CIS3931 - Intro to JAVA

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CIS3931 - Intro to JAVA

• Lecture Notes Set 10
• 28-June-05
• GUI Programming Buttons and Actions.

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In this set of notes

• Adding a button to a frame
• The content pane of a frame
• A little on layout managers
• Actions listeners
• Registering a listener
• Using actionPerformed()
• Changing the background color of a frame

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Buttons

• The swing class that defines buttons is called
  JButton
• JButton is a kind of container
• Can contain other components
• Think of it like a mini JFrame
• Can add pictures to the button

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Adding a button to a frame

• Create the button by calling the new constructor
  on the button
• JButton button new JButton(ClickMe)
• Add the button to the content pane
• getContentPane().add(button)

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Button Example Code

• See ButtonDemo.java

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ContentPane

• The content pane is a container that represents
  the main rectangle of the frame.
• The add method of the content pane puts an object
  (or another container) into the frame.
• The content pane is referenced via the
  getContentPane() method of the frame.
• getContentPane().add(OBJECT/CONTAINER)

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Layout Managers

• Layout Manager decides where objects will go
  (and how big they will be) when they are added to
  a frame.
• Sometimes the default choices arent always the
  best
• If you dont specify one, a default layout
  manager is chosen.
• ALWAYS choose the layout manager BEFORE you add
  components to the frame.

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FlowLayout Manager

• Part of the java.awt package.
• Puts components into the frame row by row in the
  order they are added.
• Automatically picks a reasonable size for the
  conponents

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FlowLayout Manager
Note The layout of the frame will change if you
add components in a different order.
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Setting the Layout Manager

• import java.awt.
• . . .
• public class ButtonDemo extends JFrame
• Button bChange //Create a button
• ButtonDemo()
• bChange new Button("Click Me!")
  //Initialize the button
• // choose the layout manager
• getContentPane().setLayout( new FlowLayout()
  )
• getContentPane().add( bChange ) //Add the
  button
• public static void main ( String args )
• ButtonDemo frm new ButtonDemo()
• . . . . .

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Sample Program with Layout Manager

• See LayoutDemo1.java

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Adding an ActionListener() to a button

• Must define complete class of a button listener
  from scratch (there is no built-in ButtonAdapter
  class)
• Must implement ActionListener
• ActionListener is an interface (not a class) that
  contains a single method
• public void actionPerformed(ActionEvent evt)

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ActionEvent

• ActionEvent is an Event object that represents an
  event (such as a button click).
• Contains information that can be used when
  responding to an event.
• Class must implement ActionListener

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ActionListener() example

• Changing the background color with a button click
• See ActionDemo1.java

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An addition to the example

• Allowing the button to change colors multiple
  times
• See ActionDemo2.java

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Demos

• Demo of completed Project 4 w/ code
• Demo of completed Project 5 w/ code