1. Java Design Patterns Java Design Patterns

2. What are design patterns? the best solution for a recurring problem a technique for making code more flexible by making it meet certain criteria

3. Why design patterns? The primary goal is to help improve the quality of the software in terms of the software being reusable, maintainable, extensible, etc. Reduce the development time

4. Three main categories of design patterns Creational: the creation of objects Creational: the creation of objects. Structural:how one object relates with another object. Structural:how one object relates with another object. Behavioral: Behavioral:communication mechanism between objects (invoke method)

5. Creational Creational Patterns 1. Factory Method:- Creates an instance of several derived classes 2. Singleton:- A class in which only a single instance can exist 3. Abstract Factory:- Creates an instance of several families of classes 4. Prototype:- A fully initialized instance to be copied or cloned 5. Builder: - Separates object construction from its representation

6. Structural Patterns 1. Adapter:- Match interfaces of different classes. 2. Bridge:- Separates an object's abstraction from its implementation. 3. Composite:- A tree structure of simple and composite objects. 4. Decorator:-Add responsibilities to objects dynamically. 5. Facade:- A single class that represents an entire subsystem. 6. Flyweight:- A fine-grained instance used for efficient sharing. 7. Proxy:- An object representing another object.

7. Patterns Behavioral Patterns 1. Mediator:- Defines simplified communication between classes. 2. Memento:- Capture and restore an object's internal state. 3. Interpreter:- A way to include language elements in a program. 4. Iterator:- Sequentially access the elements of a collection. 5. Chain of Resp: - A way of passing a request between a chain of objects. 6. Command:- Encapsulate a command request as an object. 7. State:- Alter an object's behavior when its state changes. 8. Strategy:- Encapsulates an algorithm inside a class. 9. Observer: - A way of notifying change to a number of classes. 10. Template Method:- Defer the exact steps of an algorithm to a subclass. 11. Visitor:- Defines a new operation to a class without change.

8. Examples

9. Singleton design pattern Creational pattern ensure that a class has only one instance, and to provide a global point of access to it Example: Class SomeClass { static SomeClass singleTonInstance = null; static SomeClass GetInstance() { if(singleTonInstance == null) singleTonInstance = new SomeClass() singleTonInstance = new SomeClass() return singleTonInstance; }}

10. Factory design pattern - example abstract class GUIFactory { public static GUIFactory getFactory() { int sys = readFromConfigFile("OS_TYPE"); return sys == 0 ? new WinFactory() : new OSXFactory(); } public abstract Button createButton(); public abstract Button createButton();} class WinFactory:GUIFactory { public override Button createButton() { return new WinButton(); }} class MacFactory:GUIFactory { public override Button createButton(){ return new MacButton(); }} abstract class Button { public string caption; public abstract void paint(); }

11. Abstract Factory design pattern - example try { Class.forName("sun.jdbc.odbc.JdbcOdbcDriver"); Connection con = DriverManager.getConnection("jdbc:odbc: dumy"; "sa", ""); try { Class.forName("sun.jdbc.odbc.JdbcOdbcDriver"); Connection con = DriverManager.getConnection("jdbc:odbc: dumy"; "sa", ""); Statement stmt = con.createStatement(); String query = "SELECT * FROM " + TABLE_NAME + ";"; ResultSet table = stmt.executeQuery(query); Statement stmt = con.createStatement(); String query = "SELECT * FROM " + TABLE_NAME + ";"; ResultSet table = stmt.executeQuery(query); } catch( Exception e ) { e.printStackTrace(); } catch( Exception e ) { e.printStackTrace(); }

12. Decorator design pattern Structural Pattern Avoid excessive sub-classing and gain run time flexibility Example: Java.IO package BufferedReader br = new BufferedReader( new InputStreamReader( new FileInputStream(inFile))); All derives from abstract io.Reader

13. Strategy design pattern Behavioral Pattern defines a family of interchangeable encapsulated algorithms that receives the same input type and provides the same output type in different manners that can be determined in run-time. static void Main( { SortedList studentRecords = new SortedList(); studentRecords.Add("Samual"); studentRecords.Add("Jimmy"); studentRecords.Add("Sandra"); studentRecords.SetSortStrategy(new QuickSort()); studentRecords.Sort(); studentRecords.SetSortStrategy(new ShellSort()); studentRecords.Sort(); SortedList studentRecords = new SortedList(); studentRecords.Add("Samual"); studentRecords.Add("Jimmy"); studentRecords.Add("Sandra"); studentRecords.SetSortStrategy(new QuickSort()); studentRecords.Sort(); studentRecords.SetSortStrategy(new ShellSort()); studentRecords.Sort(); }

14. Strategy design pattern - example abstract class SortStrategy { public abstract void Sort(ArrayList list) } class QuickSort : SortStrategy { public override void Sort(ArrayList list) { list.Sort(); // Default is Quicksort public override void Sort(ArrayList list) { list.Sort(); // Default is Quicksort }} class ShellSort : SortStrategy { public override void Sort(ArrayList list) { //list.ShellSort(); not-implemented public override void Sort(ArrayList list) { //list.ShellSort(); not-implemented } }

15. class SortedList { private ArrayList list = new ArrayList(); private SortStrategy sortstrategy; public void SetSortStrategy(SortStrategy sortstrategy) { this.sortstrategy = sortstrategy; } public void Add(string name) { private ArrayList list = new ArrayList(); private SortStrategy sortstrategy; public void SetSortStrategy(SortStrategy sortstrategy) { this.sortstrategy = sortstrategy; } public void Add(string name) { list.Add(name); } public void Sort() { public void Sort() { sortstrategy.Sort(list); sortstrategy.Sort(list); }} Strategy design pattern - example

16. Facade design pattern Structural Pattern Provide a unified interface to a set of interfaces in a subsystem without damaging the genric form of the sub system.

18. The End