How to get a list of specific fields values from objects stored in a list?

Question

Say I have an list of objects with two fields field1 and field2, both of String type.

How do I get a list of all field1 values wit

Answer 1

Ancient question, but I came upon it while looking to see if I could improve on a similar solution.

You can implement the List interface without creating a fleshed-out ArrayList, and thus not iterating over the parent object.

final List entities = getEntities()
final List field1 = new AbstractList() {
    public String get(int index) {
        return entities.get(index).getField1();
    }
    public int size() {
        return entities.size();
    }
}

That gives you a List without iterating over the parent object.

Random access to the derived List will be as expensive as random access to the underlying List; if you're using an implementation of List that does not provide quick random access, you might have to jump through a couple of hoops (i.e. implementing more methods of List. But this should work for 99% of the cases where you need a light-weight adapter.