A concurrency problem can begin with an ordinary requirement: let two independent operations make progress without forcing one to wait for the other. The requirement is easy to state. Choosing among Thread , ExecutorService , Future , CompletableFuture , locks, atomics, queues, and synchronizers is harder when they look like competing answers to the same question. These APIs answer different questions. Some describe work, some decide how it runs, some represent results, and others pr...