Merging lists under same objects in a list using Java streams

Question

I have two objects like following:

public class A {
    private Integer id;
    private String name;
    private List list;

    public A(Integer id         


        

Answer 1

Collection merge(List list) {
    return list.stream()
            .collect(Collectors.toMap(a -> a.id, Function.identity(), this::merge))
            .values();
}

A merge(A a1, A a2) {
    if (!a1.name.equals(a2.name)) {
        throw new IllegalArgumentException("We assumed same id means same name");
    }
    return new A(a1.id, a1.name, union(a1.list, a2.list));
}

List union(List l1, List l2) {
    List result = new ArrayList<>(l1);
    result.addAll(l2);
    return result;
}