What is CompletableFuture.isCompletedExceptionally() in Java?

isCompletedExceptionally() is an instance method of the CompletableFuture. It checks whether the future is completed with any exceptions. The possible reasons for a future to complete exceptionally are:

Any of the stages of the future get cancelled.
The completeExceptionally method is invoked explicitly.
There is an abrupt termination of any of the stages of the future.

import java.util.concurrent.*;
public class Main {
    public static void main(String[] args) throws ExecutionException, InterruptedException {
        CompletableFuture<Integer> completableFuture = new CompletableFuture<>();
        RuntimeException runtimeException = new RuntimeException("Runtime Exception");
        boolean flag = completableFuture.completeExceptionally(runtimeException);
        if(flag) System.out.println("Future moved to completed stage");
        if(completableFuture.isCompletedExceptionally()) System.out.println("Future completed exceptionally");
        Integer result = completableFuture.get();
        System.out.println("Result - " + result);
    }
}

Explanation

Line 1: We import the relevant classes.
Line 5: We create a completable future that is incomplete using the constructor of the CompletableFuture class.
Line 6: We define the exception to be thrown.
Line 7: Using the completeExceptionally() method, the future we create in line 5 is moved to the completed stage. This is done with the exception defined in line 6. The completeExceptionally() method returns a Boolean stored in the flag variable.
Line 8: Depending on the flag value, we print whether the future was moved to the completed stage or not.
Line 9: We check whether the future was completed exceptionally using isCompletedExceptionally().
Line 10: We retrieve the value of the future using the get() method. At this point, the exception defined in line 6 is thrown.
Line 11: We print the retrieved value.

What is CompletableFuture.isCompletedExceptionally() in Java?

Syntax

Parameters

Return value

Code

Explanation