CS 112 Introduction to Programming

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CS 112 Introduction to Programming

• Lecture 24
• Inheritance
• Http//zoo.cs.yale.edu/classes/cs112/

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Outline

• Admin.
• Inheritance

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Admin.

• Assignment 5
• Linked online
• Midterm one
• Highest 59
• Lowest 33
• Median/average 50/49

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Recap Inheritance

• Inheritance allows a software developer to derive
  a new class from an existing one
• The existing class is called the parent class, or
  superclass, or base class
• The derived class is called the child class, or
  subclass, or derived class.
• As the name implies, the child inherits
  characteristics of the parent
• That is, the child class inherits the methods and
  data defined for the parent class

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Inheritance

• Inheritance relationships are often shown
  graphically in a class diagram, with the arrow
  pointing to the parent class

Inheritance should create an is-a relationship,
meaning the child is a more specific version of
the parent
public - private
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Declaring a Derived Class

• Define a new class DerivedClass which extends
  BaseClass
• class BaseClass
• // class contents
• class DerivedClass BaseClass
• // class contents
• Base class the parent class if omitted the
  parent is the Object class

See Book.cs, Dictionary.cs, BookInheritance.cs
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Outline

• Admin.
• inheritance
• why inheritance?
• how to implement inheritance?
• controlling inheritance

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Controlling Inheritance

• A child class inherits the methods and data
  defined for the parent class however, whether a
  data or method member of a parent class is
  accessible in the child class depends on the
  visibility modifier of a member
• Variables and methods declared with private
  visibility are not accessible in the child class
• a private data member defined in the parent class
  is still part of the state of a derived class
• Variables and methods declared with public
  visibility are accessible but public variables
  violate our goal of encapsulation
• There is a third visibility modifier that helps
  in inheritance situations protected

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The protected Modifier

• Variables and methods declared with protected
  visibility in a parent class are only accessible
  by a child class or any class derived from that
  class
• The details of each modifier are linked on the
  schedule page
• Example
• Book.cs
• Dictionary.cs
• TestBookDictionary.cs

• public
• private
• protected

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Outline

• Admin.
• inheritance
• why inheritance?
• how to implement inheritance?
• controlling inheritance
• constructors

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Calling Parents Constructor in a Childs
Constructor the base Reference

• Constructors are not inherited, even though they
  have public visibility
• The first thing a derived class does is to call
  its base class constructor, either explicitly or
  implicitly
• implicitly it is the default constructor
• yet we often want to use a specific constructor
  of the parent to set up the "parent's part" of
  the object
• Syntax use basepublic DerivedClass
  BaseClass public DerivedClass() base()
  //

• Example2
• See FoodItem.cs
• See Pizza.cs
• See FoodAnalysis.cs

• Example1
• See Book.cs
• See Dictionary.cs
• See BookConstructor.cs

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Outline

• Admin.
• Inheritance
• why inheritance?
• how to implement inheritance?
• controlling inheritance
• calling parents constructor
• overriding methods

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Defining Methods in the Child Class Overriding
Methods

• A child class can override the definition of an
  inherited method in favor of its own
• That is, a child can redefine a method that it
  inherits from its parent
• The new method must have the same signature as
  the parent's method, but can have different code
  in the body
• The type of the object executing the method
  determines which version of the method is invoked

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Overriding Methods Syntax

• override keyword is needed if a derived-class
  method overrides a base-class method
• If a base class method is going to be overridden
  it should be declared virtual

• Example
• Thought.cs
• Advice.cs
• ThoughtAndAdvice.cs

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Overloading vs. Overriding

• Overloading deals with multiple methods in the
  same class with the same name but different
  signatures
• Overloading lets you define a similar operation
  in different ways for different data

• Overriding deals with two methods, one in a
  parent class and one in a child class, that have
  the same signature
• Overriding lets you define a similar operation in
  different ways for different object types