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懂得i++和++i

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懂得i++和++i

案例

代码1

package org.huangao.other.dome1;

import org.junit.Test;

public class Dome1 {
    @Test
    public void test1() {
        int j = 0;
        j = j++;
        System.out.println(j);
    }

    @Test
    public void test2() {
        int j = 0;
        j = ++j;
        System.out.println(j);
    }
}

结果

test1:0
test2:1

代码2

package org.huangao.other.dome1;

import org.junit.Test;

public class Dome1 {
    @Test
    public void test1() {
        int j = 0;
//        j = j++;
        j++;
        System.out.println(j);
    }

    @Test
    public void test2() {
        int j = 0;
//        j = ++j;
        ++j;
        System.out.println(j);
    }
}

结果

test1:1
test2:1

​ 可以初步分析是 j 值 对于 j++ 赋值的地方有所差异

分析

代码1的字节码

D:\jdk1.8.0_211\bin\javap.exe -c org.huangao.other.dome1.Dome1
Compiled from "Dome1.java"
public class org.huangao.other.dome1.Dome1 {
  public org.huangao.other.dome1.Dome1();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return

  public void test1();
    Code:
       0: iconst_0
       1: istore_1
       2: iload_1
       3: iinc          1, 1
       6: istore_1
       7: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
      10: iload_1
      11: invokevirtual #3                  // Method java/io/PrintStream.println:(I)V
      14: return

  public void test2();
    Code:
       0: iconst_0
       1: istore_1
       2: iinc          1, 1
       5: iload_1
       6: istore_1
       7: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
      10: iload_1
      11: invokevirtual #3                  // Method java/io/PrintStream.println:(I)V
      14: return
}

Process finished with exit code 0

分析

​ 首先我们先看test1 方法

iconst_0 代表将int型 0 推送至栈顶

istore_1 将栈顶 int 型数值存入第二个本地变量 即 j=0

iload_1 将第二个int 型本地变量 推送至栈顶 即 j=0

iinc 1,1 将局部变量表中1号元素自增1,此时j=1

istore_1 将栈顶 int 型数值存入第二个本地变量

getstatic

iload_1

invokevirtual

return

​ 接下来看test2方法

iconst_0 将 int 型 0 推入栈顶

istore_1 将栈顶 int 型 数值存入第二个本地变量

iinc 1,1 将局部变量表1 号元素 自增1 ,此时j=1

iload_1 将第二个 int 型本地变量推送至栈顶

istore_1 将栈顶 int 型数值 存入第二个本地变量

getstatic

.....

结论

本来局部变量表中的j已经完成了自增,但是在进行赋值的时候,是将操作栈中的数据弹出,直接进行覆盖操作

​ 如果站在JVM的层次来说 对于 i++ 和 ++i

  1. i++ 是先被操作数栈拿去了(先执行了load指令),然后再在局部变量表中完成了自增,但是操作数栈中还是自增前的值
  2. 而++i是在局部变量表中完成了自增(先执行了innc指令),然后再被load进行了操作数栈,所以操作数栈中保存的是自增后的值

课后思考

    @Test
    public void test3() {
        int i = 4;
        int b = i++;
        int a = ++i;
    }
    
    
      public void test3();
    Code:
       0: iconst_4
       1: istore_1
       2: iload_1
       3: iinc          1, 1
       6: istore_2
       7: iinc          1, 1
      10: iload_1
      11: istore_3
      12: return
    @Test
    public void test3() {
        int i = 4;
        i = i++;
        System.out.println(i);
        i = ++i;
        System.out.println(i);
    }
    4
    5
    
      public void test3();
    Code:
       0: iconst_4
       1: istore_1
       2: iload_1
       3: iinc          1, 1
       6: istore_1
       7: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
      10: iload_1
      11: invokevirtual #3                  // Method java/io/PrintStream.println:(I)V
      14: iinc          1, 1
      17: iload_1
      18: istore_1
      19: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
      22: iload_1
      23: invokevirtual #3                  // Method java/io/PrintStream.println:(I)V
      26: return

    @Test
    public void test3() {
        int i = 4;
        i = ++i;
        System.out.println(i);
        i = i++;
        System.out.println(i);
    }
    
    5
    5
      public void test3();
    Code:
       0: iconst_4
       1: istore_1
       2: iinc          1, 1
       5: iload_1
       6: istore_1
       7: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
      10: iload_1
      11: invokevirtual #3                  // Method java/io/PrintStream.println:(I)V
      14: iload_1
      15: iinc          1, 1
      18: istore_1
      19: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
      22: iload_1
      23: invokevirtual #3                  // Method java/io/PrintStream.println:(I)V
      26: return

标签:++,int,懂得,println,istore,iload,public
来源: https://www.cnblogs.com/huan30/p/12950612.html