Confusion with Java generics and subclassing

Question

I have the following:

• An abstract class BaseScore
• LevelScore which is a subclass of BaseScore

I then have:

• An abstract class BaseScoreRecord

  public abstract class BaseScoreRecord<T extends BaseScore> {
       public BaseScoreRecord(){
          init();
       }
  
       protected T[] mScoreHistory;
       protected int count=0;
       protected float finalScore;
  
       protected abstract void calcFinalScore();
       protected abstract void init();
  
       public float getFinalScore() {
           return mFinalScore;
       }
  
       public void addScoreRecord(T score){
           mScoreHistory[count] = score;
           count++;
       }
  }
  

• ...and MyScoreRecord which is a subclass of BaseScoreRecord

  public class MyScoreRecord extends BaseScoreRecord<LevelScore> {
       public MyScoreRecord(){
          init();
       }
  
       @Override
       protected void calcFinalScore(){
           //Does some stuff to calc final score from mScoreHistory
       }
  
       @Override
       protected void init(){
           mScoreHistory = new LevelScore[10];
           for (int i=0;i<mScoreHistory.length;i++){
               mScoreHistory[i] = new LevelScore();
           }
       };
  }
  

So, then I have BaseGame which has the following line:

protected BaseScoreRecord<? extends BaseScore> endGameRecord;

...and MyGame which is a subclass of BaseGame. MyGame includes the following methods:

@Override
protected void gameInit(){
    endGameRecord = new MyScoreRecord();  //endGameRecord initialised
}


protected void addLevelScore(LevelScore ls){
    mEndGameRecord.addScoreRecord(ls);
}

I get a compiler error on this last method:

The method addScoreRecord(capture#3-of ? extends BaseScore) 
in the type BaseScoreRecord <capture#3-of ? extends BaseScore>
is not applicable for the arguments (LevelScore)

...so I am evidentially doing something wrong regarding the generics, wildcards and the subclassing. If I lose the generics in BaseGame and MyGame it all seems to run without any nasty ClassCastException errors. Any pointers would be much appreciated. Maybe I have made things too complicated for myself?

Answer 1

You should declare that MyScoreRecord is actually a subclass of BaseScoreRecord, and parameterize it properly:

public class MyScoreRecord extends BaseScoreRecord<LevelScore>

If you are using Eclipse, for example, you will get warnings because the @Override annotations are not actually overriding anything. The above will also make the compiler enforce that LevelScore extends BaseScore.