[JAVA] CompletableFuture Getting Started 2 (Try to make CompletableFuture)

Completable Future Start (Future) Continued


As shown in the figure, Completable Furure is It implements the Future and CompletionStage interfaces. CompletionStage is an interface introduced in Java 8.

CompletableFuture represents asynchronous processing and retains the result when it becomes available.

What is different from Future?


A Future that can be completed explicitly (by setting its value and status). Supports dependent functions and actions that occur upon completion and can be used as a Completion Stage. CompletableFuture@Oracle

This "support for dependent functions and actions that occur on completion" is a major feature of Completable Future.

In addition to the feature of Future," you will not have to wait for time-consuming processing " You can do the following:

When the result of the process becomes available You can issue notifications and execute callbacks defined by lambda expressions and method references.

Basic usage

public Future<Double> getDoubleAsync(String variable){
    //Create a Completable Future that holds the calculation results
    CompletableFuture<Double> futureValue = new CompletableFuture<>();
    new Thread( () -> { //Asynchronous execution of processing in another thread
        try {
       //Time-consuming process
            double price = doSomeLongComputation(variable);
       //Set the resulting value
        } catch (Exception ex) {
    //Returns Future without waiting for calculation
    return futureValue;

How to call the above method

//The return type is Future<Double>
Future<Double> futureValue = getDoubleAsync("test");
try {
    //Call here. Blocked if value is not available
    double value = futureValue.get();
    System.out.printf("value is %.2f%n", value);
} catch (ExecutionException | InterruptedException e) {
    throw new RuntimeException(e);

When using SupplyAsync

The CompletableFuture class has multiple factory methods.

supplyAsync(Supplier supplier) Returns a new CompletableFuture in which the task running in ForkJoinPool.commonPool () calls the specified supplier and completes asynchronously using the value obtained. supplyAsync@Oracke

public static Future<Double> getDoubleAsyncHandy(String arg) {
    return CompletableFuture.supplyAsync(() -> doSomeLongComputation(arg));

You can easily write CompletableFuture as above by using the supplyAsync method.

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