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Thread: Null Error

  1. #1
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    Default 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;
       }
    }


  2. #2
    Super Moderator copeg's Avatar
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    Default Re: Null Error

    The getAverage method is declared to return a double primitive not an object, so you cannot return null. A few other alternatives would be to throw an exception (perhaps an IllegalStateException), use an assertion, or return something like Double.MAX_VALUE. I would opt for the first but leave it up to you to decide - however it is done it should be documented in the comments (which your code lacks)

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