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Differences between Future and CompletableFuture

Java Async


In the realm of asynchronous and concurrent programming in Java, Future and CompletableFuture serve as essential tools for managing and executing asynchronous tasks.

Both constructs offer ways to represent the result of an asynchronous computation, but they differ significantly in terms of functionality, flexibility, and ease of use. Understanding the distinctions between Future and CompletableFuture is crucial for Java developers aiming to design robust and efficient asynchronous systems.

At its core, a Future represents the result of an asynchronous computation that may or may not be complete. It allows developers to submit tasks for asynchronous execution and obtain a handle to retrieve the result at a later point. While Future provides a basic mechanism for asynchronous programming, its capabilities are somewhat limited in terms of composability, exception handling, and asynchronous workflow management.

On the other hand, CompletableFuture introduces a more advanced and versatile approach to asynchronous programming in Java. It extends the capabilities of Future by offering a fluent API for composing, combining, and handling asynchronous tasks with greater flexibility and control. CompletableFuture empowers developers to construct complex asynchronous workflows, handle exceptions gracefully, and coordinate the execution of multiple tasks seamlessly.

In this article, we will dive deeper into the differences between Future and CompletableFuture, exploring their respective features, use cases, and best practices. By understanding the distinct advantages and trade-offs of each construct, developers can make informed decisions when designing asynchronous systems and leveraging concurrency in Java applications. Let's embark on a journey to explore the nuances of Future and CompletableFuture in the Java ecosystem.

Use Cases for Future

  1. Parallel Processing: Use Future to parallelize independent tasks across multiple threads and gather results asynchronously. For example, processing multiple files concurrently.

  2. Asynchronous IO: When performing IO operations that are blocking, such as reading from a file or making network requests, you can use Future to perform these operations in separate threads and continue with other tasks while waiting for IO completion.

  3. Task Execution and Coordination: Use Future to execute tasks asynchronously and coordinate their completion. For example, in a web server, handle multiple requests concurrently using futures for each request processing.

  4. Timeout Handling: You can set timeouts for Future tasks to avoid waiting indefinitely for completion. This is useful when dealing with resources with unpredictable response times.

Use Cases for CompletableFuture

  1. Async/Await Pattern: CompletableFuture supports a fluent API for chaining asynchronous operations, allowing you to express complex asynchronous workflows in a clear and concise manner, similar to the async/await pattern in other programming languages.

  2. Combining Results: Use CompletableFuture to combine the results of multiple asynchronous tasks, either by waiting for all tasks to complete (allOf) or by combining the results of two tasks (thenCombine, thenCompose).

  3. Exception Handling: CompletableFuture provides robust exception handling mechanisms, allowing you to handle exceptions thrown during asynchronous computations gracefully using methods like exceptionally or handle.

  4. Dependency Graphs: You can build complex dependency graphs of asynchronous tasks using CompletableFuture, where the completion of one task triggers the execution of another, allowing for fine-grained control over the execution flow.

  5. Non-blocking Callbacks: CompletableFuture allows you to attach callbacks that are executed upon completion of the future, enabling non-blocking handling of results or errors.

  6. Completing Future Manually: Unlike Future, you can complete a CompletableFuture manually using methods like complete, completeExceptionally, or cancel. This feature can be useful in scenarios where you want to provide a result or handle exceptional cases explicitly.


Creation and Completion

Future code example of creation and completion.

ExecutorService executor = Executors.newSingleThreadExecutor();
Future<Integer> future = executor.submit(() -> {
    return 10;

CompletableFuture code example of creation and completion.

CompletableFuture<Integer> completableFuture = CompletableFuture.supplyAsync(() -> {
    try {
    } catch (InterruptedException e) {
    return 10;

In CompletableFuture, supplyAsync method allows for asynchronous execution without the need for an external executor service an shown in the first example.

Chaining Actions

Example below in how to chain actions using Future.

Future<Integer> future = executor.submit(() -> 10);
Future<String> result = future.thenApply(i -> "Result: " + i);

Now, an example using CompletableFuture in how to chain actions.

CompletableFuture<Integer> completableFuture = CompletableFuture.supplyAsync(() -> 10);
CompletableFuture<String> result = completableFuture.thenApply(i -> "Result: " + i);

CompletableFuture offers a fluent API (thenApply, thenCompose, etc.) to chain actions, making it easier to express asynchronous workflows.

Exception Handling

Handling exception using Future

Future<Integer> future = executor.submit(() -> {
    throw new RuntimeException("Exception occurred");

Handling exception using CompletableFuture

CompletableFuture<Integer> completableFuture = CompletableFuture.supplyAsync(() -> {
    throw new RuntimeException("Exception occurred");

CompletableFuture allows for more flexible exception handling using methods like exceptionally or handle.

Waiting for Completion

// Future
Integer result = future.get();

// CompletableFuture
Integer result = completableFuture.get();

Both Future and CompletableFuture provide the get() method to wait for the completion of the computation and retrieve the result.

Combining Multiple CompletableFutures

CompletableFuture<Integer> future1 = CompletableFuture.supplyAsync(() -> 10);

CompletableFuture<Integer> future2 = CompletableFuture.supplyAsync(() -> 20);

CompletableFuture<Integer> combinedFuture = future1.thenCombine(future2, (x, y) -> x + y);

CompletableFuture provides methods like thenCombine, thenCompose, and allOf to perform combinations or compositions of multiple asynchronous tasks.


In the dynamic landscape of asynchronous and concurrent programming in Java, both Future and CompletableFuture stand as indispensable tools, offering distinct advantages and use cases. While Future provides a basic mechanism for representing the result of asynchronous computations, its capabilities are somewhat limited when it comes to composability, exception handling, and asynchronous workflow management. On the other hand, CompletableFuture emerges as a powerful and flexible alternative, extending the functionalities of Future with a fluent API for composing, combining, and handling asynchronous tasks with greater control and elegance.

The choice between Future and CompletableFuture hinges on the specific requirements and complexities of the task at hand. For simple asynchronous operations or when working within the confines of existing codebases, Future may suffice. However, in scenarios that demand more sophisticated asynchronous workflows, exception handling, or task coordination, CompletableFuture offers a compelling solution with its rich feature set and intuitive API.


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