j++，是一个依赖于java里面的“中间缓存变量机制”来实现的，
通俗的说：
++在前就是“先加后赋”（++j）
++在后就是“先赋后加” (j++)
public class Test1 {
public static void main(String[] args) {
int i = 10;
int a = 0;
int b = 0;
int c = 0;
int d = 0;
// a = i++ + i++; //21
/*
* temp1 = i; 10
* i = i + 1; 11
* temp2 = i; 11
* i = i + 1; 12
* a = temp1 + temp2 = 21;
*/
// b = ++i + ++i; //23
/*
* i = i + 1; 11
* temp1 = i; 11
* i = i + 1; 12
* temp2 = i; 12
* b = temp1 + temp2 = 23;
*/
// c = ++i + i++; //22
/*
* i = i + 1; 11
* temp1 = i; 11
* temp2 = i 11
* i = i + 1; 12
* c = temp1 + temp2 = 22
*/
d = i++ + ++i; //22
/*
* temp1 = i; 10
* i = i + 1; 11
* i = i + 1; 12
* temp2 = i; 12
* d = temp1 + temp2 = 22;
*/
System.out.println(a);
System.out.println(b);
System.out.println(c);
System.out.println(d);
}
}