Can I use a Comparator without implementing Comparable?

Question

Is it possible to use a Comparator without implementing the Comparable class? For instance, if I had the following:

MyClass {

     Comparator comp;

     OrderedListInheritance(Comparator c) {
          this.comp = c;
     }

}

Could I then use comp to compare two objects? If so, how would I go about doing that?

Thanks...

Answer 1

You don't use Comparable. You use Comparator.

Comparable is an interface implemented by objects to specify their sort order with other objects of the same type.

Comparator is a generic interface that simply takes two objects and tells you their sort order. So you can do:

public class Student {
  private final int id;
  private final String name;
  private final int age; 

  public Student(int id, String name, int age) {
    this.id = id;
    this.name = name;
    this.age = age;
  }

  public int getId() { return id; }
  public String getName() { return name; }
  public int getAge() { return age; }
}

with:

public class AgeComparator implements Comparator<Student> {
  public int compare(Student s1, Student s2) {
    if (s1.getAge() == s2.getAge()) {
      return 0;
    } else {
      return s1.getAge() < s2.getAge() ? -1 : 1;
  }
}

and:

List<Student> students = new ArrayList<Student>();
students.add(new Student(1, "bob", 15));
students.add(new Student(2, "Jane", 14));
students.add(new Student(3, "Gary", 16));

SortedSet<Student> set1 = new TreeSet<Student>(new AgeComparator());
set1.addAll(students);
for (Student student : set1) {
  // age order
}

Answer 2

Comparator<T> has public int compare(T lhs, T rhs). So use that method to compare objects.

Also, the sorted collections will accept a Comparator as an argument so you can (for example) say:

Comparator<Integer> comparator = new Comparator<Integer>() {
  @Override public int compare(Integer lhs, Integer rhs) {
    if (rhs.intValue() < lhs.intValue())
      return -1;
    else if (rhs.intValue() > lhs.intValue())
      return 1;
    else 
      return 0;
  }
};
new TreeMap<Integer, Integer>(comparator);

To create a tree map where the sort order is (int this case) reversed.

Answer 3

Yes.

Comparator and Comparable are two separate and independent entities, only their purpose is similar.

In your code simply do: comp.compare(obj1, obj2)