Best Way to Swap Values in Java

Though not all Java applications require variables to be swapped, but those which do require are not given proper thoughts about correct and optimized implementation.

Method-1: Using temporary Variable

The most common way to swap two values is by using a temporary variable as shown below:

int a=10,b=5,c;
c=a; (c=10,a=10,b=5)
a=b; (c=10,a=5,b=5)
b=c; (c=10,a=5,b=10)

If the use of temporary variable c is not allowed, then we have the following two options:


Method-2.1 : Using the addition and subtraction operations

int a=10,b=5;
a=a+b;
b=a-b;
a=a-b

The problem with the above solution is that the addition and subtraction can cause overflows. An overflow is a condition when the value of a variable is increased/decreased beyond the maximum/minimum value of that variable. Though the output of such a program may show that the variables have been swapped but it is not guaranteed as per Specification of various programming languages including Java.

Example Code:

package example.java;
class Test{
    public static void main (String args[]) {
            int a=1999999999,b=1899999999;
            a = a + b;
            System.out.println("After operation 1, a = " + a +
                 " b = " + b);
            b = a - b;
            System.out.println("After operation 2, a = " + a +
                 " b = " + b);
            a = a - b;
            System.out.println("After operation 3, a = " + a +
                 " b = " + b);
   }
}

Output:

After operation 1, a =  -394967298 b = 1899999999

After operation 2, a =  -394967298 b = 1999999999

After operation 3, a =  1899999999 b = 1999999999

Here we can see that the overflow had occurred after first arithmetic operation and hence the behaviour after that is unpredictable.

Method-2.2 : Using the addition and subtraction operations

Here is a way out avoid the arithmetic overflow while swapping variables using arithmetic operations.

Example Code:

package example.java;
class Test{
    public static void main (String args[]) {
            int a=1999999999,b=1899999999;
            a = a - b;
            System.out.println("After operation 1, a = " + a +
                 " b = " + b);
            b = a + b;
            System.out.println("After operation 2, a = " + a +
                 " b = " + b);
            a = b - a;
            System.out.println("After operation 3, a = " + a +
                 " b = " + b);
   }
}

Output:

After operation 1, a =  100000000 b = 1899999999

After operation 2, a =  100000000 b = 1999999999

After operation 3, a =  1899999999 b = 1999999999

The operation 3 in the above program should be noticed that it is a = b – a and not a = a - b

Method-3: Using the XOR operator

The XOR operator can also be used swap two variables using the logic shown below:

int a=10,b=5;
a=a^b;
b=a^b;
a=a^b

A program to justify the above solution is:

Example Code:

package example.java;
class Test{
    public static void main (String args[]) {
            int a=1999999999,b=1899999999;
            a = a ^ b;
            System.out.println("After operation 1, a = " + a +
                 " b = " + b);
            b = a ^ b;
            System.out.println("After operation 2, a = " + a +
                 " b = " + b);
            a = a ^ b;
            System.out.println("After operation 3, a = " + a +
                 " b = " + b);
   }
}

Output:

After operation 1, a =  101327104 b = 1899999999

After operation 2, a =  101327104 b = 1999999999

After operation 3, a =  1899999999 b = 1999999999