thenCompose and thenCombine

Definition

thenCompose chains a step that itself returns a CompletableFuture, so the second call depends on the first result. thenCombine joins two independent futures that run at the same time, once both are done.

Example code

A dependent call, and two independent calls combined

import java.util.concurrent.CompletableFuture;

class Demo {
    static CompletableFuture<Integer> userId(String name) { return CompletableFuture.supplyAsync(() -> name.length()); }
    static CompletableFuture<String> orders(int id) { return CompletableFuture.supplyAsync(() -> id + " orders"); }
    static CompletableFuture<Integer> price() { return CompletableFuture.supplyAsync(() -> 120); }
    static CompletableFuture<Integer> shipping() { return CompletableFuture.supplyAsync(() -> 30); }

    public static void main(String[] args) {
        // thenCompose: the second call needs the first result
        String result = userId("Asha").thenCompose(Demo::orders).join();
        System.out.println(result);

        // thenCombine: two independent calls running at the same time
        int total = price().thenCombine(shipping(), Integer::sum).join();
        System.out.println(total);
    }
}

// Output:
// 4 orders
// 150