CompletableFuture in Java

CompletableFuture is Java’s non-blocking Future that can be completed explicitly and chained with callbacks, transformations, and combinations.

Q: What is CompletableFuture?
A: A java.util.concurrent.CompletableFuture that implements Future and CompletionStage and supports async pipelines without blocking.

Q: Which package is it in?
A: java.util.concurrent, added in Java 8.

Q: How does it differ from Future?
A: Future only supports blocking get(); CompletableFuture adds completion, callbacks, chaining, and combining.

Q: How do you create an already completed one?
A: CompletableFuture.completedFuture(value) or CompletableFuture.failedFuture(ex) (Java 9+).

Q: How do you run a task asynchronously?
A: CompletableFuture.supplyAsync(supplier) for a result, or runAsync(runnable) for no result.

Q: Which thread pool does supplyAsync use by default?
A: The common ForkJoinPool, unless you pass an Executor.

Q: How do you transform a result?
A: thenApply for sync mapping, thenApplyAsync to run the mapping on another thread.

Q: How do you consume a result without returning a value?
A: thenAccept or thenAcceptAsync.

Q: How do you run something after completion regardless of the result?
A: thenRun or thenRunAsync.

Q: How do you handle exceptions?
A: exceptionally to recover, handle to see both result and exception, or whenComplete for a side effect.

Q: How do you chain dependent async calls?
A: thenCompose (flatMap-style), not thenApply, when the next step itself returns a CompletableFuture.

Q: How do you combine two independent futures?
A: thenCombine for both results, thenAcceptBoth to consume both, or runAfterBoth when both finish.

Q: How do you take the first completed of two?
A: applyToEither, acceptEither, or runAfterEither.

Q: How do you wait for many futures?
A: CompletableFuture.allOf(...) for all, or anyOf(...) for the first.

Q: How do you complete it manually?
A: complete(value), completeExceptionally(ex), or obtrudeValue / obtrudeException to force overwrite.

Q: Is get() still blocking?
A: Yes. Prefer callbacks or join() only when you intentionally want to block; join() throws unchecked exceptions.

Q: Can it be cancelled?
A: Yes. cancel(mayInterruptIfRunning) completes it exceptionally with CancellationException if not already done.

Q: Why did Spring move from ListenableFuture to it?
A: CompletableFuture is the JDK standard and already covers callbacks, chaining, and error handling that ListenableFuture added.