Object Oriented Programming

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Object Oriented Programming

• Lecture 2
• Inheritance, Polymorphism, First Design pattern,
  Interface, Exceptions, Enumeration Types
• www2.hh.se/staff/jebe/oop2006/

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Abstract Data types

• We have seen that we can define new data types by
  creating new classes
• A Class implements a Contractual Interface (the
  public parts of the class)
• A Class has a state representation (variables
  should preferrably be private)
• The implementation of the class
• Implement the methods (accessors,mutators)
• Implement methods for testing (repOk())

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The Rational Class

• Desired functionality? (The contractual
  interface, ie the methods required)
• Constructors (establish the invariant)
• Rational(), Rational(int num), Rational(int
  num,int den)
• Arithmetic operations (methods)
• Plus(Rational a, Rational b)
• Minus(Rational a, Rational b)
• Times(Rational a, Rational b)
• Div(Rational a, Rational b)
• Other methods
• toString(), equals(Object x), compareTo(Object
  x), repOk()

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The Rational Class

• The implementation
• Class Rational
• private int num
• private int den
• public Rational()
• this(0,1)
• public Rational(int num, int den)
• if(den 0) throw ArithmeticException(Zero
  denominator)
• else
• this.num denlt0 ? num,num
• this.den Math.abs(den)
• simplify()

State representation, hidden (encapsulated)
Make sure the invariant is established!!
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Extending Rationals to Complex Numbers

• Complex numbers is an extension of Rational
  numbers
• The real and imaginary components are Rationals!
  (1/2 j1/2)
• Complex arithmetic can be formed as a composite
  of Rational arithmetic (1/2 j1/2) (1/2 j1/2)
  (1/1 j1/1)
• The idea We let Complex inherit Rational
  properties by extending the Rational Class

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Extending to Complex Numbers

• public class Complex extends Rational
• //State representation
• private Rational real private Rational imag
• / Constructor for injecting Rational Complex
  vector components /
• public Complex(Rational re, Rational im)
• real re
• imag im
• / Arithmetic Plus for Complex valued numbers
  /
• public Complex plus(Complex c1, Complex c2)
• return new Complex(plus(this.real,c.real()),plus
  (this.im,c.im()))
• ...
• ...

Plus in Complex overloads the inherited plus in
Class Rational
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Recapitulation

• When we create software models of real world
  systems, we use Classes to define
• Abstract Data Types (immutable, variables for
  representing the abstract data)
• Objects (mutable, instance variables that hold a
  state, methods implements the algorithms)
• In Java - Integer, Float, Double (Wrapper
  classes) are example of immutables
• Wrapper Classes are used to wrap primitive
  datatypes into objects when needed

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Recapitulation

• In both cases important terms are
• Hiding the implementation (encapsulation)
• Prevents misuse
• Programming to an interface (Abstraction)
• Modularity, Extendability, Reusability...
• Establishing and preserving the invariant
• Objects must be in a legal state after creation
  and after manipulation by a method
• The equals method
• To compare State equality of objects

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Interfaces

• Interface is a special type of Class
• An Interface declares only abstract methods that
  must be defined in a class
• Thus, an interface has no implemented methods
  itself methods are defined in the implementing
  class
• Only constants are allowed to be specified
• Interfaces can inherit from other Interfaces
• Super Interfaces and Sub Interfaces

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Interface Example

• Interface Drawable
• public int aValue 1
• public void draw(Graphics g)
• public int getWidth()
• public int getHeight()
• Public Class Square implements Drawable
• int xSide,ySide,posX,posY
• public void draw() g.drawRect(posX,posY,xSide,y
  Side)
• public int getWidth() return ySide
• public int getHeight() return xSide

Constants allowed
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Abstract Classes

• An abstract class has at least one method
  implemented
• Other methods can be abstract
• Thus, and abstract class is a class with partial
  implementation
• Like with an Interface, abstract methods must be
  implemented by the class that extends the
  abstract class

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Abstract Class Example

• public abstract class Shape
• protected Point location
• protected Shape(Point loc) location loc
  
• public void move(int dx,int
  dy)location.move(dx,dy)
• public abstract void draw()
• Public Class Square extends Drawable
• int xSide,ySide
• public Square(int x,int y) super(new
  Point(x,y))
• public void draw() g.drawRect((int)location.get
  X(), (int)location.getY(), xSide, ySide)
• public int getWidth() return ySide
• public int getHeight() return xSide

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Polymorphism

• The term polymorphic origins from Greek
• Poly many
• Morphos forms/shapes
• Roughly, polymorphic objects are objects with
  features of a similar kind
• That is - objects that shares an interface but
  has different algorithms

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Example - Polymorphic shapes in a drawing program
Common functionality for shapes...
Shape
Circle
Square
Rectangle
...how to draw a shape is defined in each
specific object of shape type
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Conversion between types

• Conversion of a subtype to a supertype is called
  widening
• Widening is always allowed to do
• Conversion of a supertype to a subtype is called
  narrowing (downcasting)
• Allowed at compile time, not always safe
• Dynamic binding
• The specific method is bound at runtime
• In the shape example, method draw is bound to a
  specific shape type during run time
• Let us see an example from an old exam...

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A few things about Throwables Exceptions

• Exceptions are unexpected or faulty conditions in
  a program
• When a fault occurs, an exception can be thrown
  and the normal program flow is interrupted
• Three kinds of Throwables
• Error (severe!)
• RuntimeException (for example array out of
  bounds)
• Exception (User defined exception should extend
  this)

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Exceptions
Serious Fault!
OutOfMemoryError
Thrown By JVM!
Error
ArithmeticException
AssertionError
Throwable
ClassCastException
RuntimeException
IndexOutOfBoundsException
NullpointerException
Exception
Io-Exception
Any program!
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Catching Exceptions

• Exceptions are caught handled with a
  try-catch-clause
• Try
• a bi
• catch (ArrayOutOfBoundsException e)
• exception handling code here...
• finally...clean up before leaving (...finally
  is not necessary)
• User defined exceptions must extend class
  Exception
• MyVeryOwnException extends Exception
• super(A terrible error has occured!)

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Throwing Exceptions

• Most Exceptions are thrown by the JVM
• But Exceptions can also be thrown anywhere in a
  program
• Public int example(void) throws MyException
• method code...
• throw new MyException()
• Method calls to methods that has a throws
  statement must be encapsulated with try-catch
  clause
• The exception should be handled in catch or be
  thrown to the object that was calling the method.

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A new type Pen Mutable Object

• What functionality do we want when using a
  pen(interface)?
• Constructor
• pen()
• Methods for changing the state of a pen(instance
  methods)
• void lift()
• void push()
• void move(double d)
• void turn(double alpha)
• Methods for assigning a drawing area to our pen
• void setPaper(Paper p)

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Pen is a mutable object

• Pen thePen
• ...
• public void recLine(int n, double length)
• if (n0)
• thePen.move(length)
• else
• int m n-1
• double shorter length/3.0
• double alpha Math.PI/3
• recLine(m,shorter)
• thePen.turn(-alpha)
• recLine(m,shorter)
• thePen.turn(2alpha)
• recLine(m,shorter)
• thePen.turn(-alpha)
• recLine(m,shorter)

Modifies the state of the pen itself! Pen is
mutable
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Lets take a look atmove(double d)

• public void move(double d)
• double oldx x
• double oldy y
• x Math.min(xdMath.cos(alpha),paper.width())
• y Math.min(ydMath.sin(alpha),paper.height())
  
• if(down)
• paper.drawLine((int)oldx,(int)oldy,(int)x,(int)y
  )

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Our first design patternObserver-Observable

• Now, lets examine the properties of a Paper.

width
A paper is a Canvas that has height and width
dimensions in pixels. It also has a paint method
to show the lines drawn by the Pen.
height
Canvas is an AWT component
The view of a paper!
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Observer ObservableSeparation of model and view

• Paper (Model)
• Add a new line
• Width
• Height
• Add Observer
• Register changes
• Notify the Observers

• Paperview (View)
• Override the paint method so that it paints the
  state of the model (lines)
• Define the method update so a change in the model
  will be reflected in the view

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Extending Observable

• The Observable class implements methods for
  keeping and interacting with a set of Observers
• Adding Observers to the Set
• Notifying Observers of changes by calling the
  Observers update method
• By extending Paper with Observable we will have
  access to all necessary methods
• We only need to know which are the Observers

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The Paper Class Observable

• public class Paper extends Observable
• private Vector lines
• private int width, height
• public Paper(int width, int height)
• this.width width
• this.height height
• lines new Vector()
• public void drawLine(int a, int b, int c, int
  d)
• lines.add(new Line(a,b,c,d))
• setChanged()
• notifyObservers()
• ...

We have added a line...
Register the change in Observer!
Notify the Observers!
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The PaperView Class

• public class PaperView extends Canvas implements
  Observer
• private Paper paperModel
• public PaperView(Paper p)
• paperModel p
• paperModel.addObserver(this)
• public void update(Observable o, Object arg)
• tryThread.sleep(200)catch(Exception
  e)
• repaint()
• public void paint(Graphics g)
• Iterator iter paperModel.iterator()
• while(iter.hasNext())
• Line line (Line)iter.next()

Called when notifying observers!
How we paint the lines!
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The Java I/O Framework

• The I/O framework is designed to be flexible and
  easy to configure
• Java has two main types of I/O
• Stream IO
• Supports reading/writing sequentially
• A stream can only be open for read or write, not
  both
• Random Access I/O
• Reading and writing at any position in a file
• File I/O can be open both for read/write
• We focus at Stream I/O
• Two kinds of streams bytes or charachter

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I/O Byte Streams

• The basic I/O streams are
• Input Stream
• read() - reads byte
• read(byte a) - reads byte array
• read(byte a, int off,
• int len) - reads part of byte array
• skip(long n) - skips over n bytes
• close() - closes the stream
• OutputStream
• write(b) - writes byte
• write(byte a) - writes byte array
• write(byte a, int off,
• int len) - writes part of byte array
• close() - closes the stream

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Reading from files

• FileInputStream/FileOutputstream are sub classes
  of Input/OutputStream byte streams (overloads the
  methods)
• But byte streams are hardly convenient to use
  directly, requires low-level manipulation
• Bitmasking and conversion
• This kind of low-level is rarely necessary
• Most data are represented by Objects

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Buffered Streams

• BufferedInputStream and BufferedOutputStream
  (extends java.io.FilterInputStream)
• Reads data via an internal buffer
• BufferedReader and BufferedWriter (extends
  java.io.Reader)
• Abstract streams for reading String/Charachter
  data
• Can be used to read entire Strings

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Object serialization

• Java supports Object serialization
• Objects can be passed as arguments to a stream
• The objects are transparently serialized to a
  sequence of bytes
• Objects must implement Serializable interface
  (java.io.Serializable)
• ObjectInputStream and ObjectOutPutStream
• FileInputStream/FileOutPutStream as arguments for
  file access

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Enumeration Types

• Enumeration types
• A finite set of distinct values
• Example of Enumerated types
• Date MonTueWedThuFriSatsun
• Direction NorthEastSouthWest
• We can use the Enumerated Types like any other
  type in our program
• Ex. Date d Monday
• The Enumerated types should be typesafe

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Enumerated types

• Some languages like C/C support enumerated
  types...
• In C or C, we would write
• enum Date Mon,Tue,Wed,Thu,Fri,Sat,Sun
• Date day1 Tue
• Date day2 Fri
• If(day1 day2)....
• Untill Java 1.5 was released, Java didnt support
  typesafe enumerated types
• Before 1.5 a common (but bad) solution Treat
  enumeration types as static int constants.

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A Type safe Idiom

• Type-Safe Enumeration Idiom
• Each value will be distinct (unique)
• A printable, descriptive name rather that just an
  Integer number
• Lets look at two examples
• An unordered Type-Safe Enumeration
• An ordered Type-Safe Enumeration

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Typesafe enumeration in Java 1.5

• Public class Card
• public enum Rank Deuce, Three, Four, Five,
  Six, Seven, Eight, Nine, Ten, Jack, Queen,
  King, Ace
• public enum Suit Clubs, Diamonds, Hearts,
  Spades
• private final Rank rank
• private final Suit suit
• private Card(Rank r, Suit s)
• rank r suit s
• public toString()

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So far in the course...

• We have defined
• Classes and Objects
• Interface and Abstract classes
• Inheritance and Polymorhpism
• Mutable and Immutable objects
• A First Design pattern
• Exceptions
• Enumeration types

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That was it for today...

• Pen and Paper example as an exercise next week
• Type safe enum can be read about in JDK 5.0
  documentation
• Next week
• More design patterns
• The Collections Framework
• Etc...