TOPIC 13B

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TOPIC 13B
Window Interfaces Using Swing Objects

• Buttons and Action Listeners

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class JButton

• Provided to create buttons in Java
• To create button
• Same technique as creating Label and Textile

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Methods Provided by the class JButton
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Methods Provided by the class JButton
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Handling an Event

• Action event
• event created when JButton is clicked
• sends a message to another object
• ActionListener
• actionPerformed method
• Event listener
• object which receives message when JButton is
  clicked
• performs some action
• some method in the listener object is invoked
  with the event as the argument
• happens automatically
• In Java, you must register the listener

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Handling An Event

• Each JButton
• Specify corresponding listener object
• i.e. registering the listener
• Define listener methods
• i.e. those methods that will be invoked when the
  event is sent to the listener

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Handling an Event

• class ActionListener
• Handles action event
• Part of package java.awt.Event
• The class ActionListener is a special type of
  class (interface)
• Must contain actionPerformed method
• public interface ActionListener
• public void actionPerformed(ActionEvent e)

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• Create a class on top of ActionListener so that
  the required object can be instantiated.
• This class must contain the necessary code for
  the method actionPerformed.
• private class CalculateButtonHandler
• implements ActionListener
• public void actionPerformed(ActionEvent e)
• // code for calculating area perimeter
• // displaying these quantities

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The complete code for the CALCULATE BUTTON handler

• private class CalculateButtonHandler
• implements ActionListener (1)
• public void actionPerformed(ActionEvent
  e) (2)
• double width, length, area, perimeter (3)
• length Double.parseDouble(lenthTF.getText(
  )) (4)
• width Double.parseDouble(widthTF.getText(
  )) (5)
• area length width (6)
• perimeter 2 (length width) (7)
• areaTF.setText( area) (8)
• perimeterTF.setText( perimeter) (9)

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• Must create the listener object in the program.
• Must associate this handler with the
  corresponding JButton
• CalculateButtonHandler cbHandler
• cbHandler new CalculateButtonHandler()
• // instantiates the object
• calculateB.addActionListener(cbHandler)

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Buttons and ActionListeners(RectangleProgram1)

• Basic steps for using a button in a Swing
  program
• Create a Button object
• Add the Button object to a container
• Create an ActionListener object that has an
  actionPerformed method
• Register the listener for the Button object
• The following slides show an example of each step.

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Create a Button Object andAdd the Button to a
Container

• JButton stopButton new JButton(Red)
• contentPane.add(stopButton)

String that will appear on the button
JButton is a predefined Swing class for buttons.
This example uses the Flow Layout so the add
method needs only one parameter.
The button will be added to this container.
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Create an ActionListener Object

• Make a class into an ActionListener
• Add the phrase implements ActionListener to the
  beginning of the class definition
• Define a method named actionPerformed

public class ButtonDemo extends JFrame
implements ActionListener . . .
public void actionPerformed(ActionEvent e)
. . .
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The actionPerformed Method

• An actionPerformed method must have only one
  parameter
• The parameter must be of type ActionEvent
• The parameter can be used to find the command for
  the ActionEvent

public void actionPerformed(ActionEvent e) if
(e.getActionCommand().equals(Red)) . . .
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Register the Listener for the Button Object

• If a button has no listener registered for it,
  there will be no response when the user clicks on
  the button.
• An example of registering a listener for a
  button

JButton stopButton new JButton(Red) stopButto
n.addActionListener(this) contentPane.add(stopBut
ton)
this refers to the object that includes this code
in a method. In this example the object is a
JFrame class that implements ActionListener.