Inheritance and Polymorphism

1
Inheritance and Polymorphism

• Chapter 9
• Spring 2006
• CS 101
• Aaron Bloomfield

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This section is not required material!!!!

• A note about inheritance
• Its not normally covered in 101
• It will be gone over in more detail in CS 201
• Ask questions if you are confused about
  inheritance
• You arent the only one!

3
Motivation

• Consider a transportation computer game
• Different types of vehicles
• Planes
• Jets, helicopters, space shuttle
• Automobiles
• Cars, trucks, motorcycles
• Trains
• Diesel, electric, monorail
• Ships
• Lets assume a class is written for each type of
  vehicle

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More on classes vs. objects
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Motivation

• Sample code for the types of planes
• fly()
• takeOff()
• land()
• setAltitude()
• setPitch()
• Note that a lot of this code is common to all
  types of planes
• They have a lot in common!
• It would be a waste to have to write separate
  fly() methods for each plane type
• What if you then have to change one you would
  then have to change dozens of methods

6
Motivation

• Indeed, all vehicles will have similar methods
• move()
• getLocation()
• setSpeed()
• isBroken()
• Again, a lot of this code is common to all types
  of vehicles
• It would be a waste to have to write separate
  move() methods for each vehicle type
• What if you then have to change one you would
  then have to change dozens of methods
• What we want is a means to specify one move()
  method, and have each vehicle type inherit that
  code
• Then, if we have to change it, we only have to
  change one copy

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Motivation
Provides move() getLocation() setSpeed() isBroken
()
Provides fly() takeOff() land() setAltitude() set
Pitch()
Provides oilChange() isInTraffic()
Provides derail() getStation()
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Motivation

• What we will do is create a parent class and a
  child class
• The child class (or subclass) will inherit the
  methods (etc.) from the parent class (or
  superclass)
• Note that some classes (such as Train) are both
  subclasses and superclasses

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Inheritance code

• class Vehicle
• ...
• class Train extends Vehicles
• ...
• class Monorail extends Train
• ...

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About extends

• If class A extends class B
• Then class A is the subclass of B
• Class B is the superclass of class A
• A is a B
• A has (almost) all the methods and variables that
  B has
• If class Train extends class Vehicle
• Then class Train is the subclass of Vehicle
• Class Vehicle is the superclass of class Train
• Train is a Vehicle
• Train has (almost) all the methods and variables
  that Vehicle has

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Object-oriented terminology

• In object-oriented programming languages, a class
  created by extending another class is called a
  subclass
• The class used for the basis is called the
  superclass
• Alternative terminology
• The superclass is also referred to as the base
  class
• The subclass is also referred to as the derived
  class

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Another example

• Consider shapes in a graphics program
• Shape class
• Circle class
• Cube class
• Dodecahedron class

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Inheritance

• Organizes objects in a top-down fashion from most
  general to least general
• Inheritance defines a is-a relationship
• A mountain bike is a kind of bicycle
• A SUV is a kind of automobile
• A border collie is a kind of dog
• A laptop is a kind of computer

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Packages

• Allow definitions to be collected together into a
  single entitya package
• The classes in our game could be added to a
  package
• Classes and names in the same package are stored
  in the same folder
• Classes in a package go into their own
  namespace and therefore the names in a
  particular package do not conflict with other
  names in other packages
• For example, a package called OtherGame might
  have a different definition of Map

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Controlling access

• Class access rights

Member Restriction this Subclass Package General
public ü ü ü ü
protected ü ü ¾ ¾
default ü ü ü ¾
private ü ¾ ¾ ¾
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Javas Mother-of-all-objectsClass Object
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Thus, everything extends Object

• Either directly or indirectly
• So what does that give us?
• Object contains the following methods
• clone()
• equals()
• toString()
• and others
• Thus, every class has those methods

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A note about equals()

• Why does the equals() method always have to have
  the following prototype
• boolean equals(Object obj)
• Many other class in the Java SDK require the user
  of equals()
• Such as the Vector class
• Those classes need to know how the equals()
  method will work in order for them to work
  properly
• Thus, it must have the same prototype

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Overriding

• Consider the following code
• class Foo // automatically extends Object
• public String toString ()
• return Foo
• ...
• Foo f new Foo()
• System.out.println (f)
• Now there are two toString() method defined
• One inherited from class Object
• One defined in class Foo
• And they both have the same prototype!
• Which one does Java call?

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Overriding

• Java will call the most specific overriden method
  it can
• toString() in Foo is more specific than
  toString() in Object
• Consider our transportation hierarchy
• Assume each class has its own toString() method
• Car extends Automobile extends Vehicle (extends
  Object)
• Assume each defines a toString() methods
• The toString() method in Vehicle is more specific
  (to vehicles) than the one in Object
• The toString() method in Automobiles is more
  specific than the ones in Vehicle or Object
• The toString() method in Car is more specific
  than the ones in Automobile, Vehicle, or Object
• Thus, for a Car object, the Car toString() will
  be called
• There are ways to call the other toString()
  methods
• This has to be specifically requested

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Overriding

• This is called overriding, because the toString()
  in Foo overrides the toString() in Object
• Note that the prototype must be EXACTLY the same
• With overloading, the parameter list must be
  DIFFERENT
• Overriding only works with inheritance
• In particular, you can only override a method
  already defined in a parent (or grandparent,
  etc.) class