Difference between b=b++ and b++ [duplicate]

Question

This question already has an answer here:

• What is x after “x = x++”? 17 answers

I was asked in an interview the below question.

int b = 0;
b = b++;
b = b++;
b = b++;
b = b++;

what will be the value of b after every line execution ? The output is coming as 0 for every line.

Why is the output not coming as 0,1,2,3 ?

Answer 1

In Java, the expression

b = b++

is equivalent to

int tmp = b;
b = b + 1;
b = tmp;

Hence the result.

(In some other languages, the exact same expression has unspecified behaviour. See Undefined behavior and sequence points.)

Answer 2

Hint:

int b = 0, c;
c = b++;
c = b++;
c = b++;
c = b++;
System.out.println(c);

c now will be 3 like you thought, but because in your question you're assigning b, it'll get 0, because as already explained, it's the same as:

int tmp = b;
b = b + 1;
b = tmp;

Answer 3

Because this is the order of execution of b = b++:

1. Get the value of b into some temp (probably into a bytecode register); this is the first part of b++, since it's a post-increment
2. Increment and store the incremented result in b (the second part of b++)
3. Assign the value from Step 1 to b (the result of the =)

Answer 4

b++ is the same as:

int temp = b;
b = b + 1;
return temp;

As you can see b++ will return his old value, but overwrites the value of b. But since you assign the returned (old) value to b, the new value will be overwritten and therefore "ignored".

A difference would be, if you write:

b = ++b; //exactly the same as just "++b"

This does the incrementation first and the new value will be returned.