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Design patterns structuralpatterns(theadapterpattern)

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  • Part of this lecture will be reserved for working through solutions to selected exercises from last week. Notes relating to this do not appear in the slides.
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    • 1. Structural Patterns The Adapter Pattern Design Patterns Chapter 8ADec 21, 2012 Design Patterns Chapter 8A1
    • 2. Objectives In this lecture, we will • Review structural design patterns • Discuss why such patterns are useful • Introduce the Adapter pattern • Discuss examples that take advantage of the adapter patternDec 21, 2012 Design Patterns Chapter 8A2
    • 3. Structural Patterns • Structural Patterns are used when it is necessary to build larger structures that are composed of other existing classes and objects – Re-use of existing implementations • Structural class patterns use inheritance • Structural object patterns use aggregation • This lecture introduces the adapter patternDec 21, 2012 Design Patterns Chapter 8A3
    • 4. What is an Adapter? • A simple example of an adapter is an electric razor adapter • Most British domestic sockets are standard three pin sockets – live, neutral and earth • Most electric razors come with a two pin plug – That will not push into a three pin socket • An electric razor adapter provides the link between the two – Modifies the interface of the razor to make it compatible with the three pin socket • What about British and Continental sockets?Dec 21, 2012 Design Patterns Chapter 8A4
    • 5. Software Adapters • In object oriented programming the same type of problem often arises • A class, XClass, has been built and works well • A system has been built that works with objects that satisfy a certain interface • The methods of the XClass provide all the functionality required of the objects that work with the system • BUT the interface of the XClass is not the interface required by the system • What is the easiest solution that re-uses the XClass? – Write an adapter!Dec 21, 2012 Design Patterns Chapter 8A5
    • 6. Dialog Boxes • How is a dialog box displayed in Java? JOptionPane.showMessageDialog ( null, “This is a TEST” ); • What is displayed? • Where does the image come from? • Can the image be changed?Dec 21, 2012 Design Patterns Chapter 8A6
    • 7. The showMessageDialog method • The declaration of this method is: public static void showMessageDialog { Component parent, Object message, String title, int messageType, Icon anIcon } • The last parameter is of type Icon; an interface – Any object that implements the Icon interface can be usedDec 21, 2012 Design Patterns Chapter 8A7
    • 8. Using an ImageIcon • The ImageIcon class implements the Icon interface and can build an icon from a GIF file • For example: JOptionPane.showMessageDialog ( null, // parent "Hello", // message "Testing GIF", // title JOptionPane.INFORMATION_MESSAGE, // messageType new ImageIcon("c:tempduke.gif") // anIcon );Dec 21, 2012 Design Patterns Chapter 8A8
    • 9. What is the Icon interface? • The definition of an Icon interface is: public interface Icon { int getIconWidth () ; int getIconHeight (); void paintIcon ( Component c, Graphics g, int x, int y); }Dec 21, 2012 Design Patterns Chapter 8A9
    • 10. A Simple Icon Class public class PeteIcon implements Icon { private int size; public PeteIcon(int aSize) { size = aSize; } public int getIconWidth() { return size; } public int getIconHeight() { return size; } public void paintIcon (Component c, Graphics g, int x, int y) { Graphics2D g2 = (Graphics2D) g; Ellipse2D.Double e = new Ellipse2D.Double (x,y,size,size); g2.setColor (Color.RED); g2.fill(e); g2.setColor(Color.BLACK); g2.drawString("Pete",size/3,size/2); } }Dec 21, 2012 Design Patterns Chapter 8A10
    • 11. Using the Icon Class • An instance of PeteIcon can be used anywhere an Icon is expected: JOptionPane.showMessageDialog ( null, "Hello", "Testing PeteIcon", JOptionPane.INFORMATION_MESSAGE, new PeteIcon(100) );Dec 21, 2012 Design Patterns Chapter 8A11
    • 12. Building a User Interface • In building a user interface frames are often used to contain a set of user interface components JFrame frame = new JFrame(); Container contentPane = frame.getContentPane(); JButton b1 = new JButton("Push"); contentPane.add(b1,BorderLayout.NORTH); JButton b2 = new JButton("STOP"); contentPane.add(b2,BorderLayout.SOUTH); frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); frame.pack(); frame.show(); • The frame has a container that manages a collection of components; a button is an example of a JComponentDec 21, 2012 Design Patterns Chapter 8A12
    • 13. What is a JComponent • JComponent is a superclass used in developing user interface components – JButton extends JComponent • JComponent provides implementations for the methods that are used by a container • Two key methods are: – paintComponent that draws the component – getPrefferedSize that returns the preferred dimension of a component • A PeteIcon knows how to draw itself and can return both a width and a height – But it cannot be used in place of a JComponent – The interface does not matchDec 21, 2012 Design Patterns Chapter 8A13
    • 14. Using an Adapter • An adapter is needed to use an instance of an Icon to fulfil the role of a JComponent – An Icon has the appropriate functionality but an inappropriate interface • The adapter must extend the JComponent class and contain an instance of an icon to do the work – Calling a method of the adapter may result in a call to a method of the contained iconDec 21, 2012 Design Patterns Chapter 8A14
    • 15. An Icon Adapter Class public class IconAdapter extends JComponent { private Icon icon; public IconAdapter (Icon aIcon) { this.icon = aIcon; } public void paintComponent(Graphics g) { icon.paintIcon(this,g,0,0); } public Dimension getPreferredSize() { return new Dimension( icon.getIconWidth(), icon.getIconHeight()); } }Dec 21, 2012 Design Patterns Chapter 8A15
    • 16. Using the Adapter • An instance of an IconAdapter can be used as a component – Built from an instance of any Icon • For example: JComponent c = new IconAdapter ( new PeteIcon(100)) ; contentPane.add(c,BorderLayout.CENTER); • Although a PeteIcon is used here any Icon could be used – The IconAdapter adapts an IconDec 21, 2012 Design Patterns Chapter 8A16
    • 17. Using the Adapter Class JFrame JComponent «interface» +paintComponent() Icon +getPreferredSize() IconAdapter PeteIcon +paintComponent() 1 1 +getPreferredSize()Dec 21, 2012 Design Patterns Chapter 8A17
    • 18. The Adapter Pattern • This is an example of the adapter pattern – Used as a structural object pattern • The situation in which the pattern is useful can be summed up as: – You want to use an existing class, called the adaptee, without modifying it – The system in which the class is to be used requires conformance to a target interface that is different to the interface of the adaptee – The adaptee has the functionality required by the target interfaceDec 21, 2012 Design Patterns Chapter 8A18
    • 19. The Adapter Pattern • Intent – Convert the interface of a class into another interface; the interface expected by a client – The adapter lets classes work together that could not do so otherwise because of incompatible interfaces • Also known as Wrapper • Applicability Use the adapter pattern when: – You want to use an existing class, and its interface does not match the one that you need – You want to create a re-useable class that co-operates with unrelated or unforeseen classesDec 21, 2012 Design Patterns Chapter 8A19
    • 20. The Adapter Pattern Participants • Target – Defines the domain-specific interface that the Client uses • Client – Collaborates with objects that conform to the Target interface • Adaptee – Defines an existing interface that needs to be adapted • Adapter – Adapts the interface of the Adaptee to the Target interfaceDec 21, 2012 Design Patterns Chapter 8A20
    • 21. The Adapter Pattern • Structure – An object adapter relies on object composition:Dec 21, 2012 Design Patterns Chapter 8A21
    • 22. The Adapter Pattern • Structure – A class adapter uses multiple inheritanceDec 21, 2012 Design Patterns Chapter 8A22
    • 23. Consequences – The Class Adapter • A class Adapter adapts an Adaptee to a Target by commiting to a concrete Adaptee class – As a consequence it is not possible to adapt a class and all of its subclasses • A class Adapter allows the Adapter to override some of the Adaptee’s behaviour – because the Adapter is a subclass of the Adaptee • A class Adapter introduces only one object and therefore no additional runtime overhead for method invocation • Now compare these trade offs with an object adapterDec 21, 2012 Design Patterns Chapter 8A23
    • 24. Consequences – The Object Adapter • An Object Adapter allows a single adapter to work with many Adpatees – The Adaptee itself and any subclasses of the Adaptee – The Adapter can add functionality to all Adaptees at once • An Object Adapter makes it harder to override Adaptee behaviour – It is necessary to subclass the Adpatee and ensure that the Adapter refers to the subclass rather than to the original AdapteeDec 21, 2012 Design Patterns Chapter 8A24
    • 25. How much adapting does an Adapter do? • In the example seen earlier in this lecture the adapter did little more than interface conversion – It changed the name of one method – It implemented another method of the target interface by calling two methods of the adaptee interface • In other examples an adapter may do much more – It may use several methods of an adaptee plus some additional code to implement a method of the target interface – It may add additional behaviour, not provided in the adaptee, in order to fulfil the interface requirements of the targetDec 21, 2012 Design Patterns Chapter 8A25
    • 26. Java and Design Patterns • The Java language contains many examples of the use of design patterns • For example consider the case where you have an input stream and you need a reader – An input stream reads bytes – A reader reads characters – The difference between the two is significant in many languages • In Java an InputStreamReader adapter is used – Reader myReader = new InputStreamReader (System.in);Dec 21, 2012 Design Patterns Chapter 8A26
    • 27. Java InputStreamReader Adapter • In this case: Adaptee InputStream Target Reader Adapter InputStreamReader Client the class that wants to read text from an input stream • As an exercise search for as many examples of the use of the adapter pattern as you can find in JavaDec 21, 2012 Design Patterns Chapter 8A27
    • 28. Summary In this lecture we have: • Reviewed structural design patterns • Discussed why such patterns are useful • Introduced the Adapter pattern • Discussed examples that take advantage of the adapter patternDec 21, 2012 Design Patterns Chapter 8A28