Question

The output of this code is:

c: 1 , id: 0

I want to know why the output is not:

c: 0 , id: 1

class A {
    private static long c;
    private final long id = c++;
    public A() {
       print("c: " + c + " , id: " + id);
    }
} 

Answer 1

Because an expression like c++ operates like this:

• Evaluate c; remember this (e.g. tmp = c😉
• Increment c
• The result of the expression is the original value (tmp)

So in this case, you start off with c = 0, so id becomes 0… but c is incremented in the process, so c is 1 after the expression has been evaluated.

See section 15.14.2 of the Java Language Specification for more details. Extract:

At run-time, if evaluation of the operand expression completes abruptly, then the postfix increment expression completes abruptly for the same reason and no incrementation occurs. Otherwise, the value 1 is added to the value of the variable and the sum is stored back into the variable. Before the addition, binary numeric promotion (§5.6.2) is performed on the value 1 and the value of the variable. If necessary, the sum is narrowed by a narrowing primitive conversion (§5.1.3) and/or subjected to boxing conversion (§5.1.7) to the type of the variable before it is stored. The value of the postfix increment expression is the value of the variable before the new value is stored.