Comparable

A class that implements the Comparable<T> interface can be used with TreeSet and TreeMap
To implement thie interface, it must implement int compareTo(Object)
It returns negative if less than the parameter, positive if greater, and zero if they are equal

It is a good idea to define equals() for such a class as follows:

class Foo implements Comparable<Foo> {
  private int value;

  public boolean equals(Object other) {
    if (other instanceof Foo) {
      return (compareTo((Foo)other) == 0);
    } else {
      return false;
    }
  }

  public int compareTo(Foo other) {
    return value - other.value;
  }
}

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