因为 java 在编译源码时, 会进行 类型擦除, 导致泛型类型被替换限定类型(无限定类型就使用 Object). 因此为保持继承和重载的多态特性, 编译器会生成 桥方法.

本文最后附录所有源码.

Pair 是个泛型类, 它具有泛型方法 setSecond(T second),
在经过编译时的 类型擦除 后变为 setSecond(Object second).

DateInterval 是 Pair<LocalDate> 的实例化子类, 它具有方法 setSecond(LocalDate second).

在 Main.main 中, 我们新建 DateInterval 对象, 并用基类 Pair<LocalDate> 来引用它,
此时调用基类的 setSecond 方法时, 我们希望它能够实现多态, 即调用 DateInterval.setSecond(LocalDate) 方法.
事实上, java 编译器通过插入 桥方法 的方式, 帮助我们实现了该功能.

反编译 DateInterval.class 会发现它具有两个 setSecond 方法:

1. void setSecond(LocalDate);
2. void setSecond(Object).

并且, 在 void setSecond(Object) 中会调用 void setSecond(LocalDate), 这就是所谓的桥方法.

附: DateInterval.class 反编译后的代码:

Compiled from "DateInterval.java"
public class com.book.chapter8.DateInterval extends com.book.chapter8.Pair<java.time.LocalDate> {public com.book.chapter8.DateInterval();Code:0: aload_01: invokespecial #1                  // Method com/book/chapter8/Pair."<init>":()V4: returnpublic void setSecond(java.time.LocalDate);Code:0: aload_11: aload_02: invokevirtual #2                  // Method getFirst:()Ljava/lang/Object;5: checkcast     #3                  // class java/time/chrono/ChronoLocalDate8: invokevirtual #4                  // Method java/time/LocalDate.compareTo:(Ljava/time/chrono/ChronoLocalDate;)I11: iflt          1914: aload_015: aload_116: invokespecial #5                  // Method com/book/chapter8/Pair.setSecond:(Ljava/lang/Object;)V19: returnpublic void setSecond(java.lang.Object);Code:0: aload_01: aload_12: checkcast     #6                  // class java/time/LocalDate5: invokevirtual #7                  // Method setSecond:(Ljava/time/LocalDate;)V8: return
}

附: 其他所有源码:

Pair.java:

package com.book.chapter8;/*** Created by zhixiao.mzx on 2016/11/5.*/
public class Pair<T> {private T first;private T second;public Pair() {first = null;second = null;}public Pair(T first, T second) {this.first = first;this.second = second;}public T getFirst() {return first;}public void setFirst(T first) {this.first = first;}public T getSecond() {return second;}public void setSecond(T second) {this.second = second;}
}

DateInterval.java:

package com.book.chapter8;import java.time.LocalDate;/*** Created by zhixiao.mzx on 2016/11/5.*/
public class DateInterval extends Pair<LocalDate> {public void setSecond(LocalDate second) {if (second.compareTo(getFirst()) >= 0) {super.setSecond(second);}}
}

Main.java:

package com.book.chapter8;import java.time.LocalDate;/*** Created by zhixiao.mzx on 2016/11/5.*/
public class Main {public static void main(String[] args) {DateInterval interval = new DateInterval();Pair<LocalDate> pair = interval;pair.setFirst(LocalDate.now());pair.setSecond(LocalDate.now());}
}