CompletableFuture

CompletableFuture is a class in the Java standard library that represents a future result of an asynchronous computation. It implements the Future and CompletionStage interfaces, enabling both blocking retrieval of a result and non-blocking composition of dependent tasks. A CompletableFuture can be completed explicitly by code calling complete or completeExceptionally, or it can be completed automatically when a submitted asynchronous task finishes.

A key feature is its support for composing asynchronous stages without blocking threads. The API provides a rich set of methods for transforming, consuming, and combining results, such as thenApply, thenAccept, thenRun, thenCompose, and handle. These methods return new CompletableFuture instances, enabling fluent pipelines where each stage starts when its predecessor completes. Exceptions can be propagated and handled with exceptionally and handle.

Factory methods include supplyAsync and runAsync to start tasks asynchronously, as well as completedFuture to create an already-completed stage. Streams of futures can be coordinated with allOf and anyOf, which complete when their component futures complete. If no executor is supplied, asynchronous tasks default to the common ForkJoinPool, but a custom Executor can be provided to control threading.

CompletableFuture is designed to be thread-safe and reusable in concurrent programming. It serves as a bridge between traditional blocking futures and a more flexible, non-blocking style of asynchronous computation, allowing developers to express complex workflows as a pipeline of dependent stages rather than nested callbacks.