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

Assuming class A has a copy constructor that effectively copies the List list attribute and a method that merges two instances of A:

public A(A another) {
    this.id = another.id;
    this.name = another.name;
    this.list = new ArrayList<>(another.list);
}

public A merge(A another) {
    list.addAll(another.list):
    return this;
}

You could achieve what you want as follows:

Map result = listOfA.stream()
    .collect(Collectors.toMap(A::getId, A::new, A::merge));

Collection result = map.values();

This uses Collectors.toMap, which expects a function that extracts the key of the map from the elements of the stream (here this would be A::getId, which extracts the id of A), a function that transforms each element of the stream to the values of the map (here it would be A::new, which references the copy constructor) and a merge function that combines two values of the map that have the same key (here this would be A::merge, which is only called when the map already contains an entry for the same key).

If you need a List instead of a Collection, simply do:

List result = new ArrayList<>(map.values());