reuse_hiearchies.ppt

James Tam
Code Reuse Through Hierarchies
You will learn about different ways of
creating class hierarchies to better
organize and group attributes and
methods in order to facilitate code
reuse

James Tam
Review: Associations Between Classes
•One type of association relationship is a ‘has-a’ relation (also
known as “aggregation”).
-E.g. 1, A car <has-a> engine.
-E.g. 2, A lecture <has-a> student.
•Typically this type of relationship exists between classes when a
class is an attribute of another class.
public class Car
{
private Engine anEngine;
private Lights carLights;
public start ()
{
anEngine.ignite ();
carLight.turnOn ();
}
}
public class Engine
{
public boolean ignite () { .. }
}
public class Lights
{
private boolean isOn;
public void turnOn () { isOn =
true;}
}

James Tam
A New Type Of Association: Is-A (Inheritance)
•An inheritance relation exists between two classes if
one class is one type of another class
Vehicle
Motorcycle Bus
Car
Mini Full sized
SUV Sports car

James Tam
What Is Inheritance?
•Taking the attributes/methods of an existing class.
Existing
class
•Attributes
•Behaviors
Inheritance: Represented as
sets
Existing class
New class
Inheritance: UML
representation
New class
•New attributes
•New behaviors
•Extend the existing class with a new class
-All non-private data and methods of the existing class are available to
the new class (but the reverse is not true).

James Tam
Inheritance Terminology
Parent class
Child class
Superclass
Subclass
Generalization
Specialization

James Tam
Explanation For Some Of The Terms: Set Theory
All people on the earth
All people in Canada
(Superset - bigger group)
(Subset - smaller group)
Superclass (Button)
Subclass (RadioButton)

James Tam
Explanation For Some Of The Terms:
Providing An Example
Generalization
Specialization
Medical
Doctor
Dermatologist
Opthamologist
Psychiatrist

James Tam
When To Employ Inheritance
•If you notice that certain behaviors or data is common among a
group of related classes.
•The commonalities may be defined by a superclass.
•What is unique may be defined by particular subclasses.

James Tam
When To Employ Inheritance
Monster
• Name
• Damage it inflicts
• Damage it can sustain
• Speed
• Move
• Make
sound
Flying monsters Stone based monsters

James Tam
Where Should Attributes And Behaviors Be
Defined?
•Rule of thumb: Put them high enough in the inheritance
hierarchy so that all the appropriate sub-classes have access to
the attribute or behavior.
•Example:
Monster
• Name
• Speed
• Move
• Make sound
Common to
all monsters
in the game

James Tam
Where Should Attributes And Behaviors Be
Defined? (2)
•Rule of thumb: Don’t put attributes and behaviors higher than
they need to be in the inheritance hierarchy otherwise some
classes will track information or perform actions that don’t
make sense.
Monster
• Name
• Speed
• Move
• Make sound
• Fly

James Tam
Using Inheritance
Format:
public class <Name of Subclass > extends <Name of Superclass>
{
// Definition of subclass – only what is unique to subclass
}
Example:
public class Dragon extends Monster
{
public void displaySpecial ()
{
System.out.println("Breath weapon: ");
}
}

James Tam
The Parent Of All Classes
•You’ve already employed inheritance
•Class Object is at the top of the inheritance hierarchy
•Inheritance from class Object is implicit
•All other classes inherit it’s data and methods
-e.g., “toString” are available to it’s child classes
•For more information about this class see the url:
http://java.sun.com/j2se/1.5/docs/api/java/lang/Object.html

James Tam
Levels Of Access Permissions
•Private “-”
-Can only access the attribute/method in the methods of the class where the
attribute is originally defined.
•Protected “#”
-Can access the attribute/method in the methods of the class where the
attribute is originally defined or the subclasses of that class.
•Public “+”
-Can access attribute/method anywhere in the program.

James Tam
Summary: Levels Of Access Permissions
Access
level
Accessible to
Same class Subclass Not a subclass
Public Yes Yes Yes
Protected Yes Yes No
Private Yes No No

James Tam
Levels Of Access Permission: An Example
public class P
{
private int num1;
protected int num2;
public int num3;
// Can access num1, num2 & num3 here.
}
public class C extends P
{
// Can’t access num1 here
}
public class Driver
{
// Can’t access num1 here.
}

James Tam
General Rules Of Thumb
•Variable attributes should not have protected access but instead
should be private.
•Most methods should be public.
•Methods that are used only by the parent and child classes
should be made protected.

James Tam
Method Overriding
•Different versions of a method can be implemented in different
ways by the parent and child class in an inheritance hierarchy.
•Methods have the same name and parameter list (identical
signature) but different bodies
public class Parent public class Child extends Parent
{ {
: : : :
public void method () public void method ()
{ {
System.out.println(“m1”); num = 1;
} }
} }

James Tam
Method Overloading Vs. Method Overriding
•Method Overloading
-Multiple method implementations for the same class
-Each method has the same name but the type, number or order of the
parameters is different (signatures are not the same)
-The method that is actually called is determined at program compile time
(early binding).
-i.e., <reference name>.<method name> (parameter list);
Distinguishes
overloaded methods

James Tam
Method Overloading Vs. Method Overriding (2)
•Example of method overloading:
public class Foo
{
public void display () { }
public void display (int i) { }
public void display (char ch) { }
}
Foo f = new Foo ();
f.display();
f.display(10);
f.display(‘c’);

James Tam
•Method Overriding
-The method is implemented differently between the parent and child
classes.
-Each method has the same return value, name and parameter list
(identical signatures).
-The method that is actually called is determined at program run time (late
binding).
-i.e., <reference name>.<method name> (parameter list);
The type of the reference
(implicit parameter “this”)
distinguishes overridden
methods

James Tam
•Example of method overriding:
public class Foo
{
public void display () { … }
: :
}
public class FooChild extends Foo
{
}
Foo f = new Foo ();
f.display();
FooChild fc = new FooChild ();
fc.display ();

James Tam
Accessing The Unique Attributes
And Methods Of The Parent
•All protected or public attributes and methods of the parent class
can be accessed directly in the child class
public class P
{
protected int num;
}
{
public void method ()
{
this.num = 1;
// OR
num = 2;
}
}

James Tam
Accessing The Non-Unique Attributes
And Methods Of The Parent
•An attribute or method exists in both the parent and child class
(has the same name in both)
•The method or attribute has public or protected access
•Must prefix the attribute or method with the “super” keyword to
distinguish it from the child class.
•Format:
super.methodName ()
super.attributeName
• Note: If you don’t preface the method attribute with the keyword “super”
then the by default the attribute or method of the child class will be
accessed.

James Tam
Accessing The Non-Unique Attributes And Methods
Of The Parent: An Example
public class P
{
protected int num;
protected void method ()
{
:
}
}

James Tam
Accessing The Non-Unique Attributes And Methods
Of The Parent: An Example (2)
{
protected int num;
{
num = 2;
super.num = 3;
super.method();
}
}

James Tam
Polymorph
•The ability to take on different forms
Images from the game Dungeon Master by FTL

James Tam
Polymorphism In Object-Orientated Theory
•An overridden method that can take on many forms
•The type of an instance (the implicit parameter) determines at
program run-time which method will be executed.
public class Foo
{
: :
}
public class FooChild extends Foo
{
}

James Tam
A Blast From The Past
Mummy
Scorpion
Dragon
Screamer
Ghost
Knight
Monsters
Weapons
Armour
Falchion
Longbow
Ninjato
Dungeon Master
:

James Tam
The Inheritance Hierarchy For The Monsters
Monster
Undead StoneBased Giggler Dragon

James Tam
The Dragon Sub-Hierarchy
Dragon
Red
Dragon
Blue
Dragon
Halitosis
Dragon

James Tam
Class DungeonMaster
•Example (The complete example can be found in the directory
/home/233/examples/hiearchies/DMExample
public class DungeonMaster
{
public static void main (String [] args)
{
BlueDragon electro = new BlueDragon ();
RedDragon pinky = new RedDragon ();
HalitosisDragon stinky = new HalitosisDragon () ;
electro.displaySpecialAbility ();
pinky.displaySpecialAbility ();
stinky.displaySpecialAbility ();
}
}

James Tam
Class Monster
public class Monster
{
// Many methods and attributes
public void displaySpecialAbility ()
{
System.out.println("No special ability");
}
}

James Tam
Class Dragon
{
{
System.out.print("Breath weapon: ");
}
}

James Tam
Class BlueDragon
public class BlueDragon extends Dragon
{
{
super.displaySpecialAbility ();
System.out.println("Lightening");
}
}

James Tam
Class HalitosisDragon
public class HalitosisDragon extends Dragon
{
{
super.displaySpecialAbility();
System.out.println("Stinky");
}
}

James Tam
Class RedDragon
public class RedDragon extends Dragon
{
{
super.displaySpecialAbility();
System.out.println("Fire");
}
}

James Tam
Changing Permissions Of
Overridden Methods
•The overridden method must have equal or stronger (less
restrictive) access permissions in the child class.
Parent
#method()
Child
+method()
Parent
#method()
Child
-method()

James Tam
Updated Scoping Rules
• When referring to an identifier in the method of a class
1. Look in the local memory space for that method
2. Look in the definition of the class
3. Look in the definition of the classes’ parent

James Tam
Updated Scoping Rules (2)
public class P
{
}
{
{
}
}
<<< First: Local >>>
<<< Second: Attribute>>>
<<< Third: Parent’s attribute >>>

James Tam
Shadowing
•Local variables in a method or parameters to a method have the
same name as instance fields.
•Attributes of the subclass have the same name as attributes of
the superclass.

James Tam
Attributes Of The Subclass Have The Same Name
As The SuperClasses’ Attributes
public class Foo
{
private int num;
public Foo () { num = 1; }
public int getNum () { return num; }
public void setNum (int newValue) {num = newValue; }
}
public class Bar extends Foo
{
public Bar ()
{
num = 10;
}
}

James Tam
As The SuperClasses’ Attributes
public class Foo
{
private int num;
}
{
public Bar ()
{
num = 10;
}
}
Insufficient access
permissions: Program
won’t compile

James Tam
As The SuperClasses’ Attributes (2)
public class Foo
{
private int num;
}
{
private int num;
public Bar ()
{
num = 1;
}
}

James Tam
The Result Of Attribute Shadowing
{
private int num;
public Bar ()
{
num = 10;
}
public int getSecondNum () { return num; }
}
public class Driver
{
public static void main (String [] arv)
{
Bar b = new Bar ();
System.out.println(b.getNum());
System.out.println(b.getSecondNum());
}
}

James Tam
Casting And Inheritance
•Because the child class IS-A parent class you can substitute
instances of a subclass for instances of a superclass.
Monster
Dragon
BlueDragon
You can substitute a
Dragon for a
Monster
You can substitute a
BlueDragon for a
Dragon

James Tam
Casting And Inheritance (2)
•You cannot substitute instances of a superclass for instances of a
subclass
Monster
Dragon
BlueDragon
You cannot
substitute a
Monster for a
Dragon
You cannot
substitute a
Dragon for a
BlueDragon
x

James Tam
Casting And Inheritance: A Previous Example
public class Monster
{
private int protection;
private int damageReceivable;
private int damageInflictable;
private int speed;
private String name;
: : :
public int getProtection () {return protection;}
: : :
}

James Tam
{
{
System.out.print("Breath weapon: ");
}
public void fly ()
{
System.out.println("Flying");
}
}

James Tam
public class BlueDragon extends Dragon
{
{
super.displaySpecialAbility ();
System.out.println("Lightening");
}
public void absorbElectricity ()
{
System.out.println("Absorbing electricity.");
}
}

James Tam
Substituting Sub And Super Classes
•You can substitute an instance of a sub class for an instance of a
super class.
Monster aMonster = new Monster ();
System.out.println(aMonster.getProtection());
System.out.println(electro.getProtection());
Dragon
BlueDragon
Monster
+getProtection ()

James Tam
Substituting Sub And Super Classes
•You cannot substitute an instance of a super class for an
instance of a sub class.
Monster aMonster = new Monster ();
electro.absorbElectricity ();
aMonster.absorbElectricity ();
Monster
Dragon
BlueDragon
+absorbElectricity()

James Tam
Casting And Inheritance
Monster aMonster;
aMonster = electro;
aMonster.fly();
aMonster.absorbElectricity();
aMonster = new Monster ();
electro = aMonster;
electro = (BlueDragon) aMonster;
electro.fly();
electro.absorbElectricity();
x
x
x
x
x
x
Monster
BlueDragon
+absorbElectricity()
Dragon
+fly()

James Tam
•When casting between classes only use the cast operator if you
are sure of the type.
Monster aMonster;
aMonster = electro;
if (aMonster instanceof BlueDragon)
{
System.out.println("AMonster is a reference to an instance of a
BlueDragon");
electro = (BlueDragon) aMonster;
electro.fly();
electro.absorbElectricity();
}

James Tam
•When casting between classes only use the cast operator if you
are sure of the type.
Monster aMonster;
aMonster = electro;
if (aMonster instanceof BlueDragon)
{
System.out.println("AMonster is actually a reference to an instance of
a BlueDragon");
((BlueDragon) aMonster).fly();
((BlueDragon) aMonster).absorbElectricity();
}

James Tam
Another Scoping Example
The complete example can be found in UNIX under:
/home/233/examples/scope:
public class ScopingExample
{
public static void main (String [] args)
{
P p1 = new P ();
C c1 = new C ();
GC gc1 = new GC ();
gc1.method1();
gc1.method2();
gc1.method3();
gc1.method();
}
}

James Tam
Another Scoping Example (2)
public class GC extends C
{
private int num1;
public GC ()
{
num1 = 1;
}
public void method1 ()
{
System.out.println("GC's method 1");
super.method1();
}
{
System.out.println("GC's method 2");
super.method2();
}

James Tam
{
int num0 = 0;
System.out.println("num0=" + num0);
System.out.println("ch=" + ch);
}
{
super.method1();
}
} // End of class GC

James Tam
{
protected int num2;
protected char ch1;
public C ()
{
ch = 'C';
num2 = 2;
}
{
System.out.println("C's method 1");
}
{
System.out.println("C's method 2");
super.method2();
}
} // End of class C

James Tam
public class P
{
protected int num3;
protected char ch;
public P ()
{
ch = 'P';
num3 = 3;
}
{
System.out.println("P's method 1");
}
{
System.out.println("P's method 2");
}
} // End of class P

James Tam
The Final Modifier (Inheritance)
•Methods preceded by the final modifier cannot be overridden
e.g., public final void displayTwo ()
•Classes preceded by the final modifier cannot be extended
-e.g., final public class ParentFoo

James Tam
Why Employ Inheritance
•To allow for code reuse
•It may result in more robust code
Existing class
New class

James Tam
Java Interfaces (Type)
•Similar to a class
•Provides a design guide rather than implementation details
•Specifies what methods should be implemented but not how
-An important design tool and agreement for the interfaces should occur
very early before program code has been written.
-(Specify the signature of methods so each part of the project can proceed
with minimal coupling between classes).
•It’s a design tool so they cannot be instantiated

James Tam
Interfaces: Format
Format for defining an interface
public interface <name of interface>
{
constants
methods to be implemented by the class that realizes this interface
}
Format for realizing / implementing the interface
public class <name of class> implements <name of interface>
{
attributes
methods actually implemented by this class
}

James Tam
Interfaces: A Checkers Example
Basic board
Regular rules
Variant rules

James Tam
Interface Board
public interface Board
{
public static final int SIZE = 8;
public void displayBoard ();
public void initializeBoard ();
public void movePiece ();
boolean moveValid (int xSource, int ySource, int xDestination,
int yDestination);
: : :
}

James Tam
Class RegularBoard
public class RegularBoard implements Board
{
public void displayBoard ()
{
:
}
public void initializeBoard ()
{
:
}

James Tam
Class RegularBoard (2)
public void movePiece ()
{
// Get (x, y) coordinates for the source and destination
if (moveValid (xS, yS, xD, yD) == true)
// Actually move the piece
else
// Don’t move piece and display error message
}
public boolean moveValid (int xSource, int ySource, int xDestination,
int yDestination)
{
if (moving forward diagonally)
return true;
else
return false;
}
} // End of class RegularBoard

James Tam
Class VariantBoard
public class VariantBoard implements Board
{
public void displayBoard ()
{
:
}
public void initializeBoard ()
{
:
}

James Tam
Class VariantBoard (2)
public void movePiece ()
{
// Get (x, y) coordinates for the source and destination
if (moveValid (xS, yS, xD, yD) == true)
// Actually move the piece
else
// Don’t move piece and display error message
}
public boolean moveValid (int xSource, int ySource, int xDestination,
int yDestination)
{
if (moving straight-forward or straight side-ways)
return true;
else
return false;
}
} // End of class VariantBoard

James Tam
Interfaces: Recapping The Example
•Interface Board
-No state (variable data) or behavior (body of the method is empty)
-Specifies the behaviors that a board should exhibit e.g., clear screen
-This is done by listing the methods that must be implemented by classes
that implement the interface.
•Class RegularBoard and VariantBoard
-Can have state and methods
-They must implement all the methods specified by interface Board (but
can also implement other methods too)

James Tam
Specifying Interfaces In UML
<< interface >>
Interface name
method specification
Class name
method implementation
Realization / Implements

James Tam
Alternate UML Representation (Lollipop Notation)
Class name
method implementation
Interface
name

James Tam
Implementing Multiple Interfaces
Class
Interface1 Interface2 Interface3

James Tam
Implementing Multiple Interfaces
Format:
public class <class name> implements <interface name 1>,
<interface name 2>, <interface name 3>…
{
}

James Tam
Multiple Implementations Vs. Multiple Inheritance
•A class can implement multiple interfaces
•Classes in Java cannot extend more than one class
•This is not possible in Java but is possible in other languages
such as C++:
class <class name 1> extends <class
name 2>, <class name 3>…
{
}

James Tam
Multiple Implementations Vs.
Multiple Inheritance (2)
•A class can implement all the methods of multiple interfaces
•Classes in Java cannot extend more than one class
•This is not possible in Java but is possible in other languages
such as C++:
Parent class 1 Parent class 2 Parent class 3
Child class

James Tam
Abstract Classes
•Classes that cannot be instantiated
•A hybrid between regular classes and interfaces
•Some methods may be implemented while others are only
specified
•Used when the parent class cannot define a complete default
implementation (implementation must be specified by the child
class).
•Format:
public abstract class <class name>
{
<public/private/protected> abstract method ();
}

James Tam
Abstract Classes (2)
•Example1:
public abstract class BankAccount
{
protected float balance;
public void displayBalance ()
{
System.out.println("Balance $" + balance);
}
public abstract void deductFees () ;
}
1) From “Big Java” by C. Horstmann pp. 449 – 500.

James Tam
Another Example For An Abstract Class
<< interface >>
Board
+SIZE:int
+displayBoard ()
+initializeBoard ()
+movePiece ()
+moveValid ()
CheckerBoard
{abstract}
+displayBoard ()
+initializeBoard ()
+movePiece ()
+moveValid ()
RegularBoard
+moveValid ()
VariantBoard
+moveValid ()

James Tam
You Should Now Know
•How the inheritance relationship works
-When to employ inheritance and when to employ other types of relations
-What are the benefits of employing inheritance
- How to create and use an inheritance relation in Java
-How casting works within an inheritance hierarchy
-What is the effect of the keyword "final" on inheritance relationships
-Issues related to methods and attributes when employing inheritance
•What is method overloading?
-How does it differ from method overriding
-What is polymorphism

James Tam
You Should Now Know (2)
•What are interfaces/types
-How do types differ from classes
-How to implement and use interfaces in Java
•What are abstract classes in Java and how do they differ from
non-abstract classes and interfaces.
•How to read/write UML notations for inheritance and interfaces.

reuse_hiearchies.ppt

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