Java Comparator syntax works, but why?

Question

I am using a Java Comparator to sort an ArrayList of SomeClass items. It works just fine, no problems. What I'm curious about is the syntax. Given:

 public static class PrioritizedPendingListComparator implements Comparator<SomeClass> {

        @Override
        public int compare(SomeClass o1, SomeClass o2) {
        ...
        }

And then, some time later,

private ArrayList<SomeClass> pendingList = new ArrayList<>(); 

... // fill it up with entries

Collections.sort(pendingList, new SomeClass.PrioritizedPendingListComparator());

I use new SomeClass.PrioritizedPendingListComparator() as the second argument to Collections.sort(). My comparator, PrioritizedPendingListComparator, is static so I don't expect to have to use new.

1. Why can I not just use SomeClass.PrioritizedPendingListComparator e.g. why do I need the new?
2. How would I know this from reading the documentation?

Answer 1

SomeClass.PrioritizedPendingListComparator is a class. You need an instance of it so you have to use new

When you define public static class the static means it doesn't implicitly have a reference to the outer class, but you still have to create it in the normal way.

One way to avoid new is to use an enum.

public enum PrioritizedPendingListComparator implements Comparator<SomeClass> {
    INSTANCE;

    @Override
    public int compare(SomeClass o1, SomeClass o2) {
    ...
    }

later

 Collections.sort(pendingList, SomeClass.PrioritizedPendingListComparator.INSTANCE);