The difference between n++ VS ++n in Java

Question

My Java teacher said it was better to use ++n instead of n++, I am not seeing the logic behind this. Does anyone know?

Answer 1

++n adds one to n, then uses it; n++ uses it then adds one. Same applies to --.

Answer 2

++n increments the value and returns the new one.

n++ increments the value and returns the old one.

Thus, n++ requires extra storage, as it has to keep track of the old value so it can return it after doing the increment.

I would expect the actual difference between these two to be negligible these days. I know a lot of compilers will optimize it so they're identical if the return of n++ isn't actually used, though I don't know of Java does that.

Answer 3

Your Java teacher is (probably) refering to the fact that preincrementation is usually a little tiny bit faster than postincrementation. I'm not even sure if that's the case in Java, because I learned about that in C course.

"We should forget about small efficiencies, say about 97% of the time: premature optimization is the root of all evil"

Donald Knuth

In everyday pracitice, I would use pre- or postincrementation basing mainly on what makes the most sense in the code.

Answer 4

They do different things.

Let's say n = 10. Then n++ increments n but returns 10.

++n increments n and returns 11.

Though I'm not sure which one requires more assembly instructions.

Answer 5

Interesting example that illustrates the difference:

public class Main {
    public static void main(String[] args) {
        for (int i=0; i<5; /*not incrementing here*/ ) 
            System.out.println(i++);
    }
}

output: 0 1 2 3 4

public class Main {
    public static void main(String[] args) {
        for (int i=0; i<5; /*not incrementing here*/ ) 
            System.out.println(++i);
    }
}

output: 1 2 3 4 5

Notice that neither for-loop performs the incrementation since it does not matter there.

Answer 6

n++ will increment the variable prior to the operation taking place, while ++n only increments the variable after the operation.

So take x = 1
        y = x++
        y = 1
        x = 2 now

So take x = 1 again
        y = ++x
        y = 2
        x = 2 now