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

If you can use vanilla Java here is a very easy solution. The list is iterated at once.

Map m = new HashMap<>();
for (A a : list) {
    if (m.containsKey(a.getId()))
        m.get(a.getId()).getList().addAll(a.getList());
    else
         m.put(a.getId(), new A(a.getId(), a.getName(), a.getList()));
}
List output =  new ArrayList<>(m.values());