Using lambdas to extract an Object attribute contained in an ArrayList

Question

I have made a class called User with two string attributes: Login and Password.

I have a database (userList, of type ArrayList<User>) with all the existing users, and I want to know if a login is already used or not.

I tried to use lambda expressions in order to do that, but it doesn't work:

ArrayList<String> loginList = null;
userListe.forEach(x->loginList.add(x.getLogin()));

How do I solve this?

What does _it doesn't work_ mean? What did you expect that code to do? Why and what did it do instead? — Sotirios Delimanolis, Sep 27 '18 at 15:04

Lorelorelore · Answer 1 · 2018-09-27T15:07:00.000

Assuming userListe is a List:

List<String> loginList = userListe
             .stream()
             .map(x -> x.getLogin())
             .collect(Collectors.toList));

Explanation:

It is better to use a List instead of ArrayList because it is always preferable using interface instead of concrete types. First, using a stream you create a flow of User objects. Using map you get a field from your objects using the appropriate getter. At last, using a collect you can collect all objects returned by map method, in this case String object.

Your loginList is null, so the first forEach will throw a NullPointerException. Also, remember that it is better to have pure function in lambdas, with no side effects. Using stream and collect allows you to get a List of objects you need without having functions with side effects.

Using lambdas to extract an Object attribute contained in an ArrayList

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