来看看java 反编译出来的东西，详细理解i++与++i

public class Inc {public static void main(String[] args) {int j = 0;j = j++ + j++;System.out.println(j);}}//1D:\Eclipse\eclipse-3.5.0\test\bin>javap -c IncCompiled from "Inc.java"public class Inc extends java.lang.Object{public Inc();  Code:   0:   aload_0   1:   invokespecial   #8; //Method java/lang/Object."<init>":()V   4:   returnpublic static void main(java.lang.String[]);  Code:   0:   iconst_0   1:   istore_1   2:   iload_1   3:   iinc    1, 1   6:   iload_1   7:   iinc    1, 1   10:  iadd   11:  istore_1   12:  getstatic       #16; //Field java/lang/System.out:Ljava/io/PrintStream;   15:  iload_1   16:  invokevirtual   #22; //Method java/io/PrintStream.println:(I)V   19:  return}public class Inc {public static void main(String[] args) {int j = 0;j = ++j + ++j;System.out.println(j);}}//3D:\Eclipse\eclipse-3.5.0\test\bin>javap -c IncCompiled from "Inc.java"public class Inc extends java.lang.Object{public Inc();  Code:   0:   aload_0   1:   invokespecial   #8; //Method java/lang/Object."<init>":()V   4:   returnpublic static void main(java.lang.String[]);  Code:   0:   iconst_0   1:   istore_1   2:   iinc    1, 1   5:   iload_1   6:   iinc    1, 1   9:   iload_1   10:  iadd   11:  istore_1   12:  getstatic       #16; //Field java/lang/System.out:Ljava/io/PrintStream;   15:  iload_1   16:  invokevirtual   #22; //Method java/io/PrintStream.println:(I)V   19:  return}public class Inc {public static void main(String[] args) {int j = 0;j = ++j + j++;System.out.println(j);}}//2D:\Eclipse\eclipse-3.5.0\test\bin>javap -c IncCompiled from "Inc.java"public class Inc extends java.lang.Object{public Inc();  Code:   0:   aload_0   1:   invokespecial   #8; //Method java/lang/Object."<init>":()V   4:   returnpublic static void main(java.lang.String[]);  Code:   0:   iconst_0   1:   istore_1   2:   iinc    1, 1   5:   iload_1   6:   iload_1   7:   iinc    1, 1   10:  iadd   11:  istore_1   12:  getstatic       #16; //Field java/lang/System.out:Ljava/io/PrintStream;   15:  iload_1   16:  invokevirtual   #22; //Method java/io/PrintStream.println:(I)V   19:  return}