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Design Patterns

Object Oriented Programming Advanced Course 2007

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Design patterns

• Patterns don't give code, but general solutions to design problems

• We need to recognize places in our designs and existing applications where we can apply them

• In the end, we need the code for the compiler!

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How to use design patterns?

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An example: a coffee machine

• You are requested to do an implementation for the GUI menu for the coffee machine

• Return value: the option chosen by the user

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An easy way

Hack a similar code example from internet

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Java example• Run modified hacked Java example in

NetBins

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Adding more options

• There are 4 types of coffee where the user can add something more

• Soy, sugar, whipped milk, cinnamon, coffee without caffeine, strong coffee, etc..

• Each set of options have different prices

• Easy solution: one class for each combination of options

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Maintenance

• In the design, you should predict the future: what if the client want to add more options or change the price?

• Maintenance: how easy will it be to do a change?

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The decorator design pattern

• Also called wrapper

• Use it to add (or decorate with) more functionalities to an object/class

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The decorator design pattern

Class for the types of coffee

Class for the extras

General class that adds

something to the coffee

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Component abstract class

1. public abstract class Beverage {2. String description = "Unknown

Beverage";3. 4. public String getDescription() {5. return description;6. }7. 8. public abstract double cost();9. }

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Decorator abstract class

1. public abstract class CondimentDecorator extends Beverage {

2. public abstract String getDescription();

3. }

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Class for coffee

1. public class Espresso extends Beverage {2. 3. public Espresso() {4. description = "Espresso";5. }6. 7. public double cost() {8. return 10;9. }10.}

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Class for the concrete decorators

1. public class Chocolate extends CondimentDecorator {2. Beverage beverage;3. 4. public Chocolate(Beverage beverage) {5. this.beverage = beverage;6. }7. 8. public String getDescription() {9. return beverage.getDescription() + ", Chocolate";10. }11. 12. public double cost() {13. return 2 + beverage.cost();14. }15.}

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Main program

1. …. 2. Beverage beverage = new Espresso();3. System.out.println(beverage.getDescription() 4. + " SEK " + beverage.cost());5. 6. Beverage beverage2 = new DarkRoast();7. beverage2 = new Chocolate(beverage2);8. beverage2 = new Chocolate(beverage2);9. beverage2 = new Whip(beverage2);10.System.out.println(beverage2.getDescription() 11. + " SEK " + beverage2.cost());12.…

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Sample output

1. Espresso SEK 14.0

2. Dark Roast Coffee, Chocolate, Chocolate, Whip SEK 17.0

3. House Blend Coffee, Soy, Chocolate, Whip SEK 13.0

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Java example

• Run decorator Java example in NetBins

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Is this a good solution?

• The objects are loosely coupled: if you add tea to the options, you don’t need to change the code for the coffee object.

• The only constant thing in software is that it is always changing.

• Is this code easy to adapt to a new reality?

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Exercise: make the program more general

• ”decorate” an empty cup with the options made by the user: coffee, milk, sugar, tea, etc.

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The decorator design pattern

Class for the empty cup

Class for the extras

General class that adds

something to the cup

The coffe types are now just decorators

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Class for the empty cup

1. public class EmptyCup extends Beverage {2. 3. public EmptyCup () {4. description = "";5. }6. 7. public double cost() {8. return 0;9. }10.}

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The GUI1. super(new BorderLayout());

2. //Create the check boxes.3. coffeeButton = new JCheckBox("Coffee");4. coffeeButton.setMnemonic(KeyEvent.VK_C);5. coffeeButton.setSelected(false);

6. milkButton = new JCheckBox("Milk");7. milkButton.setMnemonic(KeyEvent.VK_M);8. milkButton.setSelected(false);

9. chocolateButton = new JCheckBox("Chocolate");10. chocolateButton.setMnemonic(KeyEvent.VK_H);11. chocolateButton.setSelected(false);

12. cappuccinoButton = new JCheckBox("Cappuccino");13. cappuccinoButton.setMnemonic(KeyEvent.VK_P);14. cappuccinoButton.setSelected(false);15. …..

Code with strong coupling!!!

Can you think of a design pattern for the GUI?

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Exercise

1. Build the GUI (show also the prices) using a design pattern

2. Can you also use the factory design pattern to ”build” the drinks?