CompletableFuturecompletedFuturedone

CompletableFuture.completedFuture is a static factory method in the Java API for CompletableFuture. It returns a new CompletableFuture that is already completed with the given non-null value, allowing immediate use in asynchronous pipelines without creating a new asynchronous task.

Behavior and semantics: The returned future is in a completed state from the moment of creation. If you attach dependent stages such as thenApply, thenAccept, or whenComplete, those actions may execute immediately in the calling thread, since there is no pending computation to perform. If you need asynchronous execution for the subsequent stages, you can use the asynchronous variants like thenApplyAsync or thenAcceptAsync, potentially with a specified executor. The method requires a non-null value; passing null typically results in a NullPointerException.

Usage and examples: A common pattern is to adapt APIs that return futures or to provide a ready-made result in a pipeline. For instance, you can write: CompletableFuture<String> f = CompletableFuture.completedFuture("ok"); and then chain further processing that will run immediately if invoked on this already-completed future.

Null handling and related methods: Because the value must be non-null, care should be taken to ensure a valid result is supplied. Related concepts include completing a future exceptionally (completeExceptionally) and, in newer Java versions, failedFuture for representing an already-failed computation. Completed futures are useful for testing, stubbing, or providing deterministic values within a broader asynchronous workflow.

Overall, completedFuture offers a simple way to produce a ready, immutable result that participates in CompletableFuture-based pipelines without initiating new asynchronous work.