AtomicReference compareAndExchange() method in Java with Examples

The compareAndExchange() method of a AtomicReference class is used to Atomically sets the value to newValue to AtomicReference object, if the current value of AtomicReference object which is referred to as the witness value is equal to the expectedValue.This method will return the witness value, which will be the same as the expected value. This method handles the operation with memory semantics of reading as if the variable was declared volatile.

Syntax:

public final V compareAndExchange(V expectedValue,
                                  V newValue)

Parameters: This method accepts expectedValue which is the expected value and newValue which is the new value to set.

Return value: This method returns the witness value, which will be the same as the expected value if successful.

Below programs illustrate the compareAndExchange() method:
Program 1:

// Java program to demonstrate
// AtomicReference.compareAndExchange() method

import java.util.concurrent.atomic.AtomicReference;

public class GFG {
    public static void main(String[] args)
    {

        // create an atomic reference object
        // which stores Integer.
        AtomicReference<Integer> ref
            = new AtomicReference<Integer>();

        // set some value
        ref.set(999);

        // apply compareAndExchange()
        Integer oldValue
            = ref.compareAndExchange(
                999,
                999999);

        // print value
        System.out.println("Witness Value= "
                           + oldValue);
    }
}

Output:

Program 2:

// Java program to demonstrate
// AtomicReference.compareAndExchange() method

import java.util.concurrent.atomic.AtomicReference;

public class GFG {
    public static void main(String[] args)
    {

        // create an atomic reference object.
        AtomicReference<String> ref
            // = new AtomicReference<String>();

            // set some value
            ref.set("GFG");

        // apply compareAndExchange()
        String oldValue
            = ref.compareAndExchange(
                "GFG",
                "Geeks for Geeks");

        // print value
        System.out.println("Witness Value= "
                           + oldValue);
    }
}

Output:

References: https://docs.oracle.com/javase/10/docs/api/java/util/concurrent/atomic/AtomicReference.html#compareAndExchange(V, V)