iterating and filtering two lists using java 8

Question

I want to iterate two lists and get new filtered list which will have values not present in second list. Can anyone help?

I have two lists - one is list of strings, and the other is list of MyClass objects.

List<String> list1;
List<MyClass> list2;

MyClass {

    MyClass(String val)
    {
        this.str = val;
    }

     String str;
     ...
     ...
}

I want filtered list of strings based on -> check second list for elements (abc) whose values not present in list1.

List<String> list1 = Arrays.asList("abc", "xyz", "lmn");

List<MyClass> list2 = new ArrayList<MyClass>();

MyClass obj = new MyClass("abc");
list2.add(obj);
obj = new MyClass("xyz");
list2.add(obj);

Now I want new filtered list -> which will have value => "lmn". i.e. values not present in list2 whose elements are in list1.

Answer 1

// produce the filter set by streaming the items from list 2
// assume list2 has elements of type MyClass where getStr gets the
// string that might appear in list1
Set<String> unavailableItems = list2.stream()
  .map(MyClass::getStr)
  .collect(Collectors.toSet());

// stream the list and use the set to filter it
List<String> unavailable = list1.stream()
  .filter(e -> unavailableItems.contains(e))
  .collect(Collectors.toList());