Thread: Null Error

The problem with the codes it that, in "getAverage" it is not allowing the "return null". I assume this is because it is declared a double. What is the proper way to return the "getAverage"?

public class StudentScores
{
   private ArrayList<Student> scores;
   public StudentScores()
   {
      scores = new ArrayList<Student>();
   }
   public void add(String name, int score)
   {
      scores.add(new Student(name, score));
   }
   public Student getHighest()
   {
      if (scores.size() == 0)
         return null;
      Student highest = scores.get(0);
      for (int i = 1; i < scores.size(); i++)
         if (scores.get(i).getScore() > highest.getScore())
            highest = scores.get(i);
      return highest;
    }
   public Student getLowest()
   {
      if (scores.size() == 0)
         return null;
      Student lowest = scores.get(0);
      for (int i = 1; i < scores.size(); i++)
         if (scores.get(i).getScore() < lowest.getScore())
            lowest = scores.get(i);
      return lowest;
   }   
     public double getAverage()
   {
      if (scores.size() == 0)
         return null; //will not return, BlueJ gives error of "incompatible types"
      Student lowest = scores.get(0);
      for (int i = 1; i < scores.size(); i++)
         if (scores.get(i).getScore() < lowest.getScore())
            lowest = scores.get(i);
      return lowest;
   }
}