Why does post-increment work on wrapper classes

Question

I was doing a review of some code and came across an instance of someone post-incrementing a member variable that was a wrapper class around Integer. I tried it myself and was genuinely surprised that it works.

Integer x = 0; 
System.out.print(x++ + ", ");
System.out.print(x);

This prints out 0, 1, not 0, 0 as I would have expected. I've looked through the language specification and can't find anything covering this. Can anyone explain to me why this works and if it's safe across multiple platforms? I would have thought that this would decompose into

Integer x = 0;
int temp1 = x.intValue();
int temp2 = temp1 + 1;
System.out.println(temp1);
temp1 = temp2;
System.out.println(x.intValue());

But apparently there's something in the specification that make it add x = temp1; before the last line

Answer 1

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.