自增，自减的概念

//++ --   自增， 自减  一元运算符
int a = 3;

int b = a++;  //执行完这行代码后，先给b赋值，再自增
//a = a + 1;
System.out.println(a);   //a = 4
System.out.println(b);   //b = 3

System.out.println("//==============================");

int c = 3;
int d = ++c;  //执行这行代码前，先自增，再给d赋值
System.out.println(c);   //c = 4
System.out.println(d);   //d = 4