Java 8+ CompletableFuture example with error handling

CompletableFutureGreeting.java

CompletableFuture.supplyAsync(()-> {
    try {
        Thread.sleep(5000);
    } catch (InterruptedException e) {
        throw new RuntimeException("Error sleeping", e);
    }
    if (System.currentTimeMillis()%2==0) {
        throw new RuntimeException("Even time...");     // 50% chance to fail
    }
    return "Hello World!";
})
.thenAcceptAsync(s-> {
    System.out.println("Result: " + s);
})
.exceptionally(e-> {
    System.err.println("Error greeting: " + e.getMessage());
    return null;
});

well done

@mrsarm is it possible to throw the exception to the caller?

@gaurav9822 no, because it's async code, but if you are building a "service" class and you want the caller to take care of the errors, you should remove the exceptionally block and maybe the thenAcceptAsync block, and return the CompletableFuture object returned by supplyAsync, so once the caller call your service method, it can handle the exception. Eg.

public CompletableFuture getGreeting() {
    return CompletableFuture.supplyAsync(()-> "Hello World");
}

Then in the caller:

getGreeting().
    .thenAcceptAsync(s-> {
        // Do whatever needs to do the caller with the result s
        System.out.println("Result: " + s);
    })
    .exceptionally(e-> {
        // Handle the exeption, the caller way...
        System.err.println("Error greeting: " + e.getMessage());
        return null;
    });