Using classes as objects

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Using classes as objects
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• // header
• public class MainClass
• public int y
• public static void main(String args )
• int x
• supportMethod( )
• // end main method
• public static supportMethod( )
• // end supportMethod
• // end Lab1MainClass

Fully independent class definition shared
variables main method not-shared method
call support method note end brackets
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Why independent

• Has a main( )
• Does not use other classes
• Not very powerful

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What is a class?

• A program, with no main
• Contains useful methods and variables, that you
  might want to use in your program, that HAS a
  main.

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Instantiation

• You or someone else writes a class
• public class someClass
• ..
• You use the class in your program
• You must make a local copy
• someClass myObject new someClass( )
• Or, two-step
• public someClass myObject
• myObject new someClass()

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What a Class does

• A collection of methods and variables.
• Every Object created from a class, operates as an
  independent, freely running program.
• Only one main allowed
• main spawns useable objects, sometimes many
  objects of the same class.

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Using a library class

• import javax.swing.JOptionPane
• import javax.swing.JButton
• public class test
• public static void main (String args )
• JOptionPane.showMessageDialog(null, Hello)
• JButton myButton new JButton( )

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Another example of using a Class What if we need
a Date

• Look for a Date (or something like it) class.
• Check which library to import
• See if it has appropriate methods.

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• import java.util.Date
• public class testpublic test( ) // a
  constructor! System.out.println("Starti
  ng test program") Date myDate new
  Date() System.out.println( myDate )
  public static void main(String args
  ) test app new test( )

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• // header
• import java.util.Random
• public class MainClass
• public int y
• public Random myGen
• public static void main(String args )
• int x
• myGen new Random()
• // end Lab1MainClass

Using a library shared variables
? Not-shared variable
?
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• import java.util.Random
• public class test1
• public Random myGen // creation
• public test1( ) //constructor
• System.out.println(test1 starting)
• myGen new Random() // binding
• // same as Random myGen new Random( )
• public static void main(String args )
• test1 app new test1()
• // end test1

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Writing Programs
Memory space
The Java libraries
Your program main
10,000s of classes to choose from
millions of methods
new
objects
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What if Libraries arent sufficient?
Memory space
The Java libraries
Your program main
10,000s to choose from
millions of methods
Write your own class
new
objects
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What we know about Objects

• Classes are of no value until
• They are instantiated

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Objects

• They are instantiated from Classes
• An object, is a Class that has been put to
  work.
• Since Classes can be used many times in the same
  program, each instance of a Class is a
  separate, usable copy.

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Copies of Classes

• Copy is an oversimplification, but the concept
  has run-time value
• A copy is created using the new statement.
• A copy of a Class is an Object.
• non-Static Classes can never be used directly.
• They must be copied to Objects
• The copy step is called instantiation.
• Your program owns and controls the copy.

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Instantiation

• Once a class is defined, making it your own for
  use, using the new keyword
• myClass myObject new myClass( )
• Creates an instance of the Class.
• the Class solves a Class of problems.

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Class constructor

• If a class contains a method that has the same
  name as the class, it gets run automatically at
  instantiation.
• e.g. MyClass MyObject new MyClass( )
• will run a method in MyClass, if its name
  MyClass( )
• Called the Class Constructor.
• It's the reason for the parenthesis in the
  instantiation statement

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Applications - The main Method

• The computer runs it, you dont.
• The computer calls the main method
• The main method uses methods, contained in
  Objects, derived from Classes that you write.

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• public static void main(String args)
• ShowColors app new ShowColors( )
• // any method in ShowColors is now useable
• // as app.MethodName( )

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Introducing, the JFrame

• Gives us a work area beside System.out.println

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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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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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• 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

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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 new Color( )
• boxColor Color.blue
• or
• 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
• boxColor JColorChooser.showDialog(
• null, Greeting, default color )

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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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A Random Number generator

• import java.util.Random
• .
• .
• .
• Int X 0
• Random myGen new Random( )
• X myGen.nextInt( 50 )

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Meaning

• myGen is an Object of the Random class.
• It has methods.
• The nextInt( num ) method returns an int
  between 1 and num.

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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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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

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what does this do?

• FlowLayout layout new FlowLayout( )
• setLayout( layout )

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Setting up a JFrame

• setSize( w, h )
• setLocation( x, y )
• setDefaultCloseOperation( EXIT_ON_CLOSE )
  
• FlowLayout layout new FlowLayout()
• setLayout( layout )
• JButton helloButton new JButton( "Hello" )
• add( helloButton )
• setVisible( true )