Introducing Design Patterns

Transcription

1 Introducing Design Patterns Prof. Dr. Eric Biel Course "UML and Design Patterns" of module "Software Engineering and Design", version October 2007 BFH/TI/UML and Design Patterns/Software Engineering and Design/October 2007/Prof. Dr. E. Dubuis Page 1 of 19 An Example Given a UML class diagram where a linked list is used by a client: Class Node looks like: 1 public class Node { 2 private Object value; private Node succ; public Node(Object o) { this.value = o; } public Object getvalue() { return this.value; } public void setsucc(node n) { this.succ = n; } 7 public Node getsucc() { return this.succ; } 8 } BFH/TI/UML and Design Patterns/Software Engineering and Design/October 2007/Prof. Dr. E. Dubuis Page 2 of 19

2 The class LinkedList looks like: 1 public class LinkedList { 2 private Node first; private int count; public boolean isempty() {... } public Object get(int index) {... } 7 public void set(int index, Object v) {... } 8 9 public Node getfirstnode() {... } 10 //.. 11 } BFH/TI/UML and Design Patterns/Software Engineering and Design/October 2007/Prof. Dr. E. Dubuis Page of 19 Some client might use an instance of LinkedList as follows: 1 { 2 LinkedList list =...; // access every element of the list: } BFH/TI/UML and Design Patterns/Software Engineering and Design/October 2007/Prof. Dr. E. Dubuis Page of 19

3 Another Example Client Interface List: int size() Object get(int index); Department -emps: ArrayList +getemployees: List +print() Employee BFH/TI/UML and Design Patterns/Software Engineering and Design/October 2007/Prof. Dr. E. Dubuis Page of 19 A client: 1 Department dep =...; 2 List employees = dep.getemployees(); // access every element of the list: BFH/TI/UML and Design Patterns/Software Engineering and Design/October 2007/Prof. Dr. E. Dubuis Page of 19

4 A New Version of Class Department 1 public class Department { // version BFH/TI/UML and Design Patterns/Software Engineering and Design/October 2007/Prof. Dr. E. Dubuis Page 7 of } BFH/TI/UML and Design Patterns/Software Engineering and Design/October 2007/Prof. Dr. E. Dubuis Page 8 of 19

5 What's the Problem? Problem for the clients: ==> the client needs to be modified!! Unless: BFH/TI/UML and Design Patterns/Software Engineering and Design/October 2007/Prof. Dr. E. Dubuis Page 9 of 19 Idea an auxiliary object with common, fixed interface needed: What we need is: a way to get the auxiliary object from any kind of object a common way to access the object a standardized interface of the auxiliary object (next slide) BFH/TI/UML and Design Patterns/Software Engineering and Design/October 2007/Prof. Dr. E. Dubuis Page 10 of 19

6 How Should This Object Look Like? a method to get the next value of the collection: a method to know if there are more other values: BFH/TI/UML and Design Patterns/Software Engineering and Design/October 2007/Prof. Dr. E. Dubuis Page 11 of 19 Solution A solution to the problem to iterate over an arbitrary collection in a standardized way is the... Interface: 1 public interface Iterator<E> { 2 [public void remove();] } BFH/TI/UML and Design Patterns/Software Engineering and Design/October 2007/Prof. Dr. E. Dubuis Page 12 of 19

7 A Collection: 1 public class SomeCollection<E> { 2 //... public Iterator<E> iterator() {... } } A Client: 1 SomeCollection<Person> c =...; 2 //... BFH/TI/UML and Design Patterns/Software Engineering and Design/October 2007/Prof. Dr. E. Dubuis Page 1 of 19 Usage of an Iterator for the LinkedList Example (w/o Generics) Assume that class LinkedList offers now an iterator: 1 public class LinkedList { 2 //... public Iterator iterator() { return new ; } } The a client can iterate over the elements of the LinkedList instance: 1 // in a client 2 LinkedList list =...; 7 BFH/TI/UML and Design Patterns/Software Engineering and Design/October 2007/Prof. Dr. E. Dubuis Page 1 of 19

8 Exercise Given class LinkedList of page, write (on paper) a non-generic iterator class. public class LinkedList { //... private class LinkedListIterator implements Iterator { } // end of LinkedListIterator } // end of Linked List BFH/TI/UML and Design Patterns/Software Engineering and Design/October 2007/Prof. Dr. E. Dubuis Page 1 of 19 Usage of an Iterator for the Department Class Given a sketch of the Department class: 1 public class Department { // version 2 private Map<Employee> emps = new HashMap<Employee>(); //... A client can iterate over the Employee objects: 1 // in a client 2 Departmet dep =...; BFH/TI/UML and Design Patterns/Software Engineering and Design/October 2007/Prof. Dr. E. Dubuis Page 1 of 19

9 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 Java : weakly consistent iterators (see Java documentation) one-time use iterator vs. reusable iterators public interface ListIterator<E> extends Iterator<E> { } //... public E previous(); who creates iterator? BFH/TI/UML and Design Patterns/Software Engineering and Design/October 2007/Prof. Dr. E. Dubuis Page 17 of 19 General Structure «interface» Iterator E «interface» Collection E hasnext: boolean next: E Client add(o: E) iterator(): Iterator<E> Concrete Iterator E Concrete Collection E hasnext(): boolean next(): E add(o: E) iterator(): Iterator<E> BFH/TI/UML and Design Patterns/Software Engineering and Design/October 2007/Prof. Dr. E. Dubuis Page 18 of 19

10 What Have We Achieved? we've got a general structure for iterating over arbitrary collections ==> behavior pattern ==> a category of design 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, 199, ISBN BFH/TI/UML and Design Patterns/Software Engineering and Design/October 2007/Prof. Dr. E. Dubuis Page 19 of 19