Provide implementation for abstract method but restrict visibility

Question

I have a method in an abstract class that calls an abstract method, for which the subclasses must provide the implementation.

public abstract class AClass {

    public void foo() {
        ...
        fooToImplement();
        ...
    }

    // DON'T CALL THIS METHOD, ONLY PROVIDE IMPLEMENTATION!
    protected abstract void fooToImplement();

}

I want to make sure that the subclasses don't call fooToImplement(), they should always use foo() instead. The behavior is something like a "private abstract" method, but that's not possible in Java.

Is there some alternative? Thanks!

Answer 1

If you don't want your subclasses to be able to call this method you could use strategy: Extract the behavior of the method into an interface and pass an implementation of this interface to the object. E.g.

IStrategy {
  public void fooToImplement();
} 

AClass {
   public AClass(IStrategy impl) {...}

    public void foo() {
      ...
      strategy.fooToImplement();
      ...
    }
}

Delegation instead of inheritance. In java 8 this would be a little bit easier.

If your implementation of IStrategy would need access to the data of the object AClass, you could try to implement it as an inner class.

Answer 2

The method has to be visible by your subclass if you want it to be overriden. You have to use a class witch does not extends AClass as caller.

public class BClass extends ACLass {
   @Override 
   protected void fooToImplement() {
      System.out.println("override me im famous");
   }
}

public class CClass {
   private BCLass bInstance;
   public void doSomething(){
      bInstance.foo();
      // !!! NO ACCESS TO fooImplement()
   }
}