Java: Why does this swap method not work?

Question

Hi. I have the following code:

public class Main {

    static void swap (Integer x, Integer y) {
        Integer t = x;
        x = y;
        y = t;
    }

    public static void main(String[] args) {
       Integer a = 1;
       Integer b = 2;
       swap(a, b);
       System.out.println("a=" + a + " b=" + b);
    } 
}

I expect it to print a=2 b=1, but it prints the opposite. So obviously the swap method doesn't swap a and b values. Why?

Answer 1

This doesn't have anything to do with immutability of integers; it has to do with the fact that Java is Pass-by-Value, Dammit! (Not annoyed, just the title of the article :p )

To sum up: You can't really make a swap method in Java. You just have to do the swap yourself, wherever you need it; which is just three lines of code anyways, so shouldn't be that much of a problem :)

    Thing tmp = a;
    a = b;
    b = tmp;

Answer 2

Everything in Java is passed by value and the values of variables are always primitives or references to object.

Answer 3

You would need to pass the parameters by reference, which it's not possible in java. Also Integers are inmutables, so you cannot exchange the values as you don't have a setValue method.

Answer 4

As Svish and others pointed out it it's call by value, not by reference in Java. Since you have no pointers in Java you need some kind of holder object to really swap values this way. For example:

static void swap(AtomicReference<Integer> a, AtomicReference<Integer> b) {

    Integer c = a.get();
    a.set(b.get());
    b.set(c);

}

public static void main(String[] args) {

    AtomicReference<Integer> a = new AtomicReference<Integer>(1);
    AtomicReference<Integer> b = new AtomicReference<Integer>(2);

    System.out.println("a = " + a);
    System.out.println("b = " + b);

    swap(a, b);

    System.out.println("a = " + a);
    System.out.println("b = " + b);

}

Answer 5

If you want to implement a swap method for Integer objects, you have to wrap the values into an array (or ArrayList) and swap inside the array. Here's an adaptation of your code:

public class Main {

    static void swap (Integer[] values) {
        if ((values == null) || (values.length != 2)) {
          throw new IllegalArgumentException("Requires an array with exact two values");
        }

        Integer t = values[0];
        values[0] = values[1];
        values[1] = t;
    }

    public static void main(String[] args) {
       Integer a = 1;
       Integer b = 2;
       Integer[] integers= new Integer[]{a,b};
       swap(integers);
       System.out.println("a=" + integers[0] + " b=" + integers[1]);
    } 
}

(Just added this answer because Svish mentioned, that "You can't really make a swap method in Java" *fg*)

Answer 6

As all the guys mentioned its a Pass-By-Value thing.

Just liked to add: you can use this method of swapping GLOBAL integers.

private void swap (){
     a ^= b;
     b ^= a;
     a ^= b;
}

It eliminates the use of another variable, and its just cooler :)

Answer 7

Well, it actually is possible using AtomicInteger (which is mutable).

import java.util.concurrent.atomic.AtomicInteger;

public class Test {
    private static void swap(AtomicInteger a, AtomicInteger b) {
        int val = a.get();
        a.set(b.get());
        b.set(val);
    }

    public static void main(String[] args) {
        AtomicInteger a = new AtomicInteger(1);
        AtomicInteger b = new AtomicInteger(2);

        swap(a, b);

        System.out.println("a: " + a + " b: " + b);
    }
}

Answer 8

Using the XOR operator is a very bad idea:

First, it is far less readable. Second, there were times when this was faster but nowadays the opposite is the case. See

Wikipedia

for reference.