CompletableFuture and Asynchronous Programming in Java

In today's world of modern application development, it is crucial to utilize multi-threading and asynchronous programming to achieve high-performance and responsive applications. Java, being one of the most widely used programming languages, offers a powerful solution for asynchronous programming with its CompletableFuture class.

What is CompletableFuture?

CompletableFuture is a class introduced in Java 8 as a part of the java.util.concurrent package. It provides a way to perform computations asynchronously and handle their results once they become available. The CompletableFuture class is designed to make asynchronous programming easier by providing built-in support for handling dependencies, error handling, and composition of multiple asynchronous tasks.

Creating CompletableFuture Instances

To create an instance of CompletableFuture, you can use the CompletableFuture.supplyAsync() method. This method takes a Supplier functional interface, which represents the computation that will be executed asynchronously. Here's an example:

CompletableFuture<String> future = CompletableFuture.supplyAsync(() -> {
    // Perform some computation asynchronously
    return "Result";

Chaining CompletableFuture Tasks

One of the key features of CompletableFuture is the ability to chain multiple asynchronous tasks together. This is achieved using the thenApply() method, which allows you to apply a function on the result of a previous computation. Here's an example:

CompletableFuture<String> future = CompletableFuture.supplyAsync(() -> "Hello")
    .thenApply(result -> result + " World")
    .thenApply(result -> result + "!");

In this example, the first CompletableFuture supplies the string "Hello". Then, the thenApply() method is called to append " World" to the result. Finally, another thenApply() method is called to add an exclamation mark to the resulting string. Each thenApply() call returns a new CompletableFuture instance, allowing you to chain multiple tasks together.

Combining Multiple CompletableFutures

CompletableFuture also provides methods to combine the results of multiple asynchronous tasks. The thenCombine() method allows you to combine the results of two CompletableFutures using a BiFunction. Here's an example:

CompletableFuture<String> oneFuture = CompletableFuture.supplyAsync(() -> "Hello");
CompletableFuture<String> twoFuture = CompletableFuture.supplyAsync(() -> "World");

CompletableFuture<String> combinedFuture = oneFuture.thenCombine(twoFuture, (result1, result2) -> result1 + " " + result2);

In this example, the thenCombine() method combines the results of the two CompletableFutures into a single result string.

Exception Handling in CompletableFuture

CompletableFuture provides various methods to handle exceptions that occur during the execution of asynchronous tasks. For example, you can use the exceptionally() method to handle exceptions and provide a fallback value. Here's an example:

CompletableFuture<String> future = CompletableFuture.supplyAsync(() -> {
    // Perform some computation asynchronously
    throw new IllegalStateException("An error occurred");
}).exceptionally(ex -> "Fallback Value");

In this example, if an exception occurs during the computation, the exceptionally() method is called, and the provided fallback value is used instead.

Waiting for CompletableFuture to Complete

To wait for a CompletableFuture to complete and retrieve its result, you can use the get() method. However, calling the get() method blocks the current thread until the CompletableFuture completes, which may not be desirable in all cases. Alternatively, you can use the join() method, which has similar behavior but doesn't throw checked exceptions. Additionally, you can also use various other methods like thenAccept(), thenRun(), and handle() to perform further actions once the CompletableFuture completes without blocking the current thread.


CompletableFuture in Java provides a powerful and convenient way to perform asynchronous programming and handle dependencies between multiple tasks. Its rich set of methods enables you to achieve complex asynchronous workflows with ease. By leveraging CompletableFuture, you can optimize the performance and responsiveness of your applications, making them more scalable and efficient.

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