java - How to cast subclass object to superclass object

Question

i have 2 classes, called superclass and subclass, i tried to cast the subclass object to superclass, but its seems does not work when i want to use the subclass object from the superclass. Please help to explain. Thanks. These are the code:-

public class superclass
{
    public void displaySuper()
    }
        System.out.println("Display Superclass");
    }
}

public class subclass extends superclass
{
    public void displaySub()
    {  
        System.out.println("Display Subclass");
    }
}


public class Testing
{    
   public static void main(String args[])
   {
      subclass sub = new subclass();
      superclass sup = (superclass) sub; 

when i tried to use the displaySub() from the subclass get error

      sup.displaySub(); //the displaySub method cant found
   }
}

Answer 1

A superclass cannot know subclasses methods.

Think about it this way:

• You have a superclass Pet

• You have two subclasses of Pet, namely: Cat, Dog

• Both subclasses would share equal traits, such as speak
• The Pet superclass is aware of these, as all Pets can speak (even if it doesn't know the exact mechanics of this operation)
• A Dog however can do things that a cat cannot, i.e. eatHomework
• If we were to cast a Dog to a Pet, the observers of the application would not be aware that the Pet is in fact a Dog (even if we know this as the implementers)
• Considering this, it would not make sense to call eatHomework of a Pet

You could solve your problem by telling the program that you know sup is of type subclass

public class Testing
{    
   public static void main(String args[])
   {
      subclass sub = new subclass();
      superclass sup = (superclass) sub; 
      subclass theSub = (subclass) sup;
      theSub.displaySub();
   }
}

You could solve the problem altogether by doing something like this:

public class superclass
{
    public void display()
    }
        System.out.println("Display Superclass");
    }
}

public class subclass extends superclass
{
    public void display()
    {  
        System.out.println("Display Subclass");
    }
}

public class Testing
{    
   public static void main(String args[])
   {
      subclass sub = new subclass();
      superclass sup = (superclass) sub; 
      sup.display();
   }
}

check out this tutorial on more info: overrides