Why does post-increment work on wrapper classes

Answer by Sir Tedd Hopp is at a very complex level for programmers who are programming only for few or couple of years.

Let me clear your doubt in simple way suppose

    Integer x=10;
    x++;
    System.out.println(x) ;

output will be 11

Because ++ either post or pre increment is doing Addition by 1 only internally

ie x+1 is what it has to perform and put back the result in the same variable.

ie x=x+1;

now we all know that + operator can only take primitives but x is object , then we have auto-unboxing and auto-boxing concept. so the expression becomes

x.intValue()+1;//step 1 auto-unboxing

x=Integer.valueOf(x.intValue()+1);//step 2 auto-boxing

Hence output comes as 11 after another step of auto-unboxing inside the println statement.

As of Java 1.5, Java performs auto-unboxing to convert "wrapper types" such as Integer to the corresponding primitive type int when necessary. Then the increment operator can work on the resulting int.

It's perfectly safe to use across platforms. The behavior is specified in §15.4.2 of the Java Language Specification (emphasis added):

The result of the postfix expression must be a variable of a type that is convertible (§5.1.8) to a numeric type, or a compile-time error occurs.

The type of the postfix increment expression is the type of the variable. The result of the postfix increment expression is not a variable, but a value.

At run-time, if evaluation of the operand expression completes abruptly, then the postfix increment expression completes abruptly for the same reason and no incrementation occurs. Otherwise, the value 1 is added to the value of the variable and the sum is stored back into the variable. Before the addition, binary numeric promotion (§5.6.2) is performed on the value 1 and the value of the variable. If necessary, the sum is narrowed by a narrowing primitive conversion (§5.1.3) and/or subjected to boxing conversion (§5.1.7) to the type of the variable before it is stored. The value of the postfix increment expression is the value of the variable before the new value is stored.

EDIT Here's a more accurate equivalent of what's going on in your example code:

Integer x = 0;
int temp = x.intValue();
x = temp + 1; // autoboxing!
System.out.println(temp + ", ");
System.out.println(x.intValue());