MIT AITI 2003 Lecture 12

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MIT AITI 2003 Lecture 12

• Inheritance

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What is Inheritance?

• Real World Example We inherit traits from our
  mother and father. We also inherit traits from
  our grandmother, grandfather, and ancestors. We
  might have similar eyes, the same smile, a
  different height.but we are essentially
  derived from our parents.
• In Software, object inheritance is more well
  defined! Object types that are derived from other
  object types resemble their parent classes by
  inheriting/copying both state/variables and
  behavior/methods.

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Picture Representation
Both variables and methods are inherited
String name void eat()
FOOD
String name void eat() int juiceLevel
inherited
FRUIT
String name int juiceLevel void eat() void
slice()
APPLE
inherited
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Inheritance

• All Java classes inherit implicitly from Object
• Classes that inherit from other classes can call
  methods from those classes, and can access their
  data members and methods (except for those that
  are private)
• Inheritance is useful because it allows you to
  reuse functionality and to manage complexity

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Inheritance Example
Object

• class A
• int x
• class B extends A
• int y
• class C extends B
• int z

A
B
C
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Inheritance Example
x
Instance of A has just A's fields. Its type is A
(and Object).
Instance of B has fields from A and B. Its type
is A and B (and Object).
Instance of C has fields from A, B and C. Its
type is A, B and C (and Object).
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Inheritance Example

• static public void main(String args)
• A aa new A()
• A ab new B() / a B is also an A /
• A ac new C() / a C is also an A /
• B bb new B()
• B bc new C()
• C cc new C()
• / not legal - an A is not a B! /
• // B ba new A() / compile error /

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Advantages of Inheritance

• Complex classes and data types are already
  provided in java packages / libraries. We want to
  reuse these classes, and modify to our
  requirements
• java.io input/ouput
  operations
• java.swing GUI components
• java.awt.geom 2D geometric objects
• java.applet Applets embedded in
  web pages
• java.util Data Structures
  collection classes (Set, List,Tree, Vector)
• java.sql Supports Database
  access
• Also we want to write very abstract classes and
  then specialize (abstract classes). Example We
  write an abstract shape class, and then keep on
  adding new shapes (next lecture).

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Type Hierarchy
Object-oriented systems allow classes to be
defined in terms of other classes. For example, a
square, triangle, and circle are all shapes. In
object-oriented terminology, a square/triangle/cir
cle are subclasses of the shape class. Similarly,
the shape class is the superclass of all shape
objects. Shape objects are also Objects.
Object
Class/Type Hierarchy
Inherit State and Behavior Specialize
Shape
Point
Circle
Triangle
Square
Pixel
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Reusing Classes

• We wrote a Point class. Now we want a ColorPoint
  class or a GraphicsPixelPoint class.
• We can just use existing variables/methods in
  the Point class (inherit these) and then write
  more code to add new functionality (extend the
  base class). We simply add a color field to our
  Point, or other graphics information for a pixel
  point.
• We can use 2D geometry package classes already in
  java.awt.geom, and extend these classes to have
  specialized behavior. (Dont need to reinvent
  Line, Circle, Rectangle classes.)

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Less Elegant Way of reuse

• public class ColorPoint
• private Point point
• private Color color
• // We wrap a color field around a Point
  object.
• public ColorPoint(Point p, Color color)
• point p
• this.color color
• DELEGATION

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Extending the Point Class/Type

• public class ColorPoint extends Point
• private Color color
• public ColorPoint(double x, double y, Color
  color)
• super(x, y) // Make into a
  Point first.
• this.color color // Add info for
  new ColorPoint.
• INHERITANCE

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Constructors Example I
public class Shape Shape()
System.out.println(I am a Shape) public
class RoundShape extends Shape RoundShape()
System.out.println(I am a Round Shape)
public class Circle extends RoundShape
Circle() System.out.println(I am a
Circle) public static void main(String
args) Circle x new Circle()

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Output of Program

• I am a Shape
• I am a Round Shape
• I am a Circle
• Default Constructors for superclasses called.
• When we construct a Circle, we are first making
  it a Shape, then a RoundShape, and finally a
  Circle.

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Constructor Example II
class Plant private String kingdom
private String genus private String
species private boolean poisonous
public Plant(String g, String s, boolean p)
kingdom "Plantae" genus g
species s poisonous p We
write a base Plant class. All Plants are kingdom
Plantae. Specific objects of the Plant type have
a genus and species.
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continued
class Tree extends Plant private double
crownSize private double trunkSize
public Tree(String g, String s, double cs, double
ts) super(g, s, false)
crownSize cs trunkSize ts An
object of Type Tree is also a Plant. Hence, it is
of kingdom Plantae. We first call the super class
constructor to make our object a Plant. Then we
add crownSize and trunkSize modifications to make
our object a Tree.
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Hiding fields
class SuperClass int inheritData
double data class SubClass extends SuperClass
float data SubClass inherits
inheritData. However, data is defined twice. In
SubClass, we now have two data fields (one
hidden) float data is newly defined. super.data
can also be accessed in subclass.
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Overriding Methods
class SuperClass boolean isTrue()
int totalSum(int x, int y) class
SubClass extends SuperClass //
OVERRIDE totalSum method above, and add
something. int totalSum(int x, int y)
super.totalSum(x, y) // Can Call
Superclass method. // Do something more
now, MODIFY System.out.println(I can now
add modifications.)
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Access Private,Public,Protected,no access
specified (package)

• Super Class may contain members (methods or data)
  of type
• Private
• Know About This Access only internally in class.
• Protected (a little unsafe, tricky, so avoided)
• Access
• To Methods of all Subclasses.
• To all Classes in same package
• Package (no access specified)
• Access by methods of classes in same package
• Public
• Know about this Access to all classes
  everywhere.

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Final and Static

• In SuperClass, can define a variable or method as
  final and static
• Static If subclass has access, then it will
  inherit the static variable or static method and
  it will be shared between all objects of a
  derived class AND base class.
• Final If a variable, method, or class is defined
  as FINAL, then it cannot be overriden in a
  subclass.

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Casting Objects
From the inheritance concept, we now also can
CAST objects into different types. OBJECT ?
POINT ? COLORPOINT // Dont need Cast if
assigning a superclass type. Point p new
Point(4.4, 5.6) Object pHolder
(Object)p Object pHolder p //
Could just write this statement. // Need Casting
if assigning a subclass type. Object x new
Point(3.0, 4.4) Point xPoint (Point) x
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When Casting is Needed
OBJECT ? POINT ? COLORPOINT In general, dont
need to cast unless we need methods of the
subclass type. Object o new ColorPoint(4.4,
3.4, new Color(red)) Can use o.toString()
and will get appropriate method. Cant use
o.getX() since o has object type, not point. So
cast to use getX() method.
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Continued
OBJECT ? POINT ? COLORPOINT Object o new
ColorPoint(4.4, 3.4, new Color(red)) Cant
use o.getX() Using Casting to Point Now Can
Use ( ( Point ) o ).getX() OR can use
ColorPoint cast to use very specific
methods ((ColorPoint) o).getColor()
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Casting Example
Vector v new Vector() v.addElement(new
Rectangle(..)) v.addElement(new
Square()) v.addElement(new Circle()) for
(int k 0 k v.elementAt(k).toString()
((v.elementAt(k)) Shape).draw() // Cast
needed
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Casting - Summary
In general casting is not encouraged becomes
too confusing. A programmer should design
superclasses properly so we dont need to call
specific methods all the time. But, many
advantages (previous Vector exercise) We can
draw all types of different shapes just by using
a general draw method. We dont have to call each
rectangle, circle, square draw method separately.
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Programming Example

• A Company has a list of Employees. It asks you to
  provide a payroll sheet for all employees.
• Has extensive data (name, department, pay amount,
  ) for all employees.
• Different types of employees manager, engineer,
  software engineer.
• You have an old Employee class but need to add
  very different data and methods for managers and
  engineers.
• Suppose someone wrote a name system, and already
  provided a legacy Employee class. The old
  Employee class had a printData() method for each
  Employee that only printed the name. We want to
  reuse it, and print pay info.

Borrowed with permission from Course 1.00 Notes
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REVIEW PICTURE
Message passing
Main event loop
Encapsulation
Employee e1
public Main() Employee e1(Mary,Wang) ...
e1.printData() // Prints Employee names. ...
printData
private
lastName firstName
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Employee class
class Employee // Data private String
firstName, lastName // Constructor
public Employee(String fName, String lName)
firstName fName lastName lName
// Method public void printData()
System.out.println(firstName " " lastName)
This is a simple super or base class.
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Inheritance
Already written
Class Employee
firstName lastName
printData()
is-a
is-a
Class Engineer hoursWorked wages
Class Manager salary
firstName lastName
firstName lastName
printData() getPay()
printData() getPay()
You next write
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Engineer class
Subclass or (directly) derived class
Class Engineer extends Employee private
double wage private double hoursWorked
public Employee(String fName, String lName,
double rate, double hours)
super(fName, lName) wage rate
hoursWorked hours public
double getPay() return wage
hoursWorked public void printData()
super.printData() // PRINT NAME
System.out.println("Weekly pay " Wage
hoursWorked)
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Manager class
Subclass or (directly) derived class
class Manager extends Employee private
double salary public Manager(String fName,
String lName, double sal) super(fName,
lName) salary sal public
double getPay() return salary
public void printData()
super.printData() System.out.println("Mon
thly salary salary)
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Inheritance
Class Manager Salary
firstName lastName
is-a
printData getPay
Class SalesManager
firstName lastName
Salary
printData getPay
salesBonus
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SalesManager Class
class SalesManager extends Manager private
double bonus // Bonus Possible as
commission. // A SalesManager gets a constant
salary of 1250.0 public SalesManager(String
fName, String lName, double b)
super(fName, lName, 1250.0) bonus b
public double getPay() return
1250.0 public void printData()
super.printData() System.out.println(Bonu
s Pay " bonus
Derived class from derived class
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Main method
public class PayRoll public static void
main(String args) // Could get Data from
tables in a Database. Engineer fred new
Engineer(Fred", "Smith", 12.0, 8.0) Manager
ann new Manager("Ann", "Brown", 1500.0)
SalesManager mary new SalesManager("Mary",
Kate", 2000.0) // Polymorphism, or late
binding Employee employees new
Employee3 employees0 fred
employees1 ann employees2 mary for
(int i0 i
()
Java knows the object type and chooses the
appropriate method at run time
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Output from main method
Fred Smith Weekly pay 96.0 Ann Brown Monthly
salary 1500.0 Mary Barrett Monthly salary
1250.0 Bonus 2000.0 Note that we could not
write employeesi.getPay() because getPay()
is not a method of the superclass Employee. In
contrast, printData() is a method of Student, so
Java can find the appropriate version.