The essence of Java is to write useful classes

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The essence of Java is to write useful classes

• You can use library classes to add complicated
  functions to your class (like JOptionPane)
• You can instantiate another class to use locally.
• You can use Java Native (Wrapper) classes to
  accomplish very common functions, like
  String.equals(exit) and
• Integer.parseInt(userInput )
• You can find a class that already does most of
  what you need, and absorb it entirely into your
  class (i.e. you can inherit all of the
  capabilities of another class)

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Extension, Hierarchy, and Inheritance
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Extension, Hierarchy, and Inheritance

• Extends in the class statement, tells which
  class serves as a parent.
• Inheritance the child class inherits public
  methods and variables from the parent class
• Hierarchy - parent to child relationship

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Extend keyword

• public class myClass extends someOtherClass
• // I can use all the public methods and
• // variables from someOtherClass in myClass.

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Other terms

• Parent / Child
• Base class / Sub class
• Super Class / Sub class
• Any class (not just an imported library class)
  can be a parent or base class.
• Find or Write classes then write new inherited
  classes for targeted capabilities.

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Java common inherited classes

• public class myClass extends JFrame
• - myClass gets a whole bunch of methods for
  displaying windows on-screen
• public class myClass extends Applet
• - applet and browser graphics
• public class myClass extends Object
• the parent of ALL classes, usually omitted

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Child is more powerful than the Parent

• The Parent has methods that are useful (i.e. the
  JFrame class handles mouse input)
• The Child can write new methods that enhance
  those capabilities (a class that extends JFrame
  can apply mouse handling to joysticks, game
  controllers, etc.)

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An example using counterClass

• public class counterClass
• public static int intervalSummer(int a, int b)
• int x
• int total 0
• for ( x a x lt b x)
• total total x
• return(total)
• // end intervalSummer method
• // end counterClass

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new class for average

• public class averagerClass extends counterClass
• public static double averager(int a, int b)
• return(ab/2.0)
• // end averager method
• // end averagerClass

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averagerClass

• has a new method averager
• still has an old method intervalSummer from
  counterClass.
• extends means everything in both classes

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in main

• averagerClass mathObject new averagerClass( )
• int x,y
• x mathObject.intervalSummer( 4, 10 ) // legal
• y mathObject.averager(10,90)

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Currently

• If you dont specify an extend source, a class
  extends the Java base class Object
• Only mildly capable
• Better to extend a useful class

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• import javax.swing.JFrame
• public class MainClass extends JFrame
• public MainClass()
• .....
• public static void main (String args )
• MainClass application new MainClass( )

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What is this?

• Main method instantiates the class that its in?
  Why?
• Because main is not considered a member of the
  class that contains it.
• The Main class serves a place-holding function
  for the computer, by keeping main in a usable
  place.
• Instantiating the Main class causes the
  constructor (if there is one) to be run.

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JFrame

• Puts graphics-capable windows on screen.
• JOptionPane uses JFrame
• Colors
• Fonts
• Drawings
• A frame for games

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the computer screen
(600, 0)

• (0, 0)

made up of tiny pixels, each with an (x, y)
location
(600, 800)
(0, 800)
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• import javax.swing.JFrame
• public class MainClass extends JFrame
• public MainClass ( )
• setLocation(100,200)
• setSize(250,250)
• setVisible(true)
• // end MainClass constructor
• public static void main (String args)
• MainClass application new MainClass( )
• // end main method
• // end class

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• import javax.swing.JFrame
• public class MainClass extends JFrame
• public MainClass ( int x, int y )
• setLocation( x, y )
• setSize(250,250)
• setVisible(true)
• // end MainClass constructor
• public static void main (String args)
• MainClass application new MainClass(100,
  200)
• // end main method
• // end class

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• import javax.swing.JFrame
• public class MainClass extends JFrame
• public MainClass ( int x, int y )
• setLocation( x, y )
• setSize(250,250)
• setVisible(true)
• // end MainClass constructor
• public static void main (String args)
• MainClass appl1 new MainClass(100, 200)
• MainClass appl2 new MainClass(200, 300)
• // end main method
• // end class

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Demo
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The JFrame method paint( )

• looks for, and runs, a special method that YOU
  write, called
• paint( Graphics g )

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• import javax.swing.
• import java.awt.
• public class PaintTest extends JFrame
• public PaintTest()
• setSize( 500,500 )
• setVisible( true )
• repaint()
• public void paint( Graphics g )
• g.drawRect(50,50,200,200) // x, y, width,
  height
• g.drawOval(50,50,100,100) // x, y, width,
  height
• public static void main( String args )
• PaintTest app new PaintTest()
• // end main

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helper classes

• remember
• try
• catch ( Exception e )
• the Exception class acted on information behind
  the scenes to help handle run-time errors.
• well, now we have
• paint ( Graphics g )

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Objects As Containers of Information
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Objects dont always DO things

• Sometimes they just HOLD things (information)
• The Color class can hold color information
• The Point class can hold cartesian coordinate
  information
• The Container class, well

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Color class

• Like the String class, and Integer class, does
  not use the new keyword.
• holds a color value (nothing more)
• e.g Color boxColor Color.blue
• then boxColor can be used to set System
  properties in Classes/Objects that need color
  (more later).

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JColorChooser returns a Color object to the
caller
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Returns an Object?

• JColorChooser fills in all of the information in
  a blank object of the Color class, and copies it
  to the Color object in the calling statement
• import javax.swing.
• myClr JColorChooser.showDialog(
• null, Greeting, default color )

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JFrame methods

• set the window up
• requires import javax.swing.JFrame
• or import javax.swing .
• setLocation( 10, 10 )
• setSize( 500,500 )
• setVisible( true )
• paint (Graphics g)

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another swing method

• colorObject JColorChooser.showDialog( null,
  greeting, default )

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Graphics methods

• draw in the window
• requires import java.awt .
• g.setColor( Color.blue )
• g.drawRect(50,50,200,200)
• g.fillOval(50,50,100,100)
• // x, y, width, height