Classes and Objects in Java

1
Classes and Objectsin Java
2
Variables and Objects

• Let Circle be a class with
• variable r that indicates its radius
• method area() that computes its area
• Declaration Circle c
• Instantiation c new Circle()
• Usage c.r 5.5
• System.out.println(c.area())

3
The complete Circle class

• public class Circle
• public double x,y // center coordinates
• public double r // radius
• // the methods
• public double circumference()
• return 23.14r
• public double area()
• return 3.14rr

4
Using the Circle class

• public class TestCircle
• public static void main(String args)
• Circle c
• c new Circle()
• c.x 2.0 c.y 2.0 c.r 5.5
• System.out.println(c.area())

5
The this keyword

• this refers to the current object
• In the Circle class, the following definitions
  for area() are equivalent
• public double area() return 3.14 r r
• public double area() return 3.14 this.r
  this.r
• Using the keyword clarifies that you are
  referring to a variable inside an object

6
Constructors

• A constructor is a special type of method
• has the same name as the class
• It is called when an object is created
• new Circle() // calls the Circle() method
• If no constructor is defined, a default
  constructor that does nothing is implied
• Constructors with parameters can be specified

7
A constructor for the Circle class

• public class Circle
• public double x,y // center coordinates
• public double r // radius
• public Circle() // sets default values for
  x, y, and r
• this.x 0.0 this.y 0.0 this.r 1.0
• ...

8
A constructor with parameters

• public class Circle
• public Circle(double x, double y, double z)
• this.x x this.y y this.r z
• // using this is now a necessity
• ...

9
Using the different constructors

• Circle c, d
• c new Circle()
• // radius of circle has been set to 1.0
• System.out.println(c.area())
• d new Circle(1.0,1.0,5.0)
• // radius of circle has been set to 5.0
• System.out.println(d.area())

10
Method Overloading

• In Java, it is possible to have several method
  definitions under the same name
• but the signatures should be different
• Signature
• the name of the method
• the number of parameters
• the types of the parameters

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Encapsulation

• A key OO concept Information Hiding
• Key points
• The user of an object should have access to
  methods that are essential the interface
• Unnecessary implementation details should be
  hidden from the user
• The object is encapsulated
• In Java, use public and private

12
The Circle class Revisited

• public class Circle
• private double x,y // center coordinates
• private double r // radius
• // ...
• // when using the Circle class ...
• Circle c
• c.r 1.0 // this statement is not allowed

13
Outside access to private data

• No direct access
• Define (public) set and get methods instead or
  initialize the data through constructors
• Why?
• If you change your minds about the names and even
  the types of these private data, the code using
  the class need not be changed

14
Set and Get Methods

• public class Circle
• // ...
• private double r // radius
• //
• public void setRadius(double r) this.r r
  
• public double getRadius() return this.r
• // ...

15
Subclasses and Inheritance

• Inheritance
• programming language feature that allows for the
  implicit definition of variables/methods for a
  class through an existing class
• In Java, use the extends keyword
• public class B extends A
• objects of subclass B now have access to
  variables and methods defined in A

16
The EnhancedCircle class

• public class EnhancedCircle extends Circle
• // as if x, y, r, area(), circumference(),
  setRadius()
• // and getRadius() automatically defined
• private int color
• public void setColor(int c) color c
• public void draw()
• public int diameter() return getRadius()2
  

17
Using a Subclass

• EnhancedCircle c
• c new EnhancedCircle() // Circle()
  constructor
• //
  implicitly invoked
• c.setColor(5)
• c.setRadius(6.6)
• System.out.println(c.area())
• System.out.println(c.diameter())
• c.draw()

18
Life Cycle of an object

• 1) Creating Objects
• - declaration, instantiation, initialization
• 2) Using Objects
• - manipulate or inspect its variables
• - call its methods
• 3) Cleaning up unused objects
• - garbage collection

19
Applets

• Writing applets from scratch
• Steps
• extend Javas Applet class
• define variables for visual objects and other
  data
• define init() method to set up visual objects,
  action() method to process events, paint() method
  for placing graphics on the applet

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Applets and Inheritance

• Java Applets that we write extend the Applet
  class (defined in package java.applet)
• Methods such as add() (for adding visual
  components) are actually methods available in the
  Applet class
• init(), action(), and paint() are also available
  but can be overridden

21
Class Hierarchy

• Subclass relationship forms a hierarchy
• Example TextField class
• TextField extends TextComponent which extends
  Component which extends Object
• Object is the topmost class in Java

22
Applet Class Hierarchy
23
Visual Programming
24
The Java Abstract Windowing Toolkit (AWT)

• Components
• Label, Button, TextField, TextArea, Panel
• Others (self-study)
• Layout Managers
• FlowLayout, GridLayout, BorderLayout
• CardLayout, GridBagLayout (self-study)

25
Components

• Button clickable visual object
• Label text
• TextField
• contains editable text
• methods setText() and getText()
• TextArea - multiline editor
• Panel
• may contain other visual components
• methods setLayout() and add()

26
Designing a Visual Interface for an Applet

• Declare variables for the different visual
  components to be placed on the applet
• In init() method,
• create visual components (use new)
• establish layout manager (use setLayout()) --
  default for applets FlowLayout
• add visual components (use add())
• nested layouts possible (use Panels)

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GUI Example

• public class HideMessage extends Applet
• Button hidebutton
• TextField message
• public void init()
• setLayout(new FlowLayout())
• hidebutton new Button(Hide)
• message new TextField(Hello)
• add(hidebutton) add(message)
• // ...

28
Events

• The Event class and Event objects
• represents an event
• target attribute that represents object that
  was involved in the event
• action() method of Applet
• called when a visual event occurs
• includes an event object as a parameter

29
Handling Events

• In action method,
• determine which object (button, text field,
  others) was involved during the event and then
  specify what happens next
• Example
• public boolean action(Event e, Object o)
• if (e.target hidebutton)
• message.hide()
• //

30
Containers

• Components that can contain other components
• Container is a class of which Panel and Applet
  are subclasses
• setLayout() and add() are container methods
• Frame another container example
• use when you want a visual application

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Label

• Label() - blank label
• Label(String str) - str is left-justified
• Label(String str, int how) - how should be any of
  Label.LEFT, Label.RIGHT or Label.CENTER
• void setText(String str)
• String getText()
• void setAlignment(int how)
• int getAlignment()

32
Button

• Button()
• Button(String str)
• void setLabel(String str)
• String getLabel()
• implement ActionListener interface
• interface defines actionPerformed() method
• obtain label by the getActionCommand()

33
TextField

• TextField()
• TextField(int numChars)
• TextField(String str)
• TextField(String str, int numChars)
• String getText()
• void setText(String str)
• String getSelectedText()
• void select(int startIndex, int endIndex)

34
TextField

• Boolean isEditable
• void setEditable(boolean canEdit)
• void setEchoChar(char ch)
• boolean echoCharIsSet()
• char getEchoChar()

35
TextArea

• TextArea()
• TextArea(int numLines, int numChars)
• TextArea(String str)
• TextArea(String str, int numLines, int numChars)
• TextArea(String str, int numLines, int numChars,
  int sBars)
• sBars can be SCROLLBARS_BOTH, SCROLLBARS_NONE,
  SCROLLBARS_HORIZONTAL_ONLY or SCROLLBARS_VERTICAL_
  ONLY

36
TextArea

• Supports the methods
• getText(), setText(), getSelectedText(),
  select(), isEditable(), setEditable
• void append(String str)
• void insert(String str, int index)
• void replaceRange(String str, int startIndex, int
  endIndex)

37
Layout Managers

• viod setLayout(LayoutManager layoutObj)
• FlowLayout
• objects are placed row by row, left to right
• GridLayout
• divides container into an m by n grid
• BorderLayout
• divides container into 5 parts
• Center, North, South, East, West

38
FlowLayout

• FlowLayout()
• FlowLayout(int how)
• FlowLayout(int how, int horz, int vert)
• how should be any of FlowLayout.LEFT,
  FlowLayout.CENTER, FlowLayout.RIGHT
• example
• // set left-aligned flow layout
• setLayout(new FlowLayout(FlowLayout.LEFT))

39
GridLayout

• GridLayout()
• GridLayout(int numRows, int numColumns)
• GridLayout(int numRows, int numColumns, int horz,
  int vert)

40
BorderLayout

• BorderLayout()
• BorderLayout(int horz, int vert)
• Regions
• BorderLayout.CENTER
• BorderLayout.EAST
• BorderLayout.WEST
• BorderLayout.NORTH
• BorderLayout.SOUTH
• void add(Component compObj, Object region)

41
Using Insets

• Leave a small amount of space between the
  container that holds your components and the
  window that contains it.
• Insets(int top, int left, int bottom, int right)

42
Window Fundamentals

• Component
• - abstract class that encapsulates all of the
  attributes of a visual component
• Container
• - subclass of component
• - has additional methods that allow other
  components to be nested within it.
• Panel
• - concrete subclass of container
• - no additional methods, just implements
  container
• - superclass of Applet

43
Window Fundamentals

• Window
• - class creates a top-level window
• Frame
• - encapsulates what is commonly thought of as a
  window
• - subclass of window with title bar, menu bar,
  borders, and resizing corners

44
Frame

• Frame()
• Frame(String title)
• void setSize(int newWidth, int newHeight)
• void setSize(Dimension newSize)
• Dimension getSize()
• void setVisible(boolean visibleFlag)
• void setTitle(String newTitle)
• close a frame window

45
Creating a Frame Object for a Java Application

• In some main() method,
• create the frame object
• to display frame, call setVisible on that object
• Example
• public static void main(String args)
• MyFrame f new MyFrame()
• f.setVisible(true)

46
Graphics in Java

• The Java AWT supports graphics
• Graphics is a class
• a Graphics object can be viewed as something that
  you can draw on e.g., the region of an applet or
  frame
• public void paint(Graphics g) method
• contains drawing commands
• called when system needs to redraw the applet or
  frame

47
Lines and Rectangles

• void drawLine(int startX, int startY, int endX,
  int endY)
• void drawRect(int top, int left, int width, int
  height)
• void fillRect(int top, int left, int width, int
  height)
• void drawRoundRect(int top, int left, int width,
  int height, int xDiam, int yDiam)
• void fillRoundRect(int top, int left, int width,
  int height, int xDiam, int yDiam)

48
Ellipses, Circles and Arcs

• void drawOval(int top, int left, int width, int
  height)
• void fillOval(int top, int left, int width, int
  height)
• void drawArc(int top, int left, int width, int
  height, int startAngle, int sweepAngle)
• void fillArc(int top, int left, int width, int
  height, int startAngle, int sweepAngle)

49
Polygons

• void drawPolygon(int x, int y, int numPoints)
• void fillPolygon(int x, int y, int numPoints)

50
Simple Animation

• Define an int variable y as an attribute for the
  applet
• Arrange it so that y is incremented by 10 when a
  button is pressed, call repaint() after
  incrementing
• Initialize y to zero in the init() method
• In paint(), g.drawString(Hello, y, 50)
• String moves as you click on the button

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About paint()

• Describes current picture for the applet (or
  frame)
• Not cumulative
• previous string in the animation example drawn
  disappears when you refresh the drawing
• repaint() calls paint() and is in fact called
  automatically every few seconds

52
Creating Classes and Objects
53
Something to think about
Answer c 150000 but z 450000 because y
points to the same object as x

• Suppose
• long a 150000
• long b a
• a a 300000
• long c b
• BankAccount x new BankAccount( 150000, Jose
  Velarde )
• BankAccount y x
• x.deposit( 300000 )
• long z y.getBalance()
• What is the final value of c? z? Why?
• (you can try running a similar example using
  Circle and setRadius / getRadius)

54
Bank Account Object
How do we define a BankAccount class?

• State (aka Fields)
• balance the current amount of money in the
  account
• owner name of person owning the account
• Behavior (aka Methods)
• new BankAccount( initialAmount, owner ) create
  account
• getBalance() returns an integer
• deposit( amount ) adds amount to balance
• withdraw( amount ) subtracts amount from balance
• getOwner() returns the name of the owner
• Usage
• declaration BankAccount x
• Instantiation x new BankAccount(150000, J.
  Velarde)
• Using fields and methods x.deposit( 600000000
  )

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BankAccount class (balance only)

• public class BankAccount
• private long balance // current amount
• // constructor
• public BankAccount( long balance )
• this.balance balance
• public long getBalance()
• return balance
• public void deposit( long amount )
• balance amount
• public void withdraw( long amount )
• balance - amount

Fields
Methods
56
Using the BankAccount class

• public class TestBankAccount
• public static void main(String args)
• BankAccount x
• x new BankAccount( 1000 )
• x.deposit( 1000 )
• System.out.println( x.getBalance() )
• x.withdraw( 500 )
• System.out.println( x.getBalance() )

57
GraphicsIOApplet

• A drawing version of InputOutputApplet
• Extend this class instead of Applet or
  InputOutputApplet
• call addGraphics() in setup()
• create initial graphics and text int
  initOutputs() method
• create and draw Shape objects
• e.g., Circle, Line, Point, Rectangle, GraphicText

58
Shapes and Methods

• Point
• new Point( x, y ), getX, getY, setX, setY, setXY
• Line
• new Line( startX, startY, endX, endY )
• getStart(), getEnd(), setStart( x, y ), setEnd(
  x, y )
• Circle
• new Circle( x, y, radius )
• getCenter(), getRadius(), setCenter( x, y ),
  setRadius( r )
• Rectangle
• new Rectangle( leftX, upperY, width, height )
• getULCorner(), getWidth(), getHeight(),
• setULCorner( x, y ), setWidth(), setHeight()
• GraphicText
• new GraphicText( leftX, upperY, text )
• getULCorner(), getText(), setULCorner( x, y ),
  setText( text )
• All Shapes have move( dx, dy ) method

59
Interfaces
Idea We can make our own Shape objects!

• The public methods of a class constitutes its
  interface to the outside world
• In Java, we can classify objects in terms of
  their interface
• e.g., Shape interface
• void move( int dx, int dy )
• void paint( java.awt.Graphics g )
• the draw(Shape) method in GraphicsIOApplet takes
  objects that implement these methods
• Point, Circle, Line, etc. each implements Shape
• has code for move and paint

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Shape interface

• public interface Shape
• public void move( int dx, int dy )
• public void paint( java.awt.Graphics g )
• An interface definition is like a class
  definition, but
• No fields
• No method bodies
• Defines what objects of type Shape can do
  (interface), but does not define how they do it
  (implementation)

61
Crosshair Class

• public class Crosshair implements Shape
• private Point center
• private int r
• private Line hline, vline
• // constructors
• public Crosshair(Point center, int r)
• set( center, r )
• public Crosshair( int x, int y, int r )
• this( new Point( x, y ), r )
• // this can be used to call another
  version // of a constructor within a constructor
• public void set( Point center, int r )
• this.center center
• this.r r
• this.hline new Line( center.getX() - r,
  center.getY(),
• center.getX() r,
  center.getY() )
• this.vline new Line( center.getX(),
  center.getY() - r,

public void move(int x, int y)
center.move(x, y) hline.move(x, y)
vline.move(x, y) public void
paint(java.awt.Graphics g) hline.paint(g)
vline.paint(g)
62
Defining the Person Class

• Use Crosshair as an example
• Fields
• Built-in Shapes (Circles, Lines, etc.)
  corresponding to different parts of body
• A Point for reference position? (or use one of
  the shapes)
• Methods
• move just call move on all the shapes
• paint just call paint on all the shapes
• raiseLeftArm(), lowerLeftArm(), raiseRightArm(),
  lowerRightArm(), raiseLeftLeg(), lowerLeftLeg()

63
Using the Person Class

• Create 3 persons, and draw them on screen
• onButtonPressed make them do jumping-jacks by
  calling raiseLeftHand(), raiseLeftFoot(), etc.
• Make the code even simpler by defining jumpUp()
  and jumpDown() methods in terms of
  raiseLeftHand(), raiseLeftFoot(), etc.
• Optional have an input field that allows you to
  create N Persons.
• use arrays of Persons (more about this next
  Lecture)
• use clear() method in GraphicsApplet to get rid
  of old drawn Shapes