Session 11
Border Layout, using Panels,
Introduction to PinBallGame
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.
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 );
}
Steps to the solution:
1. Add a button to the CannonWorld.
2. Add a listener class to CannonWorld.
The listener’s 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...
• Shouldn’t 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.
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.
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
North
West
Center
South
East
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.
Using Panels with BorderLayout
• BorderLayout seems very restrictive, but
you can use a panel to hold several
components in each “direction”.
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.
Pinball Game Construction Kit
• Study a cleaned up and refactored version
of Budd’s 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.
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…
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 easier—as well as isolating repeated code.
Looks the same, but it’s 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.
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:
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.
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.
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. (Let’s go back to the code
(also p 104))
Mouse Events in the Pin Ball
Game
In the PinBallGame class, we have the following class relationship:
MouseListener
^
|
|
implements
|
MouseAdapter
^
|
|
extends
|
MouseKeeper
• What does a MouseKeeper do?
• What does a MouseKeeper do?
– If the mouse is pressed in the“shooting area”,
then it creates and launches a new pinball.
• What does a MouseAdapter do?
• What does a MouseAdapter do?
– Nothing, in response to any sort of mouse
event.
• Why do you suppose that Java’s creators
call it an “adapter”?
• Why do you suppose that Java’s creators
call it an “adapter”?
– They use a common design from the world as
an analogy.
• Why do you suppose Java’s creators
bothered to define the MouseListener
interface? Why not have everyone extend
MouseAdapter?
• Why do you suppose Java’s 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?