Example1.java

import java.util.concurrent.CompletableFuture;

class Example1 {
    private final Database database;

    Example1(Database database) {
        this.database = database;
    }

    CompletableFuture<Boolean> activateUser(Long userId) {
        return database.findUser(userId).thenCompose((u) -> {
            if (u != null && !u.isActive()) {
                return database.activateUser(userId).thenApply((r) -> true);
            } else {
                return CompletableFuture.completedFuture(false);
            }
        });
    }
}

interface Database {
    CompletableFuture<User> findUser(Long id);

    CompletableFuture<Void> activateUser(Long id);
}

interface User {
    boolean isActive();
}