Border Layout, using Panels,

1
Session 11

• Border Layout, using Panels,
• Introduction to PinBallGame

2
An Exercise

• Add to our CannonWorld a button that allows the
  user to change the color of the cannonball from
  blue to red to yellow to blue....
• Place the button on the lefthand side of the
  window.

3
An Exercise

• Place the button on the left-hand side of the
  window.
• public CannonWorld()
• ...
• Button changeColor new Button( "change
  ball color" )
• changeColor.addActionListener( new
  ChangeColorButtonListener() )
• add( "West", changeColor )

4
Steps to the solution

• 1. Add a button to the CannonWorld.
• 2. Add a listener class to CannonWorld.
• The listeners actionPerformed method needs to
  tell the ball to take the next color. I could
  have written an if expression that toggles the
  colors in sequence, but...
• Shouldnt the ball be controlling its own
  color, rather than the application using the
  ball?
• So I decided to have the CannonBall respond to a
• nextColor() message and toggle its own color.

5
More on a Solution to the Exercise

• The next question is, do I add a nextColor()
  method directly to the CannonBall class, or do I
  create a subclass?
• In general, if we need a new kind of object, then
  we should create a new kinds of object.
• (If a particular method turns out to be more
  generally useful than we anticipate, then we can
  always refactor the solution so that the method
  lives higher in the class hierarchy.)
• So
• 3. Implement a ChangingCannonBall class.
• 4. Use instances of ChangingCannonBall in the
  CannonWorld.

6
Layout Managers

• Frame is a subclass of the Container class
• so it can hold objects such as buttons, scroll
  bars, etc.
• layout manager assigns locations to these objects
  within the container
• the default LayoutManager is BorderLayout

7
Layout Managers

• Five standard types of layout managers
• BorderLayout
• GridLayout - creates an rectangular array of
  components
• FlowLayout - places components in rows left to
  right, top to bottom
• CardLayout - stacks components vertically with
  only one visible at any one time
• GridBagLayout - most general, but most complex.
  It allows for a nonuniform grid of squares with
  components placed in various positions within
  each square.

8
Using Panels with BorderLayout

• BorderLayout seems very restrictive, but you can
  use a panel to hold several components in each
  direction.

9
Class Interaction

• Relationships between classes
• 1. composition
• A Ball contains a Color.
• An instance of Color is a part of a Ball.
• 2. inheritance
• A MovableBall is a kind of Ball.
• A CannonWorld is a kind of Frame.
• 3. association
• A BoundedBall interacts with the Frame that
  contains it.
• A CannonWorld interacts with a ColorWheel.

10
Pinball Game Construction Kit

• Study a cleaned up and refactored version of
  Budds code from Section 7.1
• Construct a list of three questions (to turn in)
  that you have about this code.
• Your questions can be about how it works, what
  some piece of code in the program does, or even
  whey the code is written in a particular way.

11
Some cool new features

• Replaced the fire button with a mouse event.
• Multiple balls can be in the air at once.
• Control is no longer in the paint method.
• But before we talk about what Budd did well

12
Say It Once and Only Once!

• In the interest of simplifying the number of
  files we need to run a program, Budd has repeated
  code that can be reused by inheritance (figure
  7.3, p 102).
• Gravity affects a PinBall in the same way that it
  affects a CannonBall. The only differences
  between a PinBall and a CannonBall are its size
  and its initial trajectory.
• So, we can say
• public class PinBall extends CannonBall
• public PinBall( int sx, int sy )
• super( sx, sy, 10,-2 Math.random()/4, -15 )
• Inheritance makes our job easieras well as
  isolating repeated code.

13
Looks the same, but its not.

• Replaced the fire button with a mouse event.
• Multiple balls can be in the air at once.
• Control is no longer in the paint method.

14
Handling Mouse Events

• The CannonWorld handles only events that are
  generated by the active components a button and
  a slider that we added to it.
• More generally, though, we will want to trap and
  respond to any mouse action, anywhere within the
  frame.
• Any listener that wishes to monitor mouse
  activity must implement the MouseListener
  interface

15

• public interface MouseListener
• public void mouseClicked ( MouseEvent e )
• public void mouseEntered ( MouseEvent e )
• public void mouseExited ( MouseEvent e )
• public void mousePressed ( MouseEvent e )
• public void mouseReleased( MouseEvent e )
• The MouseListener interface is part of the
  package java.awt.event. It specifies what an
  object must do to be a mouse listener within the
  Java Event Model.

16
How the MouseListener works.

• Think back to how we implemented Button
  listeners
• private class FireButtonListener implements
  ActionListener
• We can do a similar thing for MouseListeners
• private class PinballGameListener implements
  MouseListener
• However, to implement an interface, the Java
  language insists that the programmer provide a
  definition for all operations.

17
MouseAdapter to the rescue

• To simplify this process, the Java library
  provides a simple class named Mouse Adapter. The
  class MouseAdapter implements the MouseListener
  interface, but uses an empty method for each
  method.
• Rather than implement a MouseListener, we may
  choose to implement a MouseAdapter. (Lets go
  back to the code (also p 104))

18
Mouse Events in the Pin Ball Game

• In the PinBallGame class, we have the following
  class relationship
• MouseListener
• implements
• MouseAdapter
• extends
• MouseKeeper

19

• What does a MouseKeeper do?

20

• What does a MouseKeeper do?
• If the mouse is pressed in theshooting area,
  then it creates and launches a new pinball.

21

• What does a MouseAdapter do?

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• What does a MouseAdapter do?
• Nothing, in response to any sort of mouse event.

23

• Why do you suppose that Javas creators call it
  an adapter?

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• Why do you suppose that Javas creators call it
  an adapter?
• They use a common design from the world as an
  analogy.

25

• Why do you suppose Javas creators bothered to
  define the MouseListener interface? Why not have
  everyone extend MouseAdapter?

26

• Why do you suppose Javas creators bothered to
  define the MouseListener interface? Why not have
  everyone extend MouseAdapter?
• Inheritance is not free. Why force programmers
  who plan to implement most or all of the
  interface to pay the extra price?