supplyAsync runs the supplier on the common ForkJoinPool and returns a future that completes when the supplier finishes. thenApply chains a synchronous transformation; the lambda runs on whichever thread completed the previous stage. get() blocks the caller until the chain finishes, so use it sparingly (only at the top of main or in tests). For real services, return the CompletableFuture itself and let the framework or caller decide when to wait.

Use thenCompose (NOT thenApply) when the next step itself returns a CompletableFuture. With thenApply you would end up with CompletableFuture<CompletableFuture<String>> and have to call .join() to flatten it; thenCompose handles that flattening for you. The mental model is flatMap for futures, the same way you would use flatMap over Optional or Stream. Reserve thenApply for pure transforms that return a plain value.

allOf returns a future that completes when EVERY input future completes; it does not, however, give you the results directly (its type is CompletableFuture<Void>). The standard idiom is allOf(futures).thenApply(v -> futures.stream().map(CompletableFuture::join).collect(...)) or, if you have already awaited, just iterate and call join() since each future is now done. The wall-clock time is roughly max(individual durations) rather than their sum, which is the whole point of fanning out. For early-fail semantics use anyOf plus your own error checking.