The newCachedThreadPool() method will create a thread pool of unbounded size, allocating a new thread anytime one is required or all existing threads are busy. This is commonly used for pools that require executing many short-lived asynchronous tasks. For long-lived processes, usage of this executor is strongly discouraged, as it could grow to encompass a large number of threads over the application life cycle.

The newFixedThreadPool() takes a number of threads and allocates them all upon creation. As long as number of tasks is less than number of threads, all tasks will be executed concurrently. If at any point the number of tasks exceeds the number of threads in the pool, they will wait in similar manner as it is with a single-thread executor. In fact, calling newFixedThreadPool() with a value of 1 is equivalent to calling newSingleThreadExecutor().

The newScheduledThreadPool() is identical to the newFixedThreadPool() method, except that it returns an instance of ScheduledExecutorService and is therefore compatible with scheduling tasks. This executor has subtle differences in the way that the scheduledAtFixedRate performs. Whereas with a single-thread executor an endless set of tasks would be scheduled over time, with a pool executor this can be avoided. If the pool size is sufficiently large, 10 for example, then as each thread finishes, it is returned to the pool and results in new threads available for the next tasks as they come up.