Future is a placeholder for a result of an operation that hasn't finished yet. Once the operation finishes, the Future will contain that result.

Limitations of futures:

• they cannot be manually completed
• no further action on a Future's result without blocking
• multiple Futures cannot be chained or combined
• there is no exception handling construct

CompletableFuture was introduced to fix the limitations of Future.

CompletableFuture is a tool for asynchronous programming in Java. Asynchronous programming is writing non-blocking code by running a task on a separate thread than the main application thread and notifying the main thread about its progress, completion, or failure.

Callbacks can be attached to the CompletableFuture using thenApply(), thenAccept() and thenRun() methods.

The CompletableFuture.get() method is blocking. It waits until the Future is completed and returns the result after its completion.

The runAsync(Runnable runnable) returns a new CompletableFuture that is asynchronously completed by a task running in the ForkJoinPool.commonPool() after it runs the given action. (It execute tasks in a thread obtained from the global ForkJoinPool.commonPool())