java 泛型--桥方法

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:

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_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<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 pair = interval; pair.setFirst(LocalDate.now()); pair.setSecond(LocalDate.now()); } }

 

分类:  java