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 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 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:
void setSecond(LocalDate);
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 {
public com.book.chapter8.DateInterval();
Code:
0: aload_0
1: invokespecial #1 // Method com/book/chapter8/Pair."":()V
4: return
public void setSecond(java.time.LocalDate);
Code:
0: aload_1
1: aload_0
2: invokevirtual #2 // Method getFirst:()Ljava/lang/Object;
5: checkcast #3 // class java/time/chrono/ChronoLocalDate
8: invokevirtual #4 // Method java/time/LocalDate.compareTo:(Ljava/time/chrono/ChronoLocalDate;)I
11: iflt 19
14: aload_0
15: aload_1
16: invokespecial #5 // Method com/book/chapter8/Pair.setSecond:(Ljava/lang/Object;)V
19: return
public void setSecond(java.lang.Object);
Code:
0: aload_0
1: aload_1
2: checkcast #6 // class java/time/LocalDate
5: invokevirtual #7 // Method setSecond:(Ljava/time/LocalDate;)V
8: 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 {
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 {
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 pair = interval;
pair.setFirst(LocalDate.now());
pair.setSecond(LocalDate.now());
}
}
