Inheritance

Inheritance

1. Code Extensibility

   • extends keyword; Java only allows 1 extension per class
     • Create new class; subclass
       • Parent-child relationship
         • Method Override
           • Write method with same signature as parent class method
             • Change existing method behaviors
         • Child has "is-a" relationship to Parent
           • Child inherits all public methods of Parent
             • Allows reference variable of parent type in assignment of a subclass instance
               • Subclass can access public and protected just not private methods from superclass
       • keyword new to create an instance
         • When subclass object is instantiated, so are all parent's classes
     • Ex: public class Child extends Parent {}
   • final keyword
     • Prevent method override; no change in behavior allowed
     • Does not permit Method Override
       • Speed improvement but design should be main concern
     • Ex: public final class Pencil{}
   • java.lang.Object
     • Ultimate Java superclass
   • keyword super represents instance of direct superclass of current object
     • Invoke superclass constructor
     • Call a hidden member or an overriden superclass method
   • Type Casting
     • Possible only to cast subclass instance to parent class; "Upcasting"
       • "Downcasting" if parent instance is cast as child class
   • instanceof keyword
     • test if object is of specified type; ex: if it's type of its superclass