How can I change Integer value when it is an argument like change array's value? [duplicate]

Question

public static void main(String[] args) {
    Integer i = new Integer(0);
    int[] arr = {1};
    p1(i);
    p2(arr);
    System.out.println(i);
    System.out.println(arr[0]);
}


public static void p1(Integer i) {
    i = 2;
}

public static void p2(int[] i) {
    i[0] = 2;
}

//output: 0, 2

How can I change the value of i like I change the value of arr?

Answer 1

You can't change the value of the variable i in main from within the p1 method, because the argument is passed by value: the parameter i in p1 is entirely separate from the i variable, it's just that they have the same value at the start of the method. Java always uses pass-by-value semantics - but when the parameter type is a class, it's a reference that is passed by value.

In fact, you're not changing the value of arr, either - it's a reference to the same array as before, but the value in the array has been changed. And that's what you can't do with Integer, because Integer is an immutable type.

If you want a mutable class like Integer, you could use AtomicInteger instead:

public static void main(String[] args) {
    AtomicInteger i = new AtomicInteger(0);
    modify(i);
    System.out.println(i);
}

private static void modify(AtomicInteger x) {
    x.set(2);
}

I would usually not do this, however - I usually try not to modify the objects that method parameters refer to. Instead, I write methods which compute a single result, and return that.