Return value from lambda function in background thread, throw NullPointerException

Question

In my use case, I want to return a user object from the lambda function as in the code below. Before posting my question, I try many similar questions like this and this but no theme solves my problem, here is my code:

public class User {
    private String name;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
}

public class AppDatabase {

    private static final int NUMBER_OF_THREADS = 4;

    public static final ExecutorService databaseWriteExecutor =
            Executors.newFixedThreadPool(NUMBER_OF_THREADS);

}

public class MyClass {

    private User mUser;

    public User findUser(){
        AppDatabase.databaseWriteExecutor.execute(() -> {
            mUser = work();
        });

        return mUser;
    }

    public User work(){
        //simulate get user from database
        User user = new User();
        user.setName("Toto");
        return user;
    }
}

public class Main {

    public static void main(String[] args) {
        MyClass myClass = new MyClass();
        User user;
        user = myClass.findUser();

        System.out.println(user.getName()); //this line: Exception in thread "main" java.lang.NullPointerException
    }
}

When I run this code, I get "Exception in thread" main "java.lang.NullPointerException". My question is how do I get the User object built by the work () function, but this function should run in a background thread like in code.

Answer 1

findUser returns right away, and it returns null because mUser hasn't been set yet. You need to either wait for it to be set, or return a Future<User> that the caller can wait on.