Inheriting the JFrame

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Inheriting the JFrame
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Three ways to make use of classes

• One step instantiation
• JFrame myWindow new JFrame( )
• Two step Instantiation
• public JFrame myWindow
• public static void method( int a)
• myWindow new JFrame( )

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third way

• public class myClass extends JFrame
• // your class inherits all of JFrames
• // methods

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

• Extends in the class statement, tells which
  larger class serves as the parent.
• Inheritance the child class inherits 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 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.
• 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
• including the ability to launch a paint( )
  method.
• 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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• import javax.swing.
• public class MainClass extends JFrame
• // stuff here
• 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?
• 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.
• What other reasons?

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JFrame

• Puts graphics-capable windows on screen.
• JOptionPane uses JFrame
• Colors
• Fonts
• Drawings
• A frame for intuitive GUI
• Adds a paint method

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Use these helper libraries

• import javax.swing.JFrame // for JFrame
• import java.awt. // for painting and animation
• import java.awt.event. // for event
  handling, more later
• import java.util.Random // always useful

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some JFrame methods

• setLocation(100,200)
• setSize(250,250)
• setVisible(true)
• setDefaultCloseOperation( EXIT_ON_CLOSE)

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• 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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• 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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• 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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If you dont extend JFrame.

• public class MainClass
• public MainClass ( )
• JFrame myFrame new JFrame( )
• myFrame.setLocation(100,200)
• myFrame.setSize(250,250)
• myFrame.setVisible(true)
• // end MainClass constructor
• public static void main (String args)
• MainClass application new MainClass( )
• // end main method
• // end class

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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 contains 10,000 variables
  that describe your computer

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the this qualifier

• Means this object, the one that were in.
• Used when a call to a method, especially in a
  child, needs to specify which object the method
  should act upon.
• this always refers to an object of some type.

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the super qualifier

• refers to the parent class
• the command super calls the parents constructor
• super.MethodName can call any method in the
  parent.

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graphics on your computer

• Since there are many computers (types,
  instances)....
• and since the programs that you write should run
  anywhere....
• then the programs that you write, should retrieve
  and use information native to the computer that
  it is running on.
• This is called context awareness

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The paint Method

• Computer runs it once (you dont), but then you
  can run it by calling repaint().
• The computer passes it an object of the Graphics
  Class, of its own making.

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• public void paint (Graphics g)
• // the computer calls the paint program
• // automatically whenever an event requires
• // the screen to be redrawn

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An event

• The user does anything moves the mouse, moves
  the window, hits a key, etc.

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Graphics!

• g.setColor
• g.fillRect
• g.drawString
• g.drawPolygon
• hundreds of methods that use your computers
  graphics capability

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demo

• A Frank Lloyd Wright style generator