k=k++; // please check my thinking?

[peter greaves]

hi all
been wondering about this over the weekend.
int k=6;
k = k++;
System.out.println(k);
prints
6
not 7. i wonder if someone could clarify? i can see the postfix operator is lower precedence than = so the psotix operation happens after the assignment which basically means k = k. what i am not sure about is why the the value of the expression (k++) == 7 is discarded. IOW what is "special" about the postfix operator in the expression k=k++ that means that k is not incremented? this code
int k = 6;
k++;
System.out.println(k);
prints
7
so clearly the k= part is involved. my guess is that the postfix is being discarded but i can't quite see why.

[Valentin Crettaz]

Please check out the following discussions about the same topic:
https://coderanch.com/t/190825/java-programmer-SCJP/certification/Array
https://coderanch.com/t/239576/java-programmer-SCJP/certification/result

[Jonathas Carrijo]

Originally posted by peter greaves:

int k=6;
k = k++;
System.out.println(k);
prints
6

Hi,
k++ returns the current value of k and then increments it.
++k increments k and then returns the incremented value.
When u write k = k++; things happen in the following sequence:
1 - the current value of k is returned (6).
2 - k is incremented, and its value becomes 7.
3 - the assignment is executed, and the value returned by the expression (6) becomes k's value.
Thus, when u print out the value of k, you get a 6, not a 7!!
Regards,
Jonathas d-_-b

[James Chegwidden]

Could not have said it any better.

[Dan Chisholm]

Originally posted by Jonathas Carrijo:

When u write k = k++; things happen in the following sequence:
1 - the current value of k is returned (6).
2 - k is incremented, and its value becomes 7.
3 - the assignment is executed, and the value returned by the expression (6) becomes k's value.
Thus, when u print out the value of k, you get a 6, not a 7!!

The above sequence is nearly correct, but I would like to suggest a minor modification.

The expression, k++, is evaluated and the result, 6, is pushed onto the stack.

The value of k is incremented to 7.

The previous value of k, 6, is popped from the stack and returned as the result of the expression.

The returned value of the postfix expression, 6, is assigned to k.

[Jonathas Carrijo]

Thanks, Dan!