review

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review

• Midterm on Wed.

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A Java Method basic program unit

• Methods are often called routines, subroutines,
  functions or procedures
• A single function from start to finish
• Can be used many times (called) from all over
• Many methods and variables are collected into a
  class
• A Class is a Java program that does one job, and
  draws on many methods and variables to accomplish
  it. Classes eventually become objects

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// public
static int ExampleMethod( ) int x, y,
Sum x 2 y 2 Sum x y
return( Sum ) //a service to users
//of this
method //
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How to call a method from elsewhere in your
program

• Calling a void method myMethod( )
• Calling a method with a return int variable -
• i.e. public static int myMethod( )
• return(6)
• Calling from main
• int LocalVariable
• LocalVariable myMethod( )

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Our First Complete Java Class (i.e. collection of
Methods and variables).
class
public class Lab1MainClass String
GreetingStr boolean AreVariablesSet
public static void main( String args ) /
ignore String args/
AreVariablesSet false AreVariablesSet
Set_Greeting( ) if (AreVariablesSet
true) then
System.out.println(GreetingStr)
public boolean Set_Greeting( )
GreetingStr Hello" return(
true ) // end class
variables
method1main
call
return
method2
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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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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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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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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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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 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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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 )

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grab a color

• public Color palette
• new Color ( 25, 120, 200 )
• // palette is a Color object
• // Color (red, green, blue )

note 25, 120, 200 is ocean blue
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graphics on a computer
X direction
( 0, 0 )
( 1024, 0 )
( ?, ? )
y direction
( 0, 768 )
( 1024, 768 )
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what is paint?

• a method in JFrame
• its a void method (returns nothing to the
  caller )
• called by the computer automatically
• that we re-write (override)
• that someone else wrote
• that uses a Graphics helper class
• that puts colors and shapes in a JFrame

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Graphics helper class?

• look up the Graphics class on the Sun Java
  website
• It must be used by a paint method in a JFrame
  class
• it contains variables and methods of its own, to
  color and draw shapes

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structure of a paint method

• public void paint( Graphics g )
• // g is the Graphics copy (object)
• // set a color
• g. setColor( pallette )
• // do something with the color
• // g.fillRect( x, y, width, height )
• g.fillRect( 50, 100, 200, 200 )

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Using Arrays to draw shapes

• int xValues 245, 245, 260, 260, 230, 230
  
• int yValues 250, 300, 300, 310, 310, 250
  
• g.setColor( Color.yellow )
• g.fillPolygon ( xValues, yValues, 6 )
• // connects the dots

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The Font class

• Constructor
• Font ( Name, Style, Size )
• Name Monospace, Arial, Helvetica
• Style uses FIELDS
• Font.BOLD, Font.PLAIN, Font.ITALIC
• Font myFont new Font(Arial, Font.PLAIN,12 )

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Writing text in Graphics

• Font is an object like Color
• Font myFont new Font ("Monospaced",
• Font.BOLD , 12)
• g.setFont( myFont )
• g.setColor( Color.red)
• g.drawString( hello" ,160, 220)

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3 new classes

• Use these two
• ActionListener a private inner class
• ActionEvent a helper class used by
  ActionListener
• To write your own
• Private inner class with actionPerformed( )
  method

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Add this to any code

• private class eventClass implements
  ActionListener
• public void actionPerformed ( ActionEvent e )
• System.out.println(Something just
  happened)
• System.out.println( e.getActionCommand( )
  )

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Add this to any code

• private class eventClass implements
  ActionListener
• public void actionPerformed ( ActionEvent e )
• System.out.println(Something just
  happened)
• System.out.println( e.getActionCommand( )
  )

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Objects of the private inner class

• Useful when this becomes an object somewhere in
  the immediate code (for instance, a class
  constructor)
• eventClass handler new eventClass( )

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Tying a component to an ActionListener

• Components JButtons, JComboBoxes, JTextFields,
  and other GUI controls.
• JButton helloButton new JButton( Hello)
• eventClass handler new eventClass( )
• helloButton.addActionListener( handler )

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• import java.awt.event.
• public class ButtonTest extends JFrame
• public ButtonTest ( )
• public static void main (String args)
• private class eventClass implements
  ActionListener
• public void actionPerformed ( ActionEvent
  e )

JFrame class
Constructor
Inner class
main method
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e.getSource( )

• Pull JButtons, JTextFields out of constructor
  brackets.
• Make them public and known to every one.
• Can be tested in if statements

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Instantiation

• public class ButtonTest extends JFrame
• JButton exitButton new JButton(I want to
  leave)
• public ButtonTest()
• // other stuff

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test in actionPerformed

• private class ButtonHandler implements
  ActionListener
• public void actionPerformed( ActionEvent e
  )
• System.out.println(e.getActionCommand())
  
• if (e.getSource( ) exitButton)
• System.out.println("Exiting")
• System.exit(0)

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• public class TextFieldTest extends JFrame
• JTextField writeBox
• public TextFieldTest()
• setLayout( new FlowLayout( ) )
• writeBox new JTextField( "Hello" )
• add( writeBox )
• eventClass handler new eventClass( )
• writeBox.addActionListener( handler )
• setVisible(true)

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A Timer Event

• Imagine a clock ticking at every tick, the
  computer generates an event.
• Computers have many internal clocks.
• Some with nano-second timing.
• 10-9
• A nanosecond is to a second as a second is to 30
  years
• The standard measure of computer time is the
  millisecond - 10-3

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Time

• second
• millisecond - 1/1000 - 10-3
• microsecond - 1/1,000,000 - 10-6
• nanosecond - 1/1,000,000,000 - 10-9

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The Timer Component

• Timer tickTimer new Timer( 1000, null )
• SomeHandler myHandler new SomeHandler( )
• tickTimer.addActionListener( myHandler )
• tickTimer.start()
• tickTimer.stop()

How many milliseconds between events
what code runs every tick