泛型 橋方法

一個泛型類 

package com.test;

import java.time.LocalDate;

public class Pair<T> {
    private T fist;
    private T second;

    public T getFist() {
        return fist;
    }

    public void setFist(T fist) {
        this.fist = fist;
    }

    public T getSecond() {
        return second;
    }

    public void setSecond(T second) {
        this.second = second;
    }


}

一個繼承泛型類的子類

package com.test;

import java.time.LocalDate;

public class Interval extends Pair<LocalDate> {
    @Override
    public void setSecond(LocalDate second){
        if (second.compareTo(getFist())>=0){
            super.setSecond(second);
        }
    }

    public static void main(String[] args) {
        Interval interval =  new Interval();
        Pair<LocalDate> pair =  interval;
        pair.setFist(LocalDate.now().minusDays(1));
        pair.setSecond(LocalDate.now());//17行
    }
}

運行字節碼

編譯器在Interval類中生成橋方法

public void setSecond（Object second）{

setSecond ((LocalDate）second);

}

運行順序： 虛擬機用pari引用的對象調用 setSecond(Object)方法，即Interval.setSecond(Object) ，這個方法是編譯器自動合成的，在方法內調用Interval.setSecond(LocalDate)

橋方法的意義 用來處理類型擦除與多態之間的衝突

橋方法也用來覆蓋

public class Employee implements Cloneable{
    public Employee clone() throws CloneNotSupportedException{..}
}

Employee類中將存在

Employee clone()

Object.clone() //編譯器將生產橋方法覆蓋父類Object中的clone方法，橋方法調用新定義的方法 

 Interval類的字節碼

// class version 52.0 (52)
// access flags 0x21
// signature Lcom/test/Pair<Ljava/time/LocalDate;>;
// declaration: com/test/Interval extends com.test.Pair<java.time.LocalDate>
public class com/test/Interval extends com/test/Pair {

  // compiled from: Interval.java

  // access flags 0x1
  public <init>()V
   L0
    LINENUMBER 5 L0
    ALOAD 0
    INVOKESPECIAL com/test/Pair.<init> ()V
    RETURN
   L1
    LOCALVARIABLE this Lcom/test/Interval; L0 L1 0
    MAXSTACK = 1
    MAXLOCALS = 1

  // access flags 0x1
  public setSecond(Ljava/time/LocalDate;)V
   L0
    LINENUMBER 8 L0
    ALOAD 1
    ALOAD 0
    INVOKEVIRTUAL com/test/Interval.getFist ()Ljava/lang/Object;
    CHECKCAST java/time/chrono/ChronoLocalDate
    INVOKEVIRTUAL java/time/LocalDate.compareTo (Ljava/time/chrono/ChronoLocalDate;)I
    IFLT L1
   L2
    LINENUMBER 9 L2
    ALOAD 0
    ALOAD 1
    INVOKESPECIAL com/test/Pair.setSecond (Ljava/lang/Object;)V
   L1
    LINENUMBER 11 L1
   FRAME SAME
    RETURN
   L3
    LOCALVARIABLE this Lcom/test/Interval; L0 L3 0
    LOCALVARIABLE second Ljava/time/LocalDate; L0 L3 1
    MAXSTACK = 2
    MAXLOCALS = 2

  // access flags 0x9
  public static main([Ljava/lang/String;)V
   L0
    LINENUMBER 14 L0
    NEW com/test/Interval
    DUP
    INVOKESPECIAL com/test/Interval.<init> ()V
    ASTORE 1
   L1
    LINENUMBER 15 L1
    ALOAD 1
    ASTORE 2
   L2
    LINENUMBER 16 L2
    ALOAD 2
    INVOKESTATIC java/time/LocalDate.now ()Ljava/time/LocalDate;
    LCONST_1
    INVOKEVIRTUAL java/time/LocalDate.minusDays (J)Ljava/time/LocalDate;
    INVOKEVIRTUAL com/test/Pair.setFist (Ljava/lang/Object;)V
   L3
    LINENUMBER 17 L3
    ALOAD 2
    INVOKESTATIC java/time/LocalDate.now ()Ljava/time/LocalDate;
    INVOKEVIRTUAL com/test/Pair.setSecond (Ljava/lang/Object;)V
   L4
    LINENUMBER 18 L4
    RETURN
   L5
    LOCALVARIABLE args [Ljava/lang/String; L0 L5 0
    LOCALVARIABLE interval Lcom/test/Interval; L1 L5 1
    LOCALVARIABLE pair Lcom/test/Pair; L2 L5 2
    // signature Lcom/test/Pair<Ljava/time/LocalDate;>;
    // declaration: pair extends com.test.Pair<java.time.LocalDate>
    MAXSTACK = 4
    MAXLOCALS = 3

  // access flags 0x1041
  public synthetic bridge setSecond(Ljava/lang/Object;)V
   L0
    LINENUMBER 5 L0
    ALOAD 0
    ALOAD 1
    CHECKCAST java/time/LocalDate
    INVOKEVIRTUAL com/test/Interval.setSecond (Ljava/time/LocalDate;)V
    RETURN
   L1
    LOCALVARIABLE this Lcom/test/Interval; L0 L1 0
    MAXSTACK = 2
    MAXLOCALS = 2
}