java.util.Comparator provides an interface which allows two objects of a given type to be compared against a user defined order algorithm.

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class MyNumberObject implements Comparator<MyNumberObject> {
  private int secretValue;
  
  public MyNumberObject(int x) {
    this.secretValue = (x*2 - 1) % 3;
  }

  @Override
  public int compare(MyNumberObject o1, MyNumberObject o2) {
    return o1.secretValue - o2.secretValue;
  }
}

java.util.Comparable provides a de-facto interface to compare two objects of a given type against a user defined order algorithm.

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public class MySpecialObject implements Comparable<MySpecialObject> {
  @Override
  public int compareTo(MySpecialObject obj) {
    return ...
  }

}

Comparable vs Comparator

  • Comparable interface can be used to provide single way of sorting whereas Comparator interface is used to provide different ways of sorting.
  • For using Comparable, Class needs to implement it whereas for using Comparator we don’t need to make any change in the class.
  • Comparable interface is in java.lang package (default) whereas Comparator interface is present in java.util package (needs to be imported).
  • We don’t need to make any code changes at client side for using Comparable, Arrays.sort() or Collection.sort() methods automatically uses the compareTo() method of the class. For Comparator, client needs to provide the Comparator class to use in compare() method. Source: journaldev

TL;DR:

  • Comparable is for a single definitive comparison whereas Comparator can be used as a utility (multiple ways), whereas
  • Comparable is implemented within the class, whereas Comparator is implemented in its own class.