Outline

1
Outline

• Inheritance
• Overriding Methods
• Keywords
• protected
• super
• final
• References revisited
• Composition vs. Inheritance

2
Recap

• this keyword
• Java Packages
• Java class library

3
Inheritance

• Derive new classes from old classes
• Improve code re-use
• Easier to manage and understand complexity
• Syntax class extends
  

4
Super and Sub classes

• The class being extended is called the
  superclass, or parent class
• The class extending the superclass is the
  subclass, or derived class
• Can be many levels of super/subclassespublic
  class B extends A public class C extends B
  public class D extends C public class
  C2 extends B etc

5
What Inheritance Means

• All public data members and methods (except
  constructors) in the superclass are inherited by
  the subclass. It is as if their definitions are
  copied into the subclasss class definition.
• Improves code re-usability- change need only be
  made once, if necessary
• No members of the subclass are visible to the
  superclass

6
Inheritance Example

• public class Person
• public String name
• public int age
• public Date birthDay
• public Person(String name, int age, Date
  birthDay)
• this.name name
• this.age age
• this.birthDay birthDay
• public void setName(String name)
• this.name name

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Example, cont

• public class Student extends Person
• public int studentID
• public String dept
• private float GPA
• public Student(String name, int age, Date
  birthDay, int studentID,
• String dept, float GPA)
• super(name, age, birthDay)
• this.studentID studentID

8
Over-riding Methods

• Redefine methods inherited from superclass to add
  or change functionality
• Onion-like view of an object
• each level of inheritance is a skin on the
  onion
• interacting with the object uses the outermost
  skin on the object a method call uses the
  definition in the most derived class

9
Method Over-riding Example

• public class Corvette
• public Roof getRoof() return new
  MetallicRoof()
• public class ConvertibleCorvetteextends Corvette
• public Roof getRoof() return new
  ConvertibleRoof()
• public void retractRoof()
• public void extendRoof()

10
Protected

• Any protected member can be seen by a subclass
• Protected members cannot be seen by unrelated
  classes
• Only class members (variables and methods) can be
  protected (classes themselves cannot be
  protected).

11
super Keyword

• Use super to refer to members of the current
  objects superclass.
• Most often used for accessing an inner layer of
  the onion I.e., for calling the superclass
  version of a method which the subclass has
  over-ridden.
• public Roof getRoof()
• if ( convertibleRoofIsBroken ) return
  super.getRoof()
• return new ConvertibleRoof()

12
super, cont

• Can be used in a constructor to access a
  super-class constructor
• Must be first line in constructor if present
• Syntax super()
• public class Student extends Person
• public Student(String name, int age, Date
  birthday, int studentID)
• super(name, age, birthday)
• this.studentID studentID

13
final, revisited

• For variables, final means unchangeable
  (constants)
• A final class cannot be extended from
• A final method cannot be over-ridden by a
  sub-class
• Used for security purposes

14
References, revisited

• A reference to an object of some derived class
  can be used as a reference of its classs
  superclass.
• E.g. a ConvertibleCorvette variable can be used
  as (assigned to, passed as a parameter, etc) a
  Corvette variable.
• X derived from YX is a Yso X can be used
  anywhere Y can

15
References, cont

• The reverse is not necessarily true!!
• public class A
• public class B extends A
• public class C extends A
• An instance of A is not an instance of B a
  reference to an A may happen to be actually a
  reference to a B, but that is not true in general
  (in fact it could be a reference to a C, or just
  an A, as well).

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Reference Examples

• A a1 new A()
• B b1 new B()
• A a2 b1
• A a3 new C()
• B b2 a1 // invalid!
• C c1 a3 // invalid!

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Object References

• All classes in Java that do not extend any other
  classes automatically extend class Object
• An Object reference can therefore refer to any
  instance of any Object
• Variables of type Object are essentially the same
  as (in C) void pointers
• Primitive data types have no class, and are not
  Objects

18
Example Class Hierarchy

Object
Student
Person
Math
Employee
System
Container
Panel
Applet
Component
Button
19
Composition vs. Inheritance

• Composition relationship has a
• Inheritance relationship is a
• E.g., a Student is a Person, and so should
  probably inherit from Person
• A House has an owner, which is likely a person,
  but the house isnt a person this is a
  composition relationship
• Composition relationships imply instance variables

20
Summary

• Inheritance
• Keywords extends, protected, super, final
• Method Overriding
• Superclass/subclass references
• Object references
• Composition vs. Inheritance

21
Next Class

• Polymorphism in Java
• MP4 is released