Java: Why does this swap method not work? [duplicate]

Answer 1

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 2

Using Scanner:

import java.util.*;
public class Swap {
 public static void main(String[] args){
  int i,temp,Num1,Num2;
  Scanner sc=new Scanner(System.in);
  System.out.println("Enter Number1 and Number2");
  Num1=sc.nextInt();
  Num2=sc.nextInt();
  System.out.println("Before Swapping Num1="+Num1+" Num2="+Num2);
  temp=Num1;
  Num1=Num2;
  Num2=temp;
  System.out.println("After Swapping Num1="+Num1+" Num2="+Num2);
 }    
}

Answer 3

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 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

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 6

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.