Learning CompleteableFuture

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ComputableFuture Class

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Asynchronous Computation in Java

Computation, in general, is based on a series of steps.  
Asynchronous Computation: actions represented as callbacks tend to be either scattered acrouss the code or deeply nested inside each other.  

Using ComputableFuture as a Simple Future

ComputableFuture class implements Future interface.

Ex: 
    we can create an instance of this class with a no-arg constructor to represent some future result, hand it out to the consumers, and complete it at some time in the future using the *complete* method.  
    The consumer may use the  *get* method to block the current thread until this result is provided. 

Below we have a method that creates a CompletableFuture instance, then spins off some computation in another thread and returns the Future immediately.

Ex: 
    public Future<String> calculateAsync() throws InterruptedException {
            CompletableFuture<String> completableFuture = new CompletableFuture<>();

            Executors.newCachedThreadPool().submit(() -> {
                Thread.sleep(500);
                completableFuture.complete("Hello");
                return null;
        });

        return completableFuture;
    }

To spin off the computation, we use the Executor API. This method of creating and completing a CompletableFuture can be used together with any concurrency mechanism or API, including raw threads.

Note: The calculateAsync method returns a Future instance.

We simply call the method, receive the Future instance, and call the get method on it when we’re ready to block for the result.

Also observe that the get method throws some checked exceptions , namely ExecutionException(encapsulating an exception that occurred during a computation)and InterruptedException(an exception signifying that thread executing a method was interrupted):

Ex:
    Future<String> completableFuture = calculateAsync();

        // ...

    String result = completableFuture.get();
    assertEquals("Hello", result);

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Source: Guide To CompletableFuture