Use only some Instance Variables

The compareTo() method takes a reference to another Monster as a parameter.

class Monster implements Comparable<Monster>
{

  . . . 
  
  public int compareTo( Monster other )
  {
  
     .... more goes here ...
  }
}

Typically, only some of the instance variables of a complex object are used by compareTo(). Say that you want monsters to be listed in ascending order of hit points. Monsters with the same number of hit points are ordered by ascending strength. Here is a partially completed method that does that:

  public int compareTo( Monster other )
  {
    int hitDifference = getHitPoints()-other.getHitPoints();
    
    if (  )
      return hitDifference;
      
    else
      return  ;
      
  }

It is easy to get this backwards. What you want is for compareTo(Monster other) to return a negative int when the monster that owns the method is less than the other monster.