Week 06 b

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Week 06 - b

• Vector Example
• Class Hierarchies, Inheritance
• Polymorphism
• Review for Test 2

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Sample Programs Referenced(code located in
course folder)

• VectorRace
• java inherit.InheritLab

• Monsters Inc.
• Monsters Inc. - Array

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Example Race Redo

• Remember the Race problem?
• Registration process
• Get names, assign numbers
• Record race results
• Get numbers and times as racers cross line
• Report results display names and times
• Project was done with array of Runner objects
• Array runners used to store individual runner
  objects
• Needed to know the maximum of runners
• Needed to keep track of the actual number of
  runners
• Referencing the individual objects is easy
• Index in brackets

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Example Race Redo

• Review the revised project done with Vector of
  runners
• Vector runners used to store individual runner
  objects
• No maximum size vector grows as needed
• The size of the vector the number of runners
• Referencing individual objects is harder
• Method calls
• Casting
• Run in BlueJ

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Inheritance
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Inheritance

• What is Inheritance?
• What does RX-15 mean?
• If we knew that RX-15 belonged to the bicycle
  class, we would have a pretty good idea
• If we further knew that it belonged to the racing
  bike sub-class, we would have even more
  information
• The subclass RacingBike "inherits" both data
  values and methods from the class Bicycle
• Instance variables currentSpeed, color
• Instantiated method changePedalingSpeed

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Hierarchy of the Bicycle Class
Bicycle
MountainBike
RacingBike
TandemBike

• numWheels might be a final static variable in
  Bicycle (2)
• numGears might be an instance variable in Bicycle
• MountainBike, RacingBike, and TandemBike
  subclasses inherit these fields as well as
  methods from Bicycle. They may contain additional
  fields and methods as well.

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Inheritance(Animated Illustrations)

• java inherit.InheritLab

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Hierarchies
Things
Living Things
Non-living Things
Animals
Plants
?
?
?
?
Invertebrates
Vertebrates
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Why Inheritance?
Superclass Data fields Methods

• If we need a class that is similar to another
  class, we could copy the class and change it
• --- OR ---
• We could EXTEND the class by creating a subclass
• The subclass can have additional data fields and
  methods
• The subclass inherits the data fields and methods
  of the superclass

Subclass1 Data fields Methods
Subclass2 Data fields Methods
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Relationships

• "Has A" (not inheritance)
• One class has an object of another class as an
  instance variable
• Examples
• The Race class had a vector of objects of the
  Runner class
• A Car class has a gas tank which was an object of
  the Tank class
• "Is A" (inheritance)
• One class is a subclass of another class

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Using Inheritance

• define one superclass Item
• define subclasses for Video and CD
• the superclass defines common attributes
• the subclasses inherit the superclass attributes
• the subclasses add their own attributes

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Inheritance in Java
no change here
public class Item ...
change here
public class Video extends Item ...
public class CD extends Item ...
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Adding more item types
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Superclass
public class Item private String title
private int playingTime private boolean
gotIt private String comment //
constructors and methods omitted.
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Subclasses
public class CD extends Item private String
artist private int numberOfTracks //
constructors and methods omitted.
public class Video extends Item private
String director // constructors and methods
omitted.
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Superclass Constructor
public class Item private String title
private int playingTime private boolean
gotIt private String comment /
Initialize the fields of the item. /
public Item(String theTitle, int time)
title theTitle playingTime time
gotIt false comment ""
// methods omitted
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Superclass constructor call

• Subclass constructors must always contain a
  'super' call.
• If none is written, the compiler inserts one
  (without parameters)
• works only if the superclass has a constructor
  without parameters
• Must be the first statement in the subclass
  constructor.

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Subclass Constructor
public class CD extends Item private String
artist private int numberOfTracks /
Constructor for objects of class CD
/ public CD(String theTitle, String
theArtist, int tracks, int time)
super(theTitle, time) artist
theArtist numberOfTracks tracks
// methods omitted
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Deeper hierarchies
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Refinement vs. Replacement

• A constructor method refines the constructor of
  the super class
• Any new or hidden fields in the subclass are
  defined by the subclass constructor, "global"
  values are established by the super class
• Regular methods replace super class methods with
  the same "signature"

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Subclasses Superclasses

• A subclass extends another class
• A subclass inherits non-private fields (data) and
  methods (behavior) from all of its ancestors
• If a variable, say xyz, is defined within a
  subclass with same name as one in direct
  ancestor, the new definition is said to hide the
  definition in the super class
• If we still want to be able to refer to both
  variables, we can reference super.xyz

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Hiding Member Variables

• class Suppper
• double aNumber
• . . .
• class Subbie extends Suppper
• int aNumber
• . . .
• aNumber (int)super.aNumber
• Recall issues of Scope

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Super-Duper Class

• All classes are eventually derived from Object
• As such, they should support standard Object
  methods
• toString()
• Printable version of data structure
• equals(obj)
• See if contents are equal as opposed to references

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The Object class
All classes inherit from Object.
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"Object" Methods

• Methods in Object are inherited by all classes.
• Any of these may be overridden.
• The toString method is most commonly overridden
• public String toString()
• Returns a string representation of the object.

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Overriding toString
public class Item ...   public String
toString() String line1 title
" (" playingTime " mins)")
if (gotIt) return line1 " "
comment) else return line1 " "
comment) // end of toString for Item
...
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Overriding Applet Methods

• The init and paint methods are pre-defined in the
  built-in class Applet, we override these methods
  when we define an Applet
• public class InputOutputApplet extends Applet
• public void init()
• yadda yadda
• // end of init method
• public void paint(Graphics g)
• moreYadda
• // end of paint method
• // end of InputOutputApplet class
• BTW, the built-in init and paint don't do anything

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Overriding vs Overloading

• A method is overloaded if it has multiple
  definitions that are distinguished from one
  another by having different numbers or types of
  arguments
• A method is overridden when a subclass gives a
  different definition of the method with the same
  number and types of arguments

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Inheritance Summary

• Inheritance helps with
• Avoiding code duplication
• Code reuse
• Easier maintenance
• Extendibility

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to the Computers

• Demo (or get help with) HW

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Java BREAK
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Monsters, Inc.

• Develop an inheritance hierarchy for monsters
• Each subclass should be more specialized than its
  superclass
• Design a Monster superclass
• All monsters should have ?
• All monsters should be able to do ?

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Monster ProjectBlueJ

• Review the Monsters Inc project in BlueJ
• Review the Monster super class
• Instance variables
• Constructor
• Methods (including toString)
• Review the MonsterApp and MonsterMixer classes
• What is the relationship between a MonsterMixer
  and a Monster? (is a or has a)

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Implementing SubClasses

• A subclass extends its superclass
• Heading for a subclass
• public class className extends superclassName
• Where have we seen this?
• Subclasses may have additional data fields and
  methods
• Design a subclass of Monster
• Data fields Do we need to repeat those in
  Monster?
• Methods Do we need to repeat those in Monster?
• What about toString? Do we write another from
  scratch?
• Constructor Do we need to assign all the
  instance variables or can we call the constructor
  from Monster?

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Inheritance and Constructors

• Can call the constructor of the super class
• Must be the first statement in the constructor
• Syntax
• super( arguments )
• Can also call other methods of the super class by
  using the keyword super
• Example
• super.toString()

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Overriding

• All monsters can move, but do they move in the
  same way?
• Can a subclass have its own move method?
• If so, which method would be called?
• Method overriding
• A method defined in a subclass blocks the
  execution of a method with the same signature in
  its superclass
• View new move methods for some subclasses
• Test in the mixer

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Overloadingreview

• We have written classes that have multiple
  constructors
• Default constructor with no parameters
• Another constructor with parameters for the
  instance variables
• Why does this work?
• Overloading allows us to write multiple methods
  in a class with the same name.
• Each method must have a unique signature.
• The signature consists of the method name and the
  parameter list.
• Review second scare method in the Monster class
• integer parameter specifies the number of times
  to write the scare message.
• Test it.

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Subtyping and Assignment
subclass objects may be assigned to superclass
variables
Vehicle v1 new Vehicle() Vehicle v2 new
Car() Vehicle v3 new Bicycle()
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Casting

• Why cant Elmer breathe?
• Elmer instantiated as type LandMonster
• Elmer declared to be of type Monster
• Monsters cant breathe
• Can cast elmer as a LandMonster
• (LiveMonster) elmer
• produces an anonymous reference to a LiveMonster
  object
• can use it to call breathe method
• ((LiveMonster) elmer).breathe()
• Try it in project
• Why did we do this?

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Array of Monsters

• Suppose we want to use an array to hold the
  collection of monsters for the mixer
• All elements of the array must be of the same
  type
• Can all be of type Monster
• Instantiate them as objects of their specialized
  subclasses
• See Monsters Inc Array
• Do the monsters report on their state correctly?
• Which toString method is used?
• Why?

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instanceof

• Suppose we want to have the specialized monsters
  perform their specialized tasks
• e.g. have the LiveMonsters breathe
• Can we call the breathe method for all monsters?
• To detect the data type of an object, use the
  instanceof operator
• object instanceof Classname
• result is either true or false
• See mixer in "Monsters Inc - Array"

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Polymorphic Variables

• Object variables in Java are polymorphic.(They
  can hold objects of more than one type.)
• They can hold objects of the declared type, or of
  subtypes of the declared type.
• Recall that Vectors hold Objects, and subtypes of
  objects

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Polymorphism

• As in C, we can overload a method by defining
  each version with a different signature.
• Signature Combination of method name and type
  and number of parameters
• True polymorphism The same method name with the
  same types and same number of parameters but with
  different results depending on which subclass the
  object belongs to.

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Design Hints for Inheritance

• Place common operations and fields in superclass
• Put sender field in Message class, not
  TextMessage and VoiceMessage subclasses
• Use inheritance to model the "is-a" relationship
• Consultant gets paid, just like employees. But
  consultant is NOT an employee, shouldn't be
  subclass
• Don't use inheritance unless all inherited
  methods make sense
• If class Holiday extends Day, day.advance()
  wouldn't make sense with Holiday
  laborDay.advance()
• Use polymorphism, not type information
• If (x is type1) action1() else if (x is type2)
  action2() etc.should use polymorphic x.action()
  if action1 and action2 represent a common concept.

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HW Assignment 6

• "Wheel of Fortune"
• Create 5 or more puzzles
• Help getting started (see handout)

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Test 2 in One Week

• Sample Test on Course Web Site
• Go To Course Schedule
• Answers on my Bulletin Board
• Please leave there for other students

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Java BREAK