CompletableFuture

CompletableFuture is Java's answer to "I have a task that takes time, and I want to do something with its result without blocking a thread to wait for it." Where the older Future API forces you to call get() and stall until the work finishes, CompletableFuture lets you describe what should happen when the result arrives and walk away. This lesson covers what problem it solves, how to create one, how to transform and chain results, how to combine multiple futures, how errors travel through a pipeline, and the cost trade-offs of running callbacks on the common pool versus a custom executor.

We covered Callable and plain Future in 13-callable-future.md, so we'll only revisit them long enough to motivate why CompletableFuture exists. Virtual threads (the modern alternative for "just write blocking code") are the topic of 23-virtual-threads.md and are out of scope here.