The CompletableFuture API consists of more than 70 methods, 38 of which are implementations of the CompletionStage interface, and five are the implementations of Future. Because the CompletableFuture class implements the Future interface, it can be treated as Future and will not break the existing functionality based on the Future API.
So, the bulk of the API comes from CompletionStage. Most of the methods return CompletableFuture (in the CompletionStage interface, they return CompletionStage, but they are converted to CompletableFuture when implemented in CompletableFuture class), which means that they allow chaining the operations similar to how the Stream methods do when only one element goes through ...
