How to test Comparator for JUnit test?

I need to test the compare() method and i am confused on how. Can I get advice on how?

public class MemberComparator implements Comparator<Member> {

    private final String clientId;

    public MemberComparator(String clientId) {
        this.clientId = clientId;
    }

    @Override
    public int compare(Member m1, Member m2) {
        if (m1.getClientId().startsWith(clientId)) {
            return m2.getClientId().startsWith(clientId) ? m1.getClientId().compareTo(m2.getClientId())
                    : -1;
        } else {
            return m2.getClientId().startsWith(clientId) ? 1
                    : m1.getClientId().compareTo(m2.getClientId());
        }
    }

}