Super and sub classes

[John Kirvan]

Is their anyway possible for me to access sub class methods in my super class.

[David Harkness]

The normal way is to declare the method in your superclass and override it in the subclass. However, if you want to call a method only defined in your subclass, I suppose you can do so by casting:

class SuperClass
{
    void superClassMethod()
    {
        if ( this instanceof SubClass )
        {
            ((SubClass) this).subClassMethod();
        }
    }
}
 
class SubClass extends SuperClass
{
    void subClassMethod()
    {
        System.out.println("Why are you calling me?");
    }
}

But my first questions would be "Why do you want to do this?" and "Are you sure this isn't a sign that the design is flawed?"

[John Kirvan]

It is a case of difficult proffessor giving difficult assignment. Yeah he could have deffinently made it a lot easier on us, oh well at least im learning.

[Stan James]

As David said, it would be more common for the superclass to implement the method to maybe do nothing at all, and for the subclass to implement it to do something meaningful.

This happens a lot in frameworks. A superclass might have all the code required to connect to a database, execute a query and return the result. Then subclasses might only provide the specific query.

public superclass
{
   public resultset doQuery()
   {
      connectToDatabase();
      sql = getSQL();
      resultset = executeQuery( sql );
      closeDatabase();
      return resultset;
   }
   public getSql()
   {
      display "you have to override this"
   }
}
 
public SubClass
{
   public String getSql()
   {
       return "select * from sometable";
   }
}
 
public SomeClientClass
{
   public doSomeWork()
   {
      resultset = new SubClass().doQuery();
      work with resultset ...
   }
}

Now, in some sense the superclass method is executing code in a subclass method. Any chance that's what your instructor wanted? If so, read up on "abstract methods" as a way to force the subclass to override getSql().

If your instructor was looking for David's solution, be worried. In a perfect world, a SuperClass should never have to know anything about any SubClasses.

Let us know how this works out in class!
[ February 04, 2005: Message edited by: Stan James ]

[Layne Lund]

Or you can make the method abstract. This is often used to force subclasses to override a method. Providing an empty method in the base (or super) class is only meaningful if it is okay for the subclass to NOT override the method and use the default "do nothing" implementation.

Layne