Question

I created an abstract base class. It contains an object which should be extended by any subclasses:

public abstract class AbstractParent {
    protected AbstractObject subMePlz;
    // ... some fields that all subclasses need
    public AbstractParent() {
        this.subMePlz = createThisInYourExtendedClass();

    }

    public abstract AbstractObject createThisInYourExtendedClass();
} 

the abstractObject:

public abstract class AbstractObject {
   // ... some fields/methods that all subclasses need 
}

What I want is to be able to use the extended field in the extended class without casting:

public class ExtendParent extends AbstractParent {
     // .. some unique fields
     public ExtendParent(){
       super();
     }


   public ConcreteObject createThisInYourExtendedClass(){
        return new ConcreteObject();
   }

   // what I want to do - no cast
   public void doSomethingWithSubMePlzWithoutCastingIt() {
        System.out.println(this.subMePlz);  
   }

   // what I end up doing - gotta cast
   public void doSomethingWithSubMePlzWithoutCastingIt() {
        System.out.println((ConcreteObject)this.subMePlz);  
   }
}

Would needing a comparator change how I should implement this? – I’m thinking a generic comparator for a list of the AbstractObjects that could be used by its subclasses.

Answer 1

It sounds like you need to make it generic:

public abstract class AbstractParent<T extends AbstractObject> {
    protected T subMePlz;
    // ... some fields that all subclasses need
    public AbstractParent() {
        this.subMePlz = createThisInYourExtendedClass();

    }

    public abstract T createThisInYourExtendedClass();
} 

public class ExtendParent extends AbstractParent<ConcreteObject> {
    ...
}

Note that calling non-private methods within a constructor is usually a bad idea – the subclass won’t have been fully initialized yet, which can make it difficult to reason about how much you can really rely on.