1. Design Patterns Yonglei Tao

2. Design Patterns  A design pattern describes a recurring design problem, a solution, and the context in which that solution works  A common way of doing things  Specialization of a design principle  Tested knowledge  A named description of a technique with its benefits and costs

3. The Strategy Pattern  Problem  how to design for varying, but related, algorithms or policies  how to design for the ability to change them  Solution  define a family of algorithms, encapsulate each one in a separate class, and make them interchangeable

4. A Sorting Example  Shellsort  No extra space needed, fast but not optimal  Very fast on nearly sorted data  Does comparatively well on small lists  Quicksort  Average case is O (n log n)  Relatively poor performance on short lists  Require a stack of log(n) in depth  Mergesort  Worst case is O (n log n)  Requires O (n) extra space  Stable

5. Sorting (cont.)  Have a sorted list container, which one gives a sort algorithm SortedList studentRecords = new SortedList( new ShellSort() ); studentRecords.add( “Sam” ); … pubic class SortedList { Object[ ] elements; SortStrategy sorter; void sort( ) { sorter.sort( elements); } }

7. More on Strategy t = getDiscount(s)

8. The Factory Pattern  Problem  a class cannot anticipate the class of objects it must create  how to handle object creation with complex creation logic or other special considerations  Solution  encapsulate the knowledge of object creation and move it out of the class that uses the object  define an interface for creating an object, but let subclasses decide which class to instantiate

9. Factory Method

10. The Singleton Pattern  Problem  objects need a global and single point of access  Solution  ensure a class has exactly one instance  define a static method that returns the singleton  Consequences  facilitates communication between objects not directly coupled  reduce coupling between objects in a system

11. // get a reference to the singleton object theSingleton = Singleton.getInstance(); The Singleton Pattern

12. Implementation class Singleton { private static Singleton _instance = null; private Singleton() { … } public static Singleton getInstance() { if ( _instance == null ) _instance = new Singleton(); return _instance; } public void otherOperations() { … } } class Program { public void aMethod() { X = Singleton.getInstance(); }

13. Factory

15. Navigational Model for Visual C++

16. Example: the Null Node in Binary Trees

17. public class BinaryNode extends Node { Node left, right; int key; public boolean includes( int value ) { if (key == value) return true; else if (value < key ) return left.includes( value ); else return right.includes(value); } … } public class NullNode extends Node { public boolean includes( int value ) { return false; } … }

18. The Composite Pattern  Problem  How to allow clients to treat individual objects and compositions of objects uniformly  Solution  Organize objects into a tree structure that represents an aggregation hierarchy

19. Structure

20. Use of Composite

21. More on Composite