）
public Demo() {
System.out.println("构造方法执行");
}
// ⚠️ 这是普通方法！（有返回类型 void）
public void Demo() {
System.out.println("普通方法执行");
}
}
public class Main {
public static void main(String[] args) {
Demo d = new Demo(); // 输出：构造方法执行
d.Demo(); // 输出：普通方法执行
}
}
⚠️ 注意：虽然语法允许定义与类名相同的普通方法，但这是极差的编码习惯！
六、构造方法与继承
6.1 子类构造调用父类构造
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<mxCell id="10" value="super(name);" style="text;html=1;strokeColor=none;fillColor=#065F46;align=left;verticalAlign=middle;fontSize=12;fontColor=#4ADE80;fontFamily=Consolas;fontStyle=1;rounded=1;" vertex="1" parent="1">
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</mxCell>
<mxCell id="13" value="💡 程序员主动写 super(参数)" style="text;html=1;strokeColor=none;fillColor=#BBF7D0;align=center;verticalAlign=middle;fontSize=12;fontColor=#166534;rounded=1;fontStyle=1;" vertex="1" parent="1">
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<mxCell id="18" value="// super();" style="text;html=1;strokeColor=none;fillColor=#1E3A5F;align=left;verticalAlign=middle;fontSize=12;fontColor=#60A5FA;fontFamily=Consolas;fontStyle=2;rounded=1;" vertex="1" parent="1">
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<mxCell id="23" value="⚡ 为什么必须这样？" style="text;html=1;strokeColor=none;fillColor=#F59E0B;align=center;verticalAlign=middle;fontSize=14;fontStyle=1;fontColor=#FFFFFF;rounded=1;" vertex="1" parent="1">
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</mxCell>
<mxCell id="27" value="继承" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;fontSize=10;fontColor=#92400E;" vertex="1" parent="1">
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</mxCell>
<mxCell id="28" value="子类对象包含父类的属性和行为，必须先初始化父类部分" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=13;fontColor=#92400E;fontStyle=0;" vertex="1" parent="1">
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</mxCell>
<mxCell id="29" value="确保父类成员在子类使用前已正确初始化 ✓" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=13;fontColor=#92400E;fontStyle=0;" vertex="1" parent="1">
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</mxCell>
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</mxGraphModel>
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6.2 代码示例
class Animal {
protected String name;
public Animal() {
System.out.println("Animal 无参构造");
}
public Animal(String name) {
this.name = name;
System.out.println("Animal 有参构造：" + name);
}
}
class Dog extends Animal {
private String breed;
// 隐式调用 super()
public Dog() {
// 编译器自动添加：super();
System.out.println("Dog 无参构造");
}
// 显式调用 super(name)
public Dog(String name, String breed) {
super(name); // ✅ 必须在第一行
this.breed = breed;
System.out.println("Dog 有参构造：" + breed);
}
}
// 测试
Dog d1 = new Dog();
// 输出：
// Animal 无参构造
// Dog 无参构造
Dog d2 = new Dog("旺财", "金毛");
// 输出：
// Animal 有参构造：旺财
// Dog 有参构造：金毛
6.3 构造方法调用顺序图
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<mxCell id="21" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=13;fontColor=#D97706;fontStyle=1;" value="Object() 开始执行" vertex="1">
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<mxGeometry height="20" width="90" x="390" y="380" as="geometry" />
</mxCell>
<mxCell id="24" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#D1FAE5;strokeColor=#10B981;strokeWidth=2;" value="" vertex="1">
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<mxCell id="26" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=13;fontColor=#047857;fontStyle=1;" value="Animal(name) 继续执行" vertex="1">
<mxGeometry height="30" width="200" x="555" y="385" as="geometry" />
</mxCell>
<mxCell id="27" parent="1" style="text;html=1;strokeColor=none;fillColor=#1E293B;align=left;verticalAlign=middle;fontSize=11;fontColor=#4ADE80;fontFamily=Consolas;rounded=1;" value="→ this.name = name" vertex="1">
<mxGeometry height="22" width="160" x="520" y="420" as="geometry" />
</mxCell>
<mxCell id="28" parent="1" style="text;html=1;strokeColor=none;fillColor=#1E293B;align=left;verticalAlign=middle;fontSize=11;fontColor=#4ADE80;fontFamily=Consolas;rounded=1;" value="→ 打印 "Animal 有参构造：旺财"" vertex="1">
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</mxCell>
<mxCell id="29" edge="1" parent="1" source="24" style="endArrow=classic;html=1;strokeWidth=2;strokeColor=#10B981;exitX=0.5;exitY=1;exitDx=0;exitDy=0;entryX=0.5;entryY=0;entryDx=0;entryDy=0;curved=1;" target="31" value="">
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<mxGeometry height="20" width="90" x="580" y="480" as="geometry" />
</mxCell>
<mxCell id="31" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#DBEAFE;strokeColor=#3B82F6;strokeWidth=2;" value="" vertex="1">
<mxGeometry height="100" width="350" x="500" y="510" as="geometry" />
</mxCell>
<mxCell id="32" parent="1" style="ellipse;whiteSpace=wrap;html=1;fillColor=#3B82F6;strokeColor=#1E40AF;strokeWidth=2;fontColor=#FFFFFF;fontSize=14;fontStyle=1;" value="5" vertex="1">
<mxGeometry height="35" width="35" x="510" y="520" as="geometry" />
</mxCell>
<mxCell id="33" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=13;fontColor=#1E40AF;fontStyle=1;" value="Dog(name, breed) 继续执行" vertex="1">
<mxGeometry height="30" width="220" x="555" y="520" as="geometry" />
</mxCell>
<mxCell id="34" parent="1" style="text;html=1;strokeColor=none;fillColor=#1E293B;align=left;verticalAlign=middle;fontSize=11;fontColor=#60A5FA;fontFamily=Consolas;rounded=1;" value="→ this.breed = breed" vertex="1">
<mxGeometry height="22" width="160" x="520" y="555" as="geometry" />
</mxCell>
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<mxGeometry height="22" width="280" x="520" y="582" as="geometry" />
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<mxCell id="36" edge="1" parent="1" source="31" style="endArrow=classic;html=1;strokeWidth=2;strokeColor=#10B981;exitX=0.5;exitY=1;exitDx=0;exitDy=0;entryX=0.5;entryY=0;entryDx=0;entryDy=0;" target="38" value="">
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</mxCell>
<mxCell id="38" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#D1FAE5;strokeColor=#10B981;strokeWidth=3;shadow=1;" value="" vertex="1">
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</mxCell>
<mxCell id="39" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;fontSize=16;fontColor=#047857;fontStyle=1;" value="✅ 对象创建成功！" vertex="1">
<mxGeometry height="50" width="250" x="550" y="645" as="geometry" />
</mxCell>
<mxCell id="40" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#1E293B;strokeColor=#475569;strokeWidth=2;" value="" vertex="1">
<mxGeometry height="240" width="320" x="60" y="460" as="geometry" />
</mxCell>
<mxCell id="41" parent="1" style="text;html=1;strokeColor=none;fillColor=#475569;align=center;verticalAlign=middle;fontSize=13;fontStyle=1;fontColor=#FFFFFF;rounded=1;" value="📊 类继承层次" vertex="1">
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</mxCell>
<mxCell id="42" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#F59E0B;strokeColor=#D97706;strokeWidth=2;fontColor=#FFFFFF;fontSize=14;fontStyle=1;" value="Object" vertex="1">
<mxGeometry height="40" width="110" x="165" y="500" as="geometry" />
</mxCell>
<mxCell id="43" edge="1" parent="1" source="44" style="endArrow=block;html=1;strokeWidth=2;strokeColor=#94A3B8;endFill=0;exitX=0.5;exitY=0;exitDx=0;exitDy=0;entryX=0.5;entryY=1;entryDx=0;entryDy=0;" target="42" value="">
<mxGeometry relative="1" as="geometry">
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<mxCell id="44" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#10B981;strokeColor=#047857;strokeWidth=2;fontColor=#FFFFFF;fontSize=14;fontStyle=1;" value="Animal" vertex="1">
<mxGeometry height="40" width="110" x="165" y="570" as="geometry" />
</mxCell>
<mxCell id="45" edge="1" parent="1" source="46" style="endArrow=block;html=1;strokeWidth=2;strokeColor=#94A3B8;endFill=0;exitX=0.5;exitY=0;exitDx=0;exitDy=0;entryX=0.5;entryY=1;entryDx=0;entryDy=0;" target="44" value="">
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<mxPoint x="220" y="630" as="sourcePoint" />
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<mxGeometry height="40" width="110" x="165" y="640" as="geometry" />
</mxCell>
<mxCell id="47" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;fontSize=10;fontColor=#94A3B8;" value="extends" vertex="1">
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</mxCell>
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<mxGeometry height="35" width="90" x="210" y="780" as="geometry" />
</mxCell>
<mxCell id="52" edge="1" parent="1" style="endArrow=classic;html=1;strokeWidth=3;strokeColor=#7C3AED;" value="">
<mxGeometry relative="1" as="geometry">
<mxPoint x="300" y="797.08" as="sourcePoint" />
<mxPoint x="360" y="797.08" as="targetPoint" />
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</mxCell>
<mxCell id="53" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#10B981;strokeColor=#047857;strokeWidth=2;fontColor=#FFFFFF;fontSize=13;fontStyle=1;" value="Animal()" vertex="1">
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</mxCell>
<mxCell id="54" edge="1" parent="1" style="endArrow=classic;html=1;strokeWidth=3;strokeColor=#7C3AED;" value="">
<mxGeometry relative="1" as="geometry">
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</mxCell>
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<mxGeometry height="35" width="90" x="510" y="780" as="geometry" />
</mxCell>
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</mxCell>
</root>
</mxGraphModel>
</diagram>
</mxfile>
6.4 常见错误：父类没有无参构造
class Parent {
public Parent(String name) { // 只有有参构造
System.out.println("Parent: " + name);
}
}
class Child extends Parent {
public Child() {
// 编译器隐式添加 super()，但 Parent 没有无参构造！
// ❌ 编译错误！
}
}
解决方案：
// 方案 1：子类显式调用父类有参构造
class Child extends Parent {
public Child() {
super("默认名称"); // ✅ 显式调用
}
}
// 方案 2：父类添加无参构造
class Parent {
public Parent() { } // ✅ 添加无参构造
public Parent(String name) {
System.out.println("Parent: " + name);
}
}
七、最佳实践
7.1 实践规则一览表
| 序号 | 规则 | 原因 |
|---|---|---|
| 1 | 始终保留无参构造 | 框架反射、序列化需要 |
| 2 | 使用 this() 避免重复 | 代码复用，统一初始化 |
| 3 | 参数校验放构造中 | 确保对象状态合法 |
| 4 | 避免调用可重写方法 | 防止子类未初始化完成 |
| 5 | 考虑使用建造者模式 | 参数过多时更清晰 |
7.2 实践 1：始终保留无参构造
public class User {
private String username;
private String email;
// ✅ 始终保留无参构造
public User() {
}
public User(String username, String email) {
this.username = username;
this.email = email;
}
}
需要无参构造的场景：
<mxfile>
<diagram id="no-arg-constructor-scenarios" name="需要无参构造的场景">
<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="750" math="0" shadow="0">
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<mxCell id="0" />
<mxCell id="1" parent="0" />
<mxCell id="2" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#F8FAFC;strokeColor=#64748B;strokeWidth=2;shadow=1;" value="" vertex="1">
<mxGeometry height="710" width="860" x="20" y="20" as="geometry" />
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<mxCell id="4" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#EEF2FF;strokeColor=#6366F1;strokeWidth=2;" value="" vertex="1">
<mxGeometry height="190" width="820" x="40" y="95" as="geometry" />
</mxCell>
<mxCell id="5" parent="1" style="ellipse;whiteSpace=wrap;html=1;fillColor=#6366F1;strokeColor=#4338CA;strokeWidth=2;fontColor=#FFFFFF;fontSize=18;fontStyle=1;" value="1" vertex="1">
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</mxCell>
<mxCell id="7" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#1E293B;strokeColor=#475569;strokeWidth=2;" value="" vertex="1">
<mxGeometry height="110" width="780" x="60" y="160" as="geometry" />
</mxCell>
<mxCell id="8" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=13;fontColor=#6B7280;fontFamily=Consolas;" value="// Spring/MyBatis 等框架" vertex="1">
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</mxCell>
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<mxGeometry height="30" width="400" x="80" y="198" as="geometry" />
</mxCell>
<mxCell id="10" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=13;fontColor=#6B7280;fontFamily=Consolas;" value="// 调用无参构造" vertex="1">
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<mxGeometry height="30" width="70" x="580" y="175" as="geometry" />
</mxCell>
<mxCell id="12" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#EF4444;strokeColor=#DC2626;strokeWidth=2;fontColor=#FFFFFF;fontSize=12;fontStyle=1;" value="MyBatis" vertex="1">
<mxGeometry height="30" width="70" x="660" y="175" as="geometry" />
</mxCell>
<mxCell id="13" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#3B82F6;strokeColor=#2563EB;strokeWidth=2;fontColor=#FFFFFF;fontSize=12;fontStyle=1;" value="Struts" vertex="1">
<mxGeometry height="30" width="70" x="740" y="175" as="geometry" />
</mxCell>
<mxCell id="14" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#FEF3C7;strokeColor=#F59E0B;strokeWidth=2;" value="" vertex="1">
<mxGeometry height="175" width="820" x="40" y="300" as="geometry" />
</mxCell>
<mxCell id="15" parent="1" style="ellipse;whiteSpace=wrap;html=1;fillColor=#F59E0B;strokeColor=#D97706;strokeWidth=2;fontColor=#FFFFFF;fontSize=18;fontStyle=1;" value="2" vertex="1">
<mxGeometry height="40" width="40" x="55" y="315" as="geometry" />
</mxCell>
<mxCell id="16" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=18;fontColor=#D97706;fontStyle=1;" value="序列化/反序列化" vertex="1">
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</mxCell>
<mxCell id="17" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#1E293B;strokeColor=#475569;strokeWidth=2;" value="" vertex="1">
<mxGeometry height="95" width="780" x="60" y="365" as="geometry" />
</mxCell>
<mxCell id="18" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=13;fontColor=#6B7280;fontFamily=Consolas;" value="// Jackson、Gson 等 JSON 库" vertex="1">
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</mxCell>
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</mxCell>
<mxCell id="20" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#0EA5E9;strokeColor=#0284C7;strokeWidth=2;fontColor=#FFFFFF;fontSize=12;fontStyle=1;" value="Jackson" vertex="1">
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</diagram>
</mxfile>
7.3 实践 2：使用 this() 避免代码重复
// ❌ 不好的写法：重复代码
public class Product {
private String name;
private double price;
private String category;
public Product() {
this.name = "未命名";
this.price = 0.0;
this.category = "未分类";
validate(); // 重复
log(); // 重复
}
public Product(String name) {
this.name = name;
this.price = 0.0;
this.category = "未分类";
validate(); // 重复
log(); // 重复
}
public Product(String name, double price, String category) {
this.name = name;
this.price = price;
this.category = category;
validate(); // 重复
log(); // 重复
}
}
// ✅ 好的写法：使用 this() 委托
public class Product {
private String name;
private double price;
private String category;
// 全参构造（核心构造方法）
public Product(String name, double price, String category) {
this.name = name;
this.price = price;
this.category = category;
validate(); // 只写一次
log(); // 只写一次
}
// 委托给全参构造
public Product() {
this("未命名", 0.0, "未分类");
}
public Product(String name) {
this(name, 0.0, "未分类");
}
public Product(String name, double price) {
this(name, price, "未分类");
}
}
7.4 实践 3：在构造方法中进行参数校验
public class BankAccount {
private String accountNumber;
private double balance;
public BankAccount(String accountNumber, double initialBalance) {
// ✅ 参数校验
if (accountNumber == null || accountNumber.trim().isEmpty()) {
throw new IllegalArgumentException("账号不能为空");
}
if (initialBalance < 0) {
throw new IllegalArgumentException("初始余额不能为负");
}
this.accountNumber = accountNumber;
this.balance = initialBalance;
}
}
// 使用
BankAccount acc1 = new BankAccount("123456", 1000); // ✅ 正常
BankAccount acc2 = new BankAccount("", 100); // ❌ 抛出异常
BankAccount acc3 = new BankAccount("123", -100); // ❌ 抛出异常
7.5 实践 4：避免在构造方法中调用可重写方法
// ❌ 危险的写法
class Parent {
public Parent() {
init(); // 调用可重写方法
}
public void init() {
System.out.println("Parent init");
}
}
class Child extends Parent {
private String name;
public Child(String name) {
super(); // 调用父类构造
this.name = name;
}
@Override
public void init() {
// ⚠️ 此时 name 还是 null！因为子类构造还没执行完
System.out.println("Child init: " + name.toUpperCase()); // NullPointerException!
}
}
问题分析图：
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解决方案：
// ✅ 使用 final 方法或 private 方法
class Parent {
public Parent() {
initInternal(); // 调用 private 方法
}
private void initInternal() { // 不能被重写
System.out.println("Parent init");
}
}
7.6 实践 5：参数过多时使用建造者模式
// ❌ 构造参数过多，难以阅读
User user = new User("张三", 20, "男", "北京", "100000",
"13800138000", "zhangsan@email.com", true);
// ✅ 使用建造者模式
User user = User.builder()
.name("张三")
.age(20)
.gender("男")
.city("北京")
.build();
建造者模式实现：
public class User {
private final String name;
private final int age;
private final String gender;
private final String city;
// 私有构造，只能通过 Builder 创建
private User(Builder builder) {
this.name = builder.name;
this.age = builder.age;
this.gender = builder.gender;
this.city = builder.city;
}
// 静态方法获取 Builder
public static Builder builder() {
return new Builder();
}
// 静态内部类 Builder
public static class Builder {
private String name;
private int age;
private String gender;
private String city;
public Builder name(String name) {
this.name = name;
return this;
}
public Builder age(int age) {
this.age = age;
return this;
}
public Builder gender(String gender) {
this.gender = gender;
return this;
}
public Builder city(String city) {
this.city = city;
return this;
}
public User build() {
return new User(this);
}
}
}
八、核心总结
8.1 知识要点速查表
| 主题 | 要点 |
|---|---|
| 默认构造 | 没有任何构造方法时编译器自动生成 |
| 消失条件 | 定义任何构造方法后默认构造消失 |
| 重载 | 同类中多个构造，参数不同 |
| this() | 调用本类其他构造，必须在第一行 |
| super() | 调用父类构造，必须在第一行 |
| 执行顺序 | Object → 父类 → 子类 |
8.2 构造方法完整生命周期图
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8.3 最佳实践清单
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