Joe Chung
Monmouth U. Homepage

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====== Introduction to Inheritance ======

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Topics we'll cover:

   * inheritance,
   * super() (superclass constructors), 
   * overriding superclass methods, 
   * "protected" access modifier, 

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===== Motivation =====

  * Suppose you will define classes to model circles, rectangles, and triangles. These classes have many features in common. What is the best way to design these classes to __avoid redundancy__? The answer is to use inheritance.

  * Suppose you are building a GUI app in Java and find yourself wishing that a GUI button could hold more information than it currently can. That is, you wish you could __add more features__ to an existing class. The answer is to use inheritance.

===== What is Inheritance? =====

  * Inheritance is one of the three pillars (foundational principles) of object-oriented programming.
    * Encapsulation/Data Hiding
    * **Inheritance**
    * Polymorphism
  * There is a parent class.
    * Child classes inherit from the parent class.
    * Other ways to express parent <-> child classes:
      * Super <-> sub classes
      * Base <-> derived classes
    * Child classes may themselves be parent classes of lower child classes
      * So, you can have a hierarchy of classes
      * Generally, the parent classes are more **general**, and the child classes are more **specific**.

{{http://blando.info/luis/personalities/zoo_example/Image1.gif}}

===== Simple Inheritance =====

    * Inheritance example: GradedActivity <- FinalExam (FinalExam.zip)
      * The keyword ''extends'' denotes an inheritance relationship between a __superclass__ (GradedActivity) and a __subclass__ (FinalExam).
      * There is no special keyword that designates a class to be a superclass.
      * The subclass ''inherits'' the ''public'' or ''protected'' instance variables and methods of the superclass.
        * See section below on the ''protected'' keyword.
        * NOTE: ''private'' instance variables and methods are NOT inherited.
      * The superclass is intended to be the ''starting point'' or ''basis'' of the development of the subclass.
      * The subclass ''inherits'' the //public// or //protected// instance variables and methods of the superclass.
        * In order for this to happen, the subclass constructor has to call the superclass constructor.
        * This call to the superclass constructor can happen explicitly or implicitly (automatically).

    * Calling superclass constructor example 1: SuperClass1 <- SubClass1 (SuperClass1.zip)
    * Calling superclass constructor example 2: SuperClass2 <- SubClass2 (SuperClass2.zip)
      * Superclass constructors are NOT inherited.
      * A superclass constructor is called explicitly or implicitly by a subclass constructor.
        * The call to the superclass constructor has to be the first thing that's done in the subclass constructor.
        * If explicitly calling the superclass constructor, use the //super// keyword.
        * If the keyword //super// is not explicitly used, the superclass's default (no-arg) constructor is automatically called at the start of a subclass constructor.
          * So, a superclass' constructor must always be called by the subclass constructor.

    * The keyword //super// refers to the superclass of the current subclass.
      * The //super// keyword can be used in two ways:
        * 1 - To call a superclass constructor
        * 2 - To call a superclass method

    * "Constructor Chaining"
        * Constructing an instance of a subclass calls a superclasses' constructor.

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===== Method Overrides =====

    * A subclass can inherit certain properties (data) and methods from the superclass.
      * But the subclass can also ...
        * Add new properties (instance variables)
        * Add new methods
        * Or //override// the methods of the superclass

    * Sometimes it is necessary for a subclass to modify the implementation (body) of a method defined in the superclass.
      * This is referred to as //method overriding//.

    * Overriding superclass methods example 1: GradedActivity <- CurvedActivity (CurvedActivity.zip)
    * Overriding superclass methods example 2: Geometric1 <- Triangle 

    * NOTE: An instance method of a superclass can only be overridden if it is accessible.
      * A //private// superclass method cannot be overridden, because it is not accessible outside its own class.

    * NOTE: Overriding vs. Overloading
      * (Overloading: See first lecture on "Classes and Object-Oriented Programming" for notes on "Overloading Methods and Constructors".)
      * A subclass method //overrides// a superclass method only if the method signature is exactly the same (same method name and same input parameter list).
      * If the method signature is different in the subclass than in the superclass, that would be an example of __overloading__ a superclass method, not //overriding// it.


{{:cs176:501b:oversomething1.png|Overriding vs. Overloading a superclass method}}

===== Protected =====

  * The //protected// visibility modifier is different from //public// and //private// and is sometimes used to control inheritance.
      * The __protected__ modifier can be applied to instance variables and methods of a class.
      * A //protected// instance variable or method in a public class can be accessed by any class in the same package (directory) or its subclasses, even if the subclasses are in a different package.
        * A //protected// instance variable in a superclass is the same as a //public// instance variable to its subclasses.
          * Some frown upon the use of //protected// for this reason and recommend that //private// continue to be used in superclasses.
      * NOTE: A subclass may override a //protected// method in its superclass and change its visibility to //public//.

    * Protected visibility modifier example: GradedActivity2 <- FinalExam2 (ProtectedDemo.zip)

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