Source: Per S Laursen. et al

1. Source: Per S Laursen. et al

2. 23 Patterns • Authors examined a large base of high-qualitycode, to identifyhowcommon software design problems weresolved • No theoreticalwork; pattens arediscovered, not invented • 23 commonstrategies for solving design problems wereidentified, and dubbeddesign patterns

3. 23 Patterns • Design patterns fall in three categories • Creational: How are objects created • Structural: How are objects combined • Behavioral: How do objects interact

4. 23 Patterns • Creational patterns • Singleton • Prototype • Builder • Factory Method • Abstract Factory

5. Creational patterns • Singleton • Ensures a class has only one instance, and provides a global point of access to it. • Factory method • Define an interface for creating an object, but let subclasses decide which class to instantiate. Patterns in programming

6. Singleton • Idea • Exactly 1 instance of a class • A single global point of access for all clients • Example class A { private static A INSTANCE = new A(); // eager initialization public static A getInstance() { return INSTANCE; } private A() { … } // more methods } • Variations • Lazy initialization • The instance is not created before it is needed. • If the instance is never needed, it does not use resources. • getInstance() method might need synchronization. • Protected constructor • Makes it possible to subclass the singleton class Patterns in programming

7. Factory method • Idea • Can return an instance of the specified class, or one of its subclasses • A constructor can only “return” an instance of its own class • Does not have to construct a new object • A constructor must create a new object • Can have any name • A constructor must have the same name as the class Patterns in programming

8. Behavioral patterns • Observer • Define a one-to-many dependency between object so that when one object changes state, all its dependents are notified (and updated) automatically. • Template method • Define a skeleton of an algorithm, deferring some steps to subclasses. Patterns in programming

9. Observer • Idea • Decouples otherwise dependent classes. • Observers register with an observable class. • When something happens (like change of state) in the observable, all observers are notified. • Usage • Swing / AWT • Visual components send events to listeners. • JavaBeans • Components sends property change events to listeners. • Variations • Data is sent with the notification. • Notification does not contain data. Data is fetched from the observable using get-methods. Patterns in programming

10. Template method • Idea • Encapsulate part of an algorithm in an abstract method implemented by subclasses. • Example abstract class SuperClass { abstract protected T doPartOfSomething(); public void doSomething() { …doPartOfSomething(); … } } class SubClass1 extends SuperClass { protected T doPartOfSomething() { … } } • Usage • Often used in frameworks • Users of the framework are supposed to extend the abstract classes. Patterns in programming

11. 23 Patterns • Structural patterns • Adapter • Bridge • Composite • Decorator • Facade • Flyweight • Proxy

12. 23 Patterns • Behavioral patterns • Chain of responsibility • Command • Interpreter • Iterator • Mediator • Memento • Observer • State • Strategy • Template Method • Visitor

13. Design Patterns • Are distilled knowledge • Learned from practical experience • Solve a problem in a given context • Provides a common vocabulary • Is not the solution to all SW design issues… • …but almost 

14. Pattern categories • Architectural patterns • High level: Affects a whole application or a major part of an application. • Design patterns • Intermediate level: Affects a few classes. • Idioms • Low level: Affects a single class or part of a single class. • Closing a resource like a file or network connection • try { … use resource … } finally { close resource } • Testing an expected exception in JUnit try { methodThatThrowsExceptions(); fail(…); } catch (SomeException ex) { /* ignore */ } Patterns in programming