Java List Stream-CodePudding

I am attempting to simplify my code but list stream is not working. I am creating an instance of a class (that contains an inner class) in a static method and trying to take items from the first list and instantiate them in the inner class and add them to a new list. My code works with fruit loops if that matters, I just want to get rid of excess loops. Below is a copy of what I tried.

instance.firstArrayList.stream()
    .map(item -> instance.new Innerclass(item))
    .collect(Collectors.toCollection(() -> instance.secondArrayList));

I am trying to do this multiple times (at least two so far) so simply setting a new ArrayList instance to this won't work for me. I know I can simply do this with a single loop but I am trying to learn how to use list streams as it has other methods that I find particularly useful and this is the first problem I have run into so far, and I cannot find examples online on this.

Apologies in advance for my poor formatting, this is my first post.

edit(Solved): in the constructor for outerclass, I redeclared the type ArrayList for secondArrayList and that is why it was not working.

CodePudding user response：

To add all results of your stream to your secondArrayList you could just

    instance.secondArrayList.addAll(instance.firstArrayList.stream()
            .map(item -> instance.new Innerclass(item))
            .toList());

Or alternatively, without intermediate list, adding elements one by one

    instance.firstArrayList.stream()
            .map(item -> instance.new Innerclass(item))
            .forEachOrdered(instance.secondArrayList::add);

Full example (demonstrating both versions):

import java.util.ArrayList;
import java.util.List;

public class Main {
    List<String> firstArrayList = new ArrayList<>();
    List<Innerclass> secondArrayList = new ArrayList<>();
    
    class Innerclass {
        private final String s;

        Innerclass(String s) {
            this.s = s;
        }
        
        @Override
        public String toString() {
            return "I: "   s;
        }
    }
    
    public static void main(String[] args) {
        
        
        Main instance = new Main();

        instance.firstArrayList.addAll(List.of("One", "Two", "Three"));
        
        instance.secondArrayList.addAll(instance.firstArrayList.stream()
                .map(item -> instance.new Innerclass(item))
                .toList());
        
        instance.firstArrayList.stream()
                .map(item -> instance.new Innerclass(item))
                .forEachOrdered(instance.secondArrayList::add);
        
        instance.secondArrayList.forEach(System.out::println);
    }

}

Output:

I: One
I: Two
I: Three
I: One
I: Two
I: Three