The type argument of Comparable should always be the type of the current class.

For example, do this:

class Foo implements Comparable<Foo> {
  public int compareTo(Foo other) { ... }
}

not this:

class Foo implements Comparable<Bar> {
  public int compareTo(Bar other) { ... }
}

Implementing Comparable for a different type breaks the API contract, which requires x.compareTo(y) == -y.compareTo(x) for all x and y. If x and y are different types, this behaviour can't be guaranteed.