Java Sub Classing: Overriding and Making New Methods

Question

Resources which teach Java seem to have conflicting answers on whether it's possible to both override a parent class's method and also create a new method from within a subclass at the same time. When I try the following:

class ABC{
     //Overridden Method
     public void disp(){
          System.out.println("disp() method of parent");
     }
}

class DEF extends ABC{
     //Overriding Method
     public void disp(){
          System.out.println("disp() method of child");
     }
     //new method in the subclass
     public void newMethod(){
          System.out.println("new method of child class");
     }
     public static void main(String args[]){
          ABC obj = new ABC();
          obj.disp();
          ABC obj2 = new DEF();
          obj2.disp();
          obj2.newMethod();
     }
}

java throws a "cannot find symbol" error. Is there something I'm missing? It seems strange that one couldn't do both at once and yet I can't seem to escape that error.

Answer 1

If you have a reference to an object, where the variable is defined as ABC, then you can only call methods that are defined on the class ABC. Even if the underlying object is of type DEF, the variable itself is defined to hold an ABC.

Try changing your code from this:

ABC obj2 = new DEF();

to this:

DEF obj2 = new DEF();

It will work because you are defining obj2 to be of type DEF.

Here's another example showing creation of a string, first with a variable defined for type String, and another with type defined as Object.

String s = new String();
s.charAt(99);             <-- this is valid on a String

Object o = new String();
o.charAt(99);             <-- same thing, but not allowed on Object

A workaround would be to use instanceof to check if obj2 is actually an instance of DEF, and if so, temporarily use it like a DEF by casting it, like this:

if (obj2 instanceof DEF) {
    ((DEF) obj2).newMethod();
}