Asynchronous execution is a programming concept that allows a program to perform multiple tasks simultaneously. In Java, CompletableFuture and ExecutorService are two popular classes that allow asynchronous execution of code.

CompletableFuture is a class introduced in Java 8 that represents a future result of an asynchronous computation. It can be used to perform an operation asynchronously and to obtain its result when it’s available. The CompletableFuture class provides a wide range of methods that enable you to chain multiple computations and handle exceptions. For example, you can use the thenApply method to perform a transformation on the result of a CompletableFuture, or the exceptionally method to handle any exceptions that occur during the computation.

ExecutorService is an interface that represents an object that manages worker threads. It provides a way to submit a task for execution and returns a Future object that can be used to track the status of the task. ExecutorService allows you to create and manage threads, so you can execute code concurrently. You can use ExecutorService to create thread pools that can be used to manage the execution of multiple tasks. This is useful when you have multiple tasks to execute, and you want to limit the number of threads that are created.