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1. Overview

A callback function is a function passed as an argument to another function and executed when that function completes or some event happens. In most programming languages, callback functions are especially useful when we're working with asynchronous code.

In this article, we'll learn the practical use cases of callback functions in Java and how we can implement them.

2. Implementing Callback Functions

Generally, we can create a callback function in Java by exposing an interface and accepting its implementation as a parameter. Such a callback can be called synchronously or asynchronously.

2.1. Synchronous Callbacks

Synchronous operations are those where one task needs to complete before another one starts.

For example, imagine such an interface:

public interface EventListener {

    String onTrigger();

The above snippet declares an EventListener interface with an onTrigger() method with a String return type. This will be our callback.

Next, let's declare a concrete class that implements this interface:

public class SynchronousEventListenerImpl implements EventListener {

    public String onTrigger(){
        return "Synchronously running callback function";

The SynchronousEventListenerImpl class implements the EventListener interface, as shown above.

Next, let's create a SynchronousEventConsumer class that composes an instance of the EventListener interface and invokes its onTrigger() method:

public class SynchronousEventConsumer {

    private final EventListener eventListener;

    // constructor

    public String doSynchronousOperation(){
        System.out.println("Performing callback before synchronous Task");
        // any other custom operations
           return eventListener.onTrigger();

The SyncronousEventConsumer class has an EventListener property that it initializes through its constructor. When the doSynchronousOperation() method is invoked, it returns the value obtained from the onTrigger() method belonging to the EventListener.

Let's write a test to demonstrate that the doSynchronousOperation() method invokes the onTrigger() method of the listener variable and obtains its returned value:

EventListener listener = new SynchronousEventListenerImpl();
SynchronousEventConsumer synchronousEventConsumer = new SynchronousEventConsumer(listener);
String result = synchronousEventConsumer.doSynchronousOperation();

assertEquals("Synchronously running callback function", result);

2.2. Asynchronous Callback Function

Asynchronous operations are operations that run in parallel to one another. Unlike synchronous operations illustrated in the previous section, asynchronous tasks are non-blocking. They don't wait for one another before performing their operations. Let's update the EventListener interface to illustrate an asynchronous callback function in Java:

public interface EventListener {

    String onTrigger();

    void respondToTrigger();

Next, let's create an implementation for the revised EventListener:

public class AsynchronousEventListenerImpl implements EventListener {

    public String onTrigger(){
        return "Asynchronously running callback function";
    public void respondToTrigger(){
        System.out.println("This is a side effect of the asynchronous trigger.");

The above class implements the EventListener interface we declared in the previous section and returns a String literal in its overridden onTrigger() method.

Next, we declare the class that asynchronously runs the onTrigger() method as a callback function:

public class AsynchronousEventConsumer{

    private EventListener listener;

    public AsynchronousEventConsumer(EventListener listener) {
        this.listener = listener;

    public void doAsynchronousOperation(){
        System.out.println("Performing operation in Asynchronous Task");

        new Thread(() -> listener.onTrigger()).start();

The AsynchronousEventConsumer class above declares a doAsynchronousOperation() method that implicitly invokes the onTrigger() method of the EventListener in a new thread.

Note that this approach of creating a new Thread for each method call is an anti-pattern and is used here for demonstration purposes. Production-ready code should rely on properly sized and tuned thread pools. Check out some of our other articles to learn more about concurrency in Java.

Let's verify that the program indeed invokes the onTrigger() method from within the doAsynchronousOperation() method:

EventListener listener = Mockito.mock(AsynchronousEventListenerImpl.class);
AsynchronousEventConsumer synchronousEventListenerConsumer = new AsynchronousEventConsumer(listener);

verify(listener, timeout(1000).times(1)).onTrigger();

2.3. Using Consumers

Consumers are functional interfaces that are commonly used in functional programming in Java. Implementations of the interface accept an argument and perform an operation with the provided argument but don't return a result.

Using Consumers, we can pass a method as an argument to another method. This allows us to invoke and run the operations of the inner method from within the parent method.

Let's consider a method that increases the value of a given number represented as age. We can pass the initial age as the first argument and a Consumer that will serve as the second method to increment the age.

Here's an illustration of how we can implement this as a callback function using Consumers:

public class ConsumerCallback {
    public void getAge(int initialAge, Consumer<Integer> callback) {

    public void increaseAge(int initialAge, int ageDifference, Consumer<Integer> callback) {
        System.out.println("===== Increase age ====");

        int newAge = initialAge + ageDifference;

In the getAge() method above, we pass the initialAge variable as an argument to the callback.accept() method. The accept() method takes an argument (in this case, an integer), and then performs any operation on the input through the method or function passed to the getAge() method as an argument at runtime.

The increaseAge() method will perform the increment on the initialAge variable. It adds the value of the initialAge to the ageDifference and then passes the result to the accept() method of the third argument, the Consumer.

Here's a demonstration of the above implementations:

ConsumerCallback consumerCallback = new ConsumerCallback();
int ageDifference = 10;
AtomicInteger newAge1 = new AtomicInteger();
int initialAge = 20;
consumerCallback.getAge(initialAge, (initialAge1) -> {
    consumerCallback.increaseAge(initialAge, ageDifference, (newAge) -> {
        System.out.printf("New age ==> %s", newAge);
assertEquals(initialAge + ageDifference, newAge1.get());

In the above snippet, we pass a function to the getAge() method. This function invokes the increaseAge() method and asserts that the value of the newAge variable equals the sum of the initialAge and ageDifference.

The callback functions in this context are the functions passed to the getAge() and increaseAge() methods. These functions are triggered to perform any custom operation after each of the getAge() and increaseAge() methods have completed their tasks.

3. Conclusion

In this article, we learned about the concept of callback functions in Java. We demonstrated how we could synchronously and asynchronously implement callback functions through interfaces. We also learned how to use the Java Consumer functional interface to perform callback operations in Java.

The code snippets provided in this article are available over on GitHub.

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