Comparator#comparing sort by the attribute of the class attribute

Newbie :

Let say I have a class ClassA, and it has ClassB as its attribute:

public ClassA
{
    private String attr;
    private ClassB classB;
}

public ClassB
{
    private String attr1;
    private int attr2;
}

From the class above, is it possible to sort the list of ClassA List<ClassA> list by attribute of ClassB using Comparator? It can be sort by the ClassA attribute easily by the code Comparator.comparing(ClassA::getAttr), but what if I want to sort the list by the attribute in ClassB, let say the attr2 attribute?

Ravindra Ranwala :

You can do it like so,

List<ClassA> sortedList = list.stream()
    .sorted(Comparator.comparing(a -> a.getClassB().getAttr2()))
    .collect(Collectors.toList());

Pass the comparing Function to the Comparator and it will create a new stream sorted according to the given Comparator. Then collect it into a List.

If you want to handle null values, it can be done like below.

final List<ClassA> sortedList = list.stream()
    .sorted(Comparator.nullsFirst(
        Comparator.comparing(a -> a.getClassB().getAttr1(), Comparator.nullsLast(String::compareTo))))
    .collect(Collectors.toList());