Writing a functioning CompletableFuture

RealAsync.java

package org.teckhooi;

import java.util.concurrent.CompletableFuture;

public class RealAsync {
    public static void main(String[] args) {
        long waitTime = 2000;

        System.out.println("Test 1 starts...");
        long timeTaken = time(() -> {
            CompletableFuture[] futures = new CompletableFuture[3];
            for (int i = 0; i < 3; i++) {
                futures[i] = CompletableFuture.completedFuture(complexProcess(waitTime));
            }
            CompletableFuture.allOf(futures).join();
        });
        System.out.println("Test 1 completes.");

        System.out.printf("First test took %dms\n", timeTaken);

        System.out.println("Test 2 starts...");

        timeTaken = time(() -> {
            CompletableFuture[] futures = new CompletableFuture[3];
            for (int i = 0; i < 3; i++) {
                futures[i] = CompletableFuture.supplyAsync(() -> complexProcess(waitTime));
            }
            CompletableFuture.allOf(futures).join();
        });

        System.out.println("Test 2 completes.");

        System.out.printf("Second test took %dms\n", timeTaken);
    }

    static public long complexProcess(long waitTime) {
        try {
            Thread.sleep(waitTime);
        } catch (InterruptedException e) {
            // Empty
        }
        return waitTime;
    }

    static public long time(Runnable block) {
        long now = System.currentTimeMillis();
        block.run();
        return System.currentTimeMillis() - now;
    }
}

The first test takes about 6s to complete while the second test takes about 2s to complete