post increment operator java

Question

I can't make heads or tails of the following code from "java puzzlers" by joshua bloch.

public class Test22{ public static void main(String args[]){ int j=0; for(int i=0;i<100;i++){ j=j++; } System.out.println(j); //prints 0 int a=0,b=0; a=b++; System.out.println(a); System.out.println(b); //prints 1 } }

I can't get the part where j prints 0. According to the author,

j=j++

is similar to

temp=j; j=j+1; j=temp;

But

a=b++

makes b 1. So it should've evaluated like this,

a=b b=b+1

By following the same logic, shouldn't

j=j++

be evaluated as,

j=j j=j+1

Where does the temp come into picture here? Any explanations would be much appreciated. << I'm breaking my head over this. ;)>> Thanks in advance.

polygenelubricants · Accepted Answer · 2010-05-01 14:58:49Z

Let's break down your own argument:

According to the author,
j=j++; 
is similar to
temp=j; j=j+1; // increment j=temp; // then assign 

Yes, you're right so far..., but here's where you got it wrong:

But
a=b++; 
makes b=1. So it should've evaluated like this,
a=b; // assign b=b+1; // then increment 

WRONG! You're not applying the rule consistently! You've changed the order from increment then assign to assign then increment!!! It's actually evaluated like this:

temp=b; b=b+1; // increment a=temp; // then assign

Basically assignments of this form:

lhs = rhs++;

is similar to doing something like this:

temp = rhs; rhs = rhs+1; // increment lhs = temp; // then assign

Apply this to a = b++;. Then apply it also to j = j++;. That's why you get the results that you get.

What you did was you came up with your own interpretation of what a = b++; does -- a WRONG interpretation that doesn't follow the above rule. That's the source of your confusion.

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