supplyAsync and thenApply

Definition

A CompletableFuture is a result that will be ready later. supplyAsync starts a task in the background. thenApply says what to do with the result when it arrives, so you build a chain of steps without blocking a thread to wait.

Example code

Start a task, then transform its result

import java.util.concurrent.CompletableFuture;

class Demo {
    static int fetchPrice() {
        try { Thread.sleep(100); } catch (InterruptedException ignored) { }   // pretend this is a slow call
        return 200;
    }

    public static void main(String[] args) {
        CompletableFuture<String> message = CompletableFuture
                .supplyAsync(Demo::fetchPrice)                  // runs in the background
                .thenApply(price -> price * 118 / 100)          // runs when the price is ready (add tax)
                .thenApply(total -> "Total: " + total);

        System.out.println(message.join());                      // wait for the final result
    }
}

// Output:
// Total: 236