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Java / Java Concurrency and Multi Threading in Java 17 and Java 21 Interview questions

Why should you use CompletableFuture instead of Future?

A plain Future only lets you check whether a task is done (isDone()) or block until it finishes (get()). There's no way to attach a callback, react to completion asynchronously, or combine it with another Future.

CompletableFuture, added in Java 8, implements both Future and CompletionStage, adding non-blocking composition.

CompletableFuture.supplyAsync(() -> fetchUser())
    .thenApply(User::getName)
    .thenCombine(fetchPrefsAsync(), (name, prefs) -> render(name, prefs))
    .exceptionally(ex -> fallbackView())
    .thenAccept(view -> display(view));

Methods like thenApply, thenCompose, and thenCombine let you chain dependent work without blocking a thread waiting on get(), exceptionally/handle give you structured error handling in the chain, and it can also be completed manually with complete(), which a plain Future cannot be.

In short, Future is a passive handle to a result; CompletableFuture is an active, composable pipeline for asynchronous work.

What can CompletableFuture do that a plain Future cannot?
Which method lets you supply a fallback value if a CompletableFuture completes exceptionally?

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