Introducing Design Patterns

Transcription

1 Introducing Design Patterns Prof. Dr. Eric Dubuis Berner Biel Course "UML and Design Patterns" of module "Software Engineering and Design", version April 2009 (X) BFH/TI/Software Engineering and Design/UML and Design Patterns/Introducing Design Patterns/April 2009 (X)/Prof. Dr. E. Dubuis Page 1 of 20

2 Learning Goals You'll learn: How a design pattern can be found The iterator pattern Purpose and goal of a design pattern GoF ( Gang of Four Gamma et al.) classical categorization of patterns Format of a pattern catalog BFH/TI/Software Engineering and Design/UML and Design Patterns/Introducing Design Patterns/April 2009 (X)/Prof. Dr. E. Dubuis Page 2 of 20

3 An Example Given a UML class diagram where a linked list is used by a client: 1 * 1 Client X LinkedList X Node +first 1 X Class Node looks like: 1 1 public class Node { 2 private Object value; 3 private Node succ; 4 public Node(Object o) { this.value = o; } 5 public Object getvalue() { return this.value; } 6 public void setsucc(node n) { this.succ = n; } 7 public Node getsucc() { return this.succ; } 8 } Object 1 +succ 1 BFH/TI/Software Engineering and Design/UML and Design Patterns/Introducing Design Patterns/April 2009 (X)/Prof. Dr. E. Dubuis Page 3 of 20

4 The class LinkedList looks like: 1 public class LinkedList { 2 private Node first; 3 private int count; 4 5 public boolean isempty() {... } 6 public Object get(int index) {... } 7 public void set(int index, Object v) {... } 8 9 public Node getfirstnode() {... } 10 //.. 11 } BFH/TI/Software Engineering and Design/UML and Design Patterns/Introducing Design Patterns/April 2009 (X)/Prof. Dr. E. Dubuis Page 4 of 20

5 Some client might use an instance of LinkedList as follows: 1 { 2 LinkedList list =...; // created and some objects added // access every element of the list: 5 6 for (Node tmp = list.getfirstnode(); 7 tmp!= null; tmp = tmp.getsucc()) { 8 Object v = tmp.getvalue(); 9 // do something with the object } } BFH/TI/Software Engineering and Design/UML and Design Patterns/Introducing Design Patterns/April 2009 (X)/Prof. Dr. E. Dubuis Page 5 of 20

6 Another Example Client 1 X 1 1 * Department X Employee Interface List: -emps: ArrayList +getemployees: List +print() int size() Object get(int index); public List getemployees() { return this.emps; } BFH/TI/Software Engineering and Design/UML and Design Patterns/Introducing Design Patterns/April 2009 (X)/Prof. Dr. E. Dubuis Page 6 of 20

7 A client: 1 Department dep =...; // created and Employee instances added 2 List employees = dep.getemployees(); 3 // access every element of the list: 4 5 for (int i = 0; i < employees.size(); i++) { 6 Employee e = (Employee) employees.get(i); 7 // do something with e... 8 } 9 10 BFH/TI/Software Engineering and Design/UML and Design Patterns/Introducing Design Patterns/April 2009 (X)/Prof. Dr. E. Dubuis Page 7 of 20

8 A New Version of Class Department 1 public class Department { // version 2 2 private Map emps = new HashMap(); // no generics used here 3 4 public void addemployee(employee e) { 5 String id = generateid(); // helper, not shown 6 this.emps.put(id, e); 7 } 8 9 // A first idea of the return type for getemployees() might be: 10 // public Map getemployees() { 11 // return this.emps; 12 // } Changing the return type of the method ==> clients have no fun! BFH/TI/Software Engineering and Design/UML and Design Patterns/Introducing Design Patterns/April 2009 (X)/Prof. Dr. E. Dubuis Page 8 of 20

9 // A second idea of the return type for getemployees() might be: 22 public Set getemployees() { // return type Set => clients need to be 23 return this.emps.values(); // modified 24 } Clients still have no fun! } BFH/TI/Software Engineering and Design/UML and Design Patterns/Introducing Design Patterns/April 2009 (X)/Prof. Dr. E. Dubuis Page 9 of 20

10 What's the Problem? Problem for the clients: If the data structure changes ==> the client needs to be modified!! And: For every collection: Access to elements in clients differ... Unless: there is a standard way to iterate over an arbitrary collection! BFH/TI/Software Engineering and Design/UML and Design Patterns/Introducing Design Patterns/April 2009 (X)/Prof. Dr. E. Dubuis Page 10 of 20

11 Idea an auxiliary object with common, fixed interface needed: Communication Diagram 1: iterator :Collection o1:object What we need is: :Client 2: new o2:object a way to get the auxiliary object from any kind of object 3: oper i x:x on:object a common way to access the object a standardized interface of the auxiliary object (next slide) BFH/TI/Software Engineering and Design/UML and Design Patterns/Introducing Design Patterns/April 2009 (X)/Prof. Dr. E. Dubuis Page 11 of 20

12 How Should This Object Look Like? a method to get the next value of the collection: without generics: Object next(); with generics: E next(); a method to know if there are more other values: boolean hasnext(); BFH/TI/Software Engineering and Design/UML and Design Patterns/Introducing Design Patterns/April 2009 (X)/Prof. Dr. E. Dubuis Page 12 of 20

13 Solution A solution to the problem to iterate over an arbitrary collection in a standardized way is the ITERATOR. Interface (using genercis only) 1 public interface Iterator<E> { 2 public E next(); 3 public boolean hasnext(); 4 [public void remove();] // not discussed here 5 } BFH/TI/Software Engineering and Design/UML and Design Patterns/Introducing Design Patterns/April 2009 (X)/Prof. Dr. E. Dubuis Page 13 of 20

14 A Collection: 1 public class SomeCollection<E> { 2 //... 3 public Iterator<E> iterator() {... } 4 } A Client: Sort of factory method 1 SomeCollection<Person> c =...; // collection initialized and filled, not shown... 2 //... 3 for (Iterator<Person> it = c.iterator(); it.hasnext(); ) { 4 Person p = it.next(); 5 //... 6 } BFH/TI/Software Engineering and Design/UML and Design Patterns/Introducing Design Patterns/April 2009 (X)/Prof. Dr. E. Dubuis Page 14 of 20

15 Usage of an Iterator for the LinkedList Example (w/o Generics) Assume that class LinkedList offers now an iterator: 1 public class LinkedList { 2 //... 3 public Iterator iterator() { return new LinkedListIterator(this); } 4 } The a client can iterate over the elements of the LinkedList instance: 1 // in a client 2 LinkedList list =...; // created and filled with objects for (Iterator it = list.iterator(); it.hasnext(); ) { 5 Object v = (Object) it.next(); // use object v } BFH/TI/Software Engineering and Design/UML and Design Patterns/Introducing Design Patterns/April 2009 (X)/Prof. Dr. E. Dubuis Page 15 of 20

16 Exercise Given class LinkedList of page 3, the Iterator class looks like: public class LinkedList { //... private class LinkedListIterator implements Iterator { private Node node; public LinkedListIterator(LinkedList l) { this.node = l.getfirstnode(); } public Object next() { if (this.node == null) { throw new NoSuchElementException(... ); public boolean hasnext() { return this.node!= null; } } Object v = this.node.getvalue(); this.node = this.node.getsucc(); return v; } // end of LinkedListIterator } // end of Linked List BFH/TI/Software Engineering and Design/UML and Design Patterns/Introducing Design Patterns/April 2009 (X)/Prof. Dr. E. Dubuis Page 16 of 20

17 Usage of an Iterator for the Department Class Given a sketch of the Department class: 1 public class Department { // version 3 2 private Map<Employee> emps = new HashMap<Employee>(); 3 //... 4 public Iterator<Employee> getemployees() { 5 return this.emps.values().iterator(); 6 } A client can iterate over the Employee objects: name of factory 1 // in a client method not always 2 Departmet dep =...; // created and filled with employees iterator for (Iterator<Employee> it = dep.getemployees(); it.hasnext(); ) { 5 Employee e = it.next(); // and use e... 6 } BFH/TI/Software Engineering and Design/UML and Design Patterns/Introducing Design Patterns/April 2009 (X)/Prof. Dr. E. Dubuis Page 17 of 20

18 Reflection underlying set of objects should NOT change while an iterator is in use or: iterator gets invalidated ==> fail-fast iterator iterator makes a copy ==> expensive Since Java 5: weakly consistent iterators (see Java documentation) one-time use iterator vs. reusable iterators (GoF iterator: reusable!) public interface ListIterator<E> extends Iterator<E> { } //... public E previous(); public boolean hasprevious(); who creates iterator? ==> the collection itself! BFH/TI/Software Engineering and Design/UML and Design Patterns/Introducing Design Patterns/April 2009 (X)/Prof. Dr. E. Dubuis Page 18 of 20

19 General Structure «interface» Iterator hasnext: boolean next: E E «call» Client «call» «interface» Collection add(o: E) iterator(): Iterator<E> E Concrete Iterator E Concrete Collection E hasnext(): boolean next(): E «new» add(o: E) iterator(): Iterator<E> BFH/TI/Software Engineering and Design/UML and Design Patterns/Introducing Design Patterns/April 2009 (X)/Prof. Dr. E. Dubuis Page 19 of 20

20 What Have We Achieved? we've got a general structure for iterating over arbitrary collections ==> behavioral pattern ==> a category of design patterns creational, structural, and behavioral patterns Catalog of Design Patterns, general (simplified) format of an entry: Problem (what's the problem to solve?) Forces (what makes the problem addressed so interesting?) Solution (what is a solution that can be applied again and again?) Name (how can we refer to the problem/solution pair?) Gamma E., Helm R., Johnson R., Vlissides J.: "Design Patterns -- Elements of Reusable Object-Oriented Software", Addison-Wesley Professional, 1995, ISBN BFH/TI/Software Engineering and Design/UML and Design Patterns/Introducing Design Patterns/April 2009 (X)/Prof. Dr. E. Dubuis Page 20 of 20