Increment operator doesn't show expected result

Question

Let's say we have following

Integer a = 0;
a = a++;
System.debug(a)

why it shows 0?

Answer 1

Incrementing modifies in place. You don't need to reassign at all. However, using the ++a operator returns the value after increment, as opposed to the a++ operator, which returns the value before increment.

Answer 2

Speaking from a technical point of view, with the postfix operator, the compiler basically does this internally:

temp = a; // rvalue storage
a = a + 1; // ++ operator
a = temp; // lvalue assignment

As Adrian says, you shouldn't assign the value, as the prefix and postfix ++ and -- operators modify the variable being referenced.

Using a prefix operator (a = ++a) would change it to:

a = a + 1; // ++ operator
temp = a; // rvalue storage
a = temp; // lvalue assignment

You should generally not use a prefix or postfix operator on the variable being assigned to (the lvalue); in practice, the increment and decrement operators are typically used to modify an accumulator or counter, not modify the value currently being assigned to.