In Java, an atomic variable is a variable that can be read and modified atomically, meaning that the variable can be updated in a thread-safe manner without the need for explicit synchronization. Atomic variables are useful in multithreaded programming because they provide a way to perform operations on shared data without the risk of race conditions and other synchronization issues.

Java provides a number of classes for atomic variables in the java.util.concurrent.atomic package, including:

• AtomicInteger: a class that provides atomic operations on int values
• AtomicLong: a class that provides atomic operations on long values
• AtomicBoolean: a class that provides atomic operations on boolean values
• AtomicReference: a class that provides atomic operations on object references
• AtomicLongArray: a class that provides atomic operations on arrays of long values

Atomic variables are implemented using low-level synchronization primitives such as compare-and-swap (CAS) operations, which allow the variable to be updated atomically without the need for explicit synchronization. CAS operations work by comparing the current value of the atomic variable with an expected value, and if they match, updating the variable to a new value.

Here's an example of using AtomicInteger to increment a counter in a multithreaded program:

import java.util.concurrent.atomic.AtomicInteger;

public class AtomicCounter {
    private AtomicInteger count = new AtomicInteger(0);

    public void increment() {
        count.incrementAndGet();
    }

    public int getCount() {
        return count.get();
    }
}

In this example, we define an AtomicCounter class that uses an AtomicInteger to implement a thread-safe counter. The increment method uses the incrementAndGet method of AtomicInteger to atomically increment the counter, and the getCount method uses the get method of AtomicInteger to retrieve the current value of the counter.

One important thing to note about atomic variables is that while they can provide thread safety for simple operations like incrementing a counter, they may not be sufficient for more complex operations that involve multiple steps. In these cases, explicit synchronization or the use of other concurrent data structures may be necessary to ensure thread safety.

Overall, atomic variables are a powerful tool for ensuring thread safety in multithreaded Java programs, and their use can help to avoid synchronization issues and race conditions that can arise when multiple threads access shared data.