What is CompletableFuture.join() in Java?

Overview

join() is an instance method of the CompletableFuture class. It is used to return the value when the future is complete or throws an unchecked exception if completed exceptionally. If the task involved in the completion of the CompletableFuture raises an exception, then this method throws a CompletionException with the underlying exception as its cause.

The join method is defined in the CompletableFuture class. The CompletableFuture class is defined in the java.util.concurrent package. To import the CompletableFuture class, check the following import statement.

import java.util.concurrent.CompletableFuture;

import java.util.concurrent.*;
public class Main {
    static void sleep(int millis){
        try {
            Thread.sleep(millis);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
    public static void main(String[] args){
        CompletableFuture<String> completableFuture = CompletableFuture.supplyAsync(() -> {
            sleep(1000);
            System.out.println("Thread execution - " + Thread.currentThread().getName());
            return "Hello-Educative";
        });
        String resultValue = completableFuture.join();
        System.out.println("Result - " + resultValue);
    }
}

What is CompletableFuture.join() in Java?

Overview

Syntax

Parameters

Return value

Example

Explanation