CallBackFuture.java

// Using a callback method
CompletableFuture<String> future4 = CompletableFuture.supplyAsync(() -> {
    System.out.println("Hello "+ Thread.currentThread().getName());
    return "Hello";
}).thenApply((s) -> { // thenApply, thenAccept or thenRun can also be used here
    System.out.println(Thread.currentThread().getName());
    return s.toUpperCase(Locale.ROOT);
});

System.out.println(future4.get());