The Decorator Design Pattern

1
Session 30

• The Decorator Design Pattern

2
PA07 Posted

• A Graphical version of Bulls and Cows (like
  Mastermind)
• Must provide the ability to set the following at
  construction of the game
• six colors to choose from in building the
  sequence
• four colors in the code, duplicate colors
  allowed.
• ten attempts to break the code. 

3
PA07 Posted

• Your program should also provide as a minimum the
  following features
• a display of the choices available to the
  code-breaker in constructing the guesses
• some input mechanism for specifying a guess
• a display of the guess from the code-breaker
• a display of the response from the code-maker
• a display containing all past guesses/responses
• a way to start a new game and quit the program
• the use of one Swing element we haven't explored
  yet (eg. JTextField, JTextArea, JCheckbox,
  JRadioButton, JMenu, etc.  Basically ANYTHING
  other than JButton or JScrollBar.

4
PA07 Posted

• You may elect to work with a single partner
• Both students must email me by Wednesday,
  November 7th.
• I will conduct in-person grading after return
  from Thanksgiving break.

5
An Exercise

• The physics department would like for us to write
  a simple ball world program that it can use to
  teach concepts of friction. In this program, we
  need for some MovableBalls to decelerate. Every
  time one of these decelerating balls moves, its
  speed decreases by 5.
• Add a DeceleratingBall class to the Ball
  hierarchy for this purpose.

6
Review A Possible Solution

• public class DeceleratingBall extends MovableBall
  
• public DeceleratingBall( int x, int y, int r,
  double dx, double dy )
• super( x, y, r, dx, dy )
• public void move()
• super.move()
• setMotion( xMotion() 0.95,
• yMotion() 0.95 )
• We create a DeceleratingBall just as we would a
  MovableBall
• DeceleratingBall b new DeceleratingBall( 10,
  15, 5, 5.0, 10.0 )

7
A New Wrinkle

• Running the program, we realize that some
  decelerating balls need to bounce off the walls
  they hit, too. So we need a class of BoundedBalls
  that decelerate.
• Can you fix the problem?

8
A New Solution

• public class DeceleratingBoundedBall extends
  BoundedBall
• public DeceleratingBoundedBall(int x, int y, int
  r,
• double dx, double dy, Frame f )
• super( x, y, r, dx, dy, f )
• public void move()
• super.move()
• setMotion( xMotion() 0.95, yMotion() 0.95
  )

9
How Good is Our Solution?

• Our approach to this family of problems is
  straightforward implement a decelerating version
  of any Ball class that needs a decelerating
  counterpart.
• What are the strengths of this approach?
• It is simple.
• It is easy to implement right now.

10
How Good is Our Solution?

• What are the weaknesses of this approach?
• It is tedious.
• It repeats codes. The move() methods in the
  class
• DeceleratingBall and the class Decelerating-Bounde
  dBall are identical!
• You may be asking yourself, So what? It works.

11
So what? It works.

• What happens if we need to change the
  deceleration factor, say, from 95 to 80?
• We must remember to make the change in two
  different classes.
• What happens if we need to add deceleration
  behavior to other MovableBalls?
• More subclasses!
• What happens if we need to add another kind of
  behavior to our ball classes, including the
  decelerating balls?
• Even more subclasses!
• Solutions that make future extensions to the
  system unbearable are probably not very good
  solutions at all...

12
The Full Ball Hierarchy
13
An Alternative Solution

• BoundedBalls respond to the same set of messages
  as MovableBalls. So they are substitutable for
  one another. Can we use this to our advantage?
• public class DeceleratingBall extends MovableBall
  
• private MovableBall workerBall
• public DeceleratingBall( MovableBall aBall )
• super()
• workerBall aBall
• public void move()
• workerBall.move()
• workerBall.setMotion(
• workerBall.xMotion() 0.95,
• workerBall.yMotion() 0.95 )

14
An Alternative Solution

• // ALL OTHER MESSAGES ARE DELEGATED
• // DIRECTLY TO THE INSTANCE VARIABLE!
• public void paint( Graphics g )
• workerBall.paint( g )
• public void setColor( Color newColor )
• workerBall.setColor( newColor )
• protected int radius()
• return workerBall.radius()
• ...
• // -------------------------------------------
• protected double xMotion()
• return workerBall.xMotion()
• ...

15
Using DeceleratingBalls

• Now, we create a DeceleratingBall by giving it a
  MovableBall to direct
• DeceleratingBall b
• new DeceleratingBall(
• new MovableBall(10, 15, 5, 2.0, 5.0 ) )
• What is the advantage of this?