Event Handlers

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Event Handlers

• called Action Listeners

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JButtons
An Event Java code that Listens for an
event Is called an Action Listener
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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
• Any class with actionPerformed( ) method

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Add this to any class

• public class EventClass extends JFrame implements
  ActionListener
• public void actionPerformed ( ActionEvent e )
• System.out.println(Something just
  happened)
• System.out.println( e.getActionCommand( )
  )

Helper class
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What does implement mean

• Called an Interface
• Just like extend, only different.
• In extend, you can choose to use as-is, or
  re-write the methods of the parent class.
• The author of a class that is implemented,
  insists that YOU write certain pre-determined
  methods for it.

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Why implement an Interface?

• Because there are probably tons of classes that
  USE the implemented class, and if youre writing
  a new one, it must fit exactly as previous ones
  did
• Heres the problem how else would you write a
  class that comfortably slips into an application
  thats already running?

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ActionListener class

• An interface written by someone else, and you
  must adhere to the contract
• Class that implements the ActionListener must
  meet one obligation
• provide a method called actionPerformed that
  receives an ActionEvent object, as a means of
  telling what just happened?.
• The actionPerformed method runs based on a
  certain specified asynchronous event

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Add this to any code

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

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ActionEvent objects

• An object of the helper ActionEvent class has the
  following property
• It gets notified of computer events.
• It is an object of a class that contains methods
  that deal with events
• e.getActionCommand()
• e.getSource()
• e.toString()

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Asynchronous

• On NO schedule
• Happens without warning

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Objects of this implemented 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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the JButton

• grabs one ActionListener object as its own
• (e.g. handler)
• sends its activity to that ActionListener

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Tying a component to an ActionListener

• Components JButtons, JComboBoxes, JTextFields,
  and other GUI controls.
• JButton helloButton new JButton( Hello)
• // the class that contains the action listener
• eventClass handler new eventClass( )
• helloButton.addActionListener( handler )
• // or if were already IN the class
• helloButton.addActionListener( this )

Has to be an object that contains an
actionPerformed method
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• Watch the demo

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e.getSource( )

• Pull Jbuttons and other JComponents out of
  constructor brackets.
• Make them public and known to every one.
• Can be tested in if statements

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test in actionPerformed

• public void actionPerformed( ActionEvent e )
• System.out.println(e.getActionCommand())
  
• if (e.getSource( ) exitButton)
• System.out.println("Exiting")
• System.exit(0)
• // end actionPerformed method

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Handlers in General
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A Component

• JButton relies on user action
• an event producer
• JButton helloButton new JButton( Hello)

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Registration

• Tying a handler object to an event producer
• helloButton.addActionListener( myHandler )
• helloButton.addActionListener( this )
• if the current class contains the
  actionPerformed method( )

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A Handler Object

• Instantiate the Private, Inner Class
• JButton exitButton new JButton( "Exit" )
• add( exitButton )
• eventClass handler new eventClass( )
• exitButton.addActionListener( handler )
• handler is an object
• Can be attached to a component
• exitButton.addActionListener( this )

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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

• Public Timer tickTimer
• tickTimer new Timer( 1000, null )
• tickTimer.addActionListener( this )
• tickTimer.start()
• tickTimer.stop()

How many milliseconds between events
what code runs every tick
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Animation Coding Example
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Main Class

• import javax.swing.
• import java.awt.
• import java.awt.event.
• public class bouncer extends JFramepublic int
  ballX 0public int ballY 0public int
  diameter 50public int width 400public int
  height 400public int counter 0public
  Timer tickTimer new Timer(10, null)

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constructor

• public bouncer() setSize( width, height
  ) setVisible( true ) setDefaultCloseOper
  ation( EXIT_ON_CLOSE )
  tickTimer.addActionListener( this )
  tickTimer.start( )

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paint method

• public void paint (Graphics g)
• // background
• g.setColor( Color.white )
• g.fillRect( 0,0, width, height)
• // ball
• g.setColor( Color.red )
• g.fillOval( ballX, ballY, diameter, diameter
  )

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actionPerformed every time the timer ticks

• public void actionPerformed(ActionEvent e)
• ballY
• ballX
• repaint( ) // like calling the paint method
• if ( counter 400 ) // inc. and test
  together
• tickTimer.stop( )

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add a bounce every time the timer goes off

• public void actionPerformed( ActionEvent e )
• ballY
• ballX
• repaint( )
• if (counter 400)
• tickTimer.stop()
• if (counter gt 200)
• ballY ballY - 2 // net effect -1