Confusion with increment operator

Question

public class F {
    int test(int e) {
        System.out.println("ok");
        return e;
    }

    public static void main(String[] args) {
        int y = 8;
        F f = new F();
        int i = f.test(y++);
        System.out.println(i);
    }
}

the output of this program is 8, which is what I expect.

public class Sa {
    public static void main(String[] args) {
        int i = 8;
        i++;
        System.out.println(i);
    }
}

For this program, the output is 9, which is surprising: why we are getting different values using the same values and the same increment operator in both programs?

You can do the following: 1) try to use a breakpoint to debug; 2) google; 3) make your question better looking. The current post shows virtually none effort. — zw324, Jun 10 '13 at 15:29

Nathan Hughes · Accepted Answer · 2013-06-10T15:33:42.313

5

y++ post-increments. That means it increments after the expression is evaluated.

When you run

i=f.test(y++)

then the value passed into the test method is the one before the increment took place.

In your other code sample i++ is evaluated by itself so the increment takes place before the println.

Change the code in your first sample to ++y and you should get 9.

edited Jun 10 '13 at 15:33

answered Jun 10 '13 at 15:27

Nathan Hughes

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score 1 · Answer 2 · answered Jun 10 '13 at 15:28

i++ is a postincrement operator, which means that it evaluates to the current value of i and then increments after use.

I would expect

int i = 8
System.out.println(i++);
System.out.println(i);

would print 8 then 9.

You might have meant ++i which is preincrement

score 0 · Answer 3 · answered Jun 10 '13 at 15:28

0

y++ is post-increment.

It pass the value and then increments.So you are getting the previous value before incrementing.

In second case you are printing the incremented value.

answered Jun 10 '13 at 15:28

Suresh Atta

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score 0 · Answer 4 · answered Jun 10 '13 at 15:38

y++ in the

i=f.test(y++)

is evaluated after the test() method is run. So, the test gets the value 8 and prints 8.

Incrementing it in the Sa class is equal to:

i++; //     is equal to i+=1     OR i = i + 1;

But, that is not why we use the incrementation (++). The purpose of it is the side-effect that it has in an expression.

Exactly in your example, you want to pass in the 8, but increment it to 9 after the test has been executed.

Confusion with increment operator

4 Answers4