ADAPTER PATTERN

1. Winter 2010 ADAPTER PATTERN Ali Zonoozi Design patterns course Advisor: Dr. Noorhoseini

2. Intent • Convert the interface of a class into another interface clients expect. • Adapter lets classes work together that couldn't otherwise because of incompatible interfaces. • Wrap an existing class with a new interface. • Also Known As -> Wrapper Amirkabir University of Technoloy Computer Engineering Department

3. Motivation Sometimes a toolkit or class library can not be used because its interface is incompatible with the interface required by an application We can not change the library interface, since we may not have its source code Even if we did have the source code, we probably should not change the library for each domain-specific application Amirkabir University of Technoloy Computer Engineering Department

4. Motivation Example: Amirkabir University of Technoloy Computer Engineering Department

5. Participants • Target (shape) • defines the domain-specific interface that Client uses. • Adapter (TextShape) • adapts the interface Adaptee to the Target interface. • Adaptee (TextView) • defines an existing interface that needs adapting. • Client (DrawingEditor) • collaborates with objects conforming to the Target interface. Amirkabir University of Technoloy Computer Engineering Department

6. Structure A class adapter uses multiple inheritance to adapt one interface to another: Amirkabir University of Technoloy Computer Engineering Department

7. Structure An object adapter relies on object composition: Amirkabir University of Technoloy Computer Engineering Department

8. Collaboration Amirkabir University of Technoloy Computer Engineering Department

9. Applicability Use the Adapter pattern when You want to use an existing class, and its interface does not match the one you need You want to create a reusable class that cooperates with unrelated classes with incompatible interfaces Amirkabir University of Technoloy Computer Engineering Department

10. Implementation • How much adapting should be done? • Simple interface conversion that just changes operation names and order of arguments • Totally different set of operations • Does the adapter provide two-way transparency? • A two-way adapter supports both the Target and the Adaptee interface. It allows an adapted object (Adapter) to appear as an Adaptee object or a Target object Amirkabir University of Technoloy Computer Engineering Department

11. Example 1 The classic round pegs and square pegs! Here's the SquarePeg class: /** * The SquarePeg class. * This is the Target class. */ public class SquarePeg { public void insert(String str) { System.out.println("SquarePeg insert(): " + str); } } Amirkabir University of Technoloy Computer Engineering Department

12. Example 1 (continued) And the RoundPeg class: /** * The RoundPeg class. * This is the Adaptee class. */ public class RoundPeg { public void insertIntoHole(String msg) { System.out.println("RoundPeginsertIntoHole(): " + msg); } } If a client only understands the SquarePeg interface for inserting pegs using the insert() method, how can it insert round pegs? A peg adapter! Amirkabir University of Technoloy Computer Engineering Department

13. Example 1 (continued) Here is the PegAdapter class: /** * The PegAdapter class. * This is the Adapter class. * It adapts a RoundPeg to a SquarePeg. * Its interface is that of a SquarePeg. */ public class PegAdapter extends SquarePeg { private RoundPegroundPeg; public PegAdapter(RoundPeg peg) {this.roundPeg = peg;} public void insert(String str) {roundPeg.insertIntoHole(str);} } Amirkabir University of Technoloy Computer Engineering Department

14. Example 1 (continued) Typical client program: // Test program for Pegs. public class TestPegs { public static void main(String args[]) { // Create some pegs. RoundPegroundPeg = new RoundPeg(); SquarePegsquarePeg = new SquarePeg(); // Do an insert using the square peg. squarePeg.insert("Inserting square peg..."); Amirkabir University of Technoloy Computer Engineering Department

15. Example 1 (continued) // Now we'd like to do an insert using the round peg. // But this client only understands the insert() // method of pegs, not a insertIntoHole() method. // The solution: create an adapter that adapts // a square peg to a round peg! PegAdapter adapter = new PegAdapter(roundPeg); adapter.insert("Inserting round peg..."); } } Client program output: SquarePeg insert(): Inserting square peg... RoundPeginsertIntoHole(): Inserting round peg... Amirkabir University of Technoloy Computer Engineering Department

16. Example 2 Consider that we have a third party library that provides sorting functionality through it's NumberSorter class. This is our Adaptee /* * This is our adaptee, a third party implementation of a * number sorter that deals with Lists, not arrays. */ public class NumberSorter { public List<Integer> sort(List<Integer> numbers) { //sort and return return new ArrayList<Integer>(); } } Amirkabir University of Technoloy Computer Engineering Department

17. Example 2 (continued) Our Client deals with primitive arrays rather than Lists. For the sake of this example, lets say we can't change the client to use Lists. We've provided a Sorter interface that expects the client input. This is our target. //this is our Target interface public interface Sorter { public int[] sort(int[] numbers); } Amirkabir University of Technoloy Computer Engineering Department

18. Example 2(continued) Finally, the SortListAdapter implements our target interface and deals with our adaptee, NumberSorter public class SortListAdapter implements Sorter { @Override public int[] sort(int[] numbers) { //convert the array to a List List<Integer> numberList = new ArrayList<Integer>(); //call the adapter NumberSortersorter = new NumberSorter(); numberList= sorter.sort(numberList); //convert the list back to an array and return return sortedNumbers; } } Amirkabir University of Technoloy Computer Engineering Department

19. Example 2 (Continued) int[] numbers = new int[]{34, 2, 4, 12, 1}; Sorter sorter = new SortListAdapter(); sorter.sort(numbers); Amirkabir University of Technoloy Computer Engineering Department

20. Example 3 Notice in Example 1 that the PegAdapter adapts a RoundPeg to a SquarePeg. The interface for PegAdapter is that of a SquarePeg. What if we want to have an adapter that acts as a SquarePeg or a RoundPeg? Such an adapter is called a two-way adapter. One way to implement two-way adapters is to use multiple inheritance, but we can't do this in Java But we can have our adapter class implement two different Java interfaces! Amirkabir University of Technoloy Computer Engineering Department

21. Example 3 (continued) Here are the interfaces for round and square pegs: /** *The IRoundPeg interface. */ public interface IRoundPeg { public void insertIntoHole(String msg); } /** *The ISquarePeg interface. */ public interface ISquarePeg { public void insert(String str); } Amirkabir University of Technoloy Computer Engineering Department

22. Example 3 (continued) Here are the new RoundPeg and SquarePeg classes. These are essentially the same as before except they now implement the appropriate interface. // The RoundPeg class. public class RoundPeg implements IRoundPeg { public void insertIntoHole(String msg) { System.out.println("RoundPeginsertIntoHole(): " + msg); } } // The SquarePeg class. public class SquarePeg implements ISquarePeg { public void insert(String str) { System.out.println("SquarePeg insert(): " + str); } } Amirkabir University of Technoloy Computer Engineering Department

23. Example 3 (continued) And here is the new PegAdapter: /** * The PegAdapter class. * This is the two-way adapter class. */ public class PegAdapter implements ISquarePeg, IRoundPeg { private RoundPegroundPeg; private SquarePegsquarePeg; public PegAdapter(RoundPeg peg) {this.roundPeg = peg;} public PegAdapter(SquarePeg peg) {this.squarePeg = peg;} public void insert(String str) {roundPeg.insertIntoHole(str);} public void insertIntoHole(String msg){squarePeg.insert(msg);} } Amirkabir University of Technoloy Computer Engineering Department

24. Example 3 (continued) A client that uses the two-way adapter: // Test program for Pegs. public class TestPegs { public static void main(String args[]) { // Create some pegs. RoundPegroundPeg = new RoundPeg(); SquarePegsquarePeg = new SquarePeg(); // Do an insert using the square peg. squarePeg.insert("Inserting square peg..."); // Create a two-way adapter and do an insert with it. ISquarePegroundToSquare = new PegAdapter(roundPeg); roundToSquare.insert("Inserting round peg..."); Amirkabir University of Technoloy Computer Engineering Department

25. Example 3 (continued) // Do an insert using the round peg. roundPeg.insertIntoHole("Inserting round peg..."); // Create a two-way adapter and do an insert with it. IRoundPegsquareToRound = new PegAdapter(squarePeg); squareToRound.insertIntoHole("Inserting square peg..."); } } Client program output: SquarePeg insert(): Inserting square peg... RoundPeginsertIntoHole(): Inserting round peg... RoundPeginsertIntoHole(): Inserting round peg... SquarePeg insert(): Inserting square peg... Amirkabir University of Technoloy Computer Engineering Department

26. Related Patterns The Bridge pattern shares structure with object adapter, but diverges on intent. Bridge is concerned with separating an object's interface from its implementation while adapter changes the interface of an existing object The Decorator pattern does not change an object's interface, but it does support recursive composition which adapter does not. In this way, Decorator is more flexible than adapter for dealing with functionality additions Amirkabir University of Technoloy Computer Engineering Department

27. Questions? Amirkabir University of Technoloy Computer Engineering Department