Java_基础_i=i++;

例：

package deep;

public class PostfixIncrementOperator {
    public static void main(String[] args) {
        int i = 10;
        i = i++;
        System.out.println(i);
    }
}

运行结果：10

首先我们看JLS中对后置加的解释：The result of the postfix increment expression is not a variable, but a value. 即后置加的结果不是一个变量，而是一个值。并且后置加的优先级在赋值之前，所以是先完成的后置加再进行的赋值。

我们先使用javac -g PostfixIncrementOperator.java编译源码，再使用javap -c -s -l -v PostfixIncrementOperator.class反汇编此class.。如下：

Classfile /C:/Users/tspoi/workspace/Test/src/deep/PostfixIncrementOperator.class

  Last modified 2015-8-7; size 584 bytes
  MD5 checksum 69c9ee93f1dbe68468d009ddf8f0a1b1
  Compiled from "PostfixIncrementOperator.java"
public class deep.PostfixIncrementOperator
  minor version: 0
  major version: 52
  flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
   #1 = Methodref          #5.#21         // java/lang/Object."<init>":()V
   #2 = Fieldref           #22.#23        // java/lang/System.out:Ljava/io/PrintStream;
   #3 = Methodref          #24.#25        // java/io/PrintStream.println:(I)V
   #4 = Class              #26            // deep/PostfixIncrementOperator
   #5 = Class              #27            // java/lang/Object
   #6 = Utf8               <init>
   #7 = Utf8               ()V
   #8 = Utf8               Code
   #9 = Utf8               LineNumberTable
  #10 = Utf8               LocalVariableTable
  #11 = Utf8               this
  #12 = Utf8               Ldeep/PostfixIncrementOperator;
  #13 = Utf8               main
  #14 = Utf8               ([Ljava/lang/String;)V
  #15 = Utf8               args
  #16 = Utf8               [Ljava/lang/String;
  #17 = Utf8               i
  #18 = Utf8               I
  #19 = Utf8               SourceFile
  #20 = Utf8               PostfixIncrementOperator.java
  #21 = NameAndType        #6:#7          // "<init>":()V
  #22 = Class              #28            // java/lang/System
  #23 = NameAndType        #29:#30        // out:Ljava/io/PrintStream;
  #24 = Class              #31            // java/io/PrintStream
  #25 = NameAndType        #32:#33        // println:(I)V
  #26 = Utf8               deep/PostfixIncrementOperator
  #27 = Utf8               java/lang/Object
  #28 = Utf8               java/lang/System
  #29 = Utf8               out
  #30 = Utf8               Ljava/io/PrintStream;
  #31 = Utf8               java/io/PrintStream
  #32 = Utf8               println
  #33 = Utf8               (I)V
{
  public deep.PostfixIncrementOperator();
    descriptor: ()V
    flags: ACC_PUBLIC
    Code:
      stack=1, locals=1, args_size=1
         0: aload_0
         1: invokespecial #1                  // Method java/lang/Object."<init>":()V
         4: return
      LineNumberTable:
        line 3: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       5     0  this   Ldeep/PostfixIncrementOperator;

  public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=2, locals=2, args_size=1
         0: bipush        10
         2: istore_1
         3: iload_1
         4: iinc          1, 1
         7: istore_1
         8: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
        11: iload_1
        12: invokevirtual #3                  // Method java/io/PrintStream.println:(I)V
        15: return
      LineNumberTable:
        line 5: 0
        line 6: 3
        line 7: 8
        line 8: 15
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      16     0  args   [Ljava/lang/String;
            3      13     1     i   I
}

我们主要看关键部分：

         0: bipush        10    //常数10入栈
         2: istore_1            //常数值出栈，i入栈
                                //至此完成i=10;
         3: iload_1             //装载变量i，10入栈
                                //这是特殊的一步，这步将i值先行保存，以备赋值使用
         4: iinc          1, 1  //变量值增加，栈内值不变
         7: istore_1            //i赋值，还是赋的10，因为栈内值未改变

So…

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