On Polymorphism and the Open-Closed Principle

Transcription

1 Berner Fachhochschule Engineering and Information Technology On Polymorphism and the Open-Closed Principle Prof. Dr. Eric Dubuis Berner Fachhochschule, Engineering and Information Biel Course "UML and Design Patterns" of module "Software Engineering and Design", academic year BFH/TI/UML and Design Patterns/Software Engineering and Design/ /Prof. Dr. E. Dubuis Page 1 of 14 Session's Goals You'll learn: Review of polymorphism The Open-Closed Principle (OCP) Polymorphism by class inheritance BFH/TI/UML and Design Patterns/Software Engineering and Design/ /Prof. Dr. E. Dubuis Page 2 of 14

2 Readings [Martin]: Agile Software Development, Robert C. Martin, Prentice Hall, ISBN BFH/TI/UML and Design Patterns/Software Engineering and Design/ /Prof. Dr. E. Dubuis Page 3 of 14 Introduction to the Problem Consider a program being able to move shapes. Suppose the following shape classes: 1 public class Rectangle { 2 public int x0 = 10; 3 public int y0 = 10; 4 public int width = 40; 5 public int height = 20; 6 // constructors not shown public class Line { 9 public int x0 = 10; 10 public int y0 = 20; 11 public int x1 = 20; 12 public int y1 = 5; // constructors not shown BFH/TI/UML and Design Patterns/Software Engineering and Design/ /Prof. Dr. E. Dubuis Page 4 of 14

3 Introduction to the Problem (cont'd) Consider a method moveallshapes(shapes) of a class Client wanting to move Line and Rectangle shapes: 1 Object[] shapes =...; // Now let's move the shapes. 4 moveallshapes(shapes); 5 6 // continued on next page BFH/TI/UML and Design Patterns/Software Engineering and Design/ /Prof. Dr. E. Dubuis Page 5 of 14 Introduction to the Problem (cont'd) 7 // continued 8 9 private static void drawallshapes(object[] shapes) { 10 for (int i = 0; i < shapes.length; i++) { 11 int dx =...; int dy =...; 12 if (shapes[i] instanceof Rectangle) { 13 Rectangle r = (Rectangle) shapes[i]; 14 moverectangle(r, dx, dy); // definition see below 15 else if (shapes[i] instanceof Line) { 16 Line l = (Line) shapes[i]; 17 moveline(l, dx, dy); // definition see below 18 else { 19 System.err.println("Error: Cannot handle shape type: " + shapes[i]); 20 BFH/TI/UML and Design Patterns/Software Engineering and Design/ /Prof. Dr. E. Dubuis Page 6 of 14

4 Introduction to the Problem (cont'd) The helper methods to move the rectangles and lines are: 1 private static void moverectangle(rectangle r, int dx, int dy) { private static void moveline(line l, int dx, int dy) { BFH/TI/UML and Design Patterns/Software Engineering and Design/ /Prof. Dr. E. Dubuis Page 7 of 14 Discussion What if: we change the representation of a line: from: to: we add new kind of shapes Big disadvantage of this approach: Changing the representation of a shape or adding other kind of shape type requires the modification of all client code! Another issue is: You can add a new kind of shape, but forget the client to handle it! BFH/TI/UML and Design Patterns/Software Engineering and Design/ /Prof. Dr. E. Dubuis Page 8 of 14

5 Open-Closed Principle [Martin] Software entities (classes, modules, functions, etc.) should be open for extension but closed for modification. (OCP) That is, method moveallshapes(shapes) does not conform to the OCP because it cannot be closed against changes in the sense of changing the representation of a particular shape or in the sense of adding new shape types. If the code for moving shapes of class Client should be extended to move lines with another representation, or if the client should be able to move new kind of shapes, then the method moveallshapes(shapes) must be changed. BFH/TI/UML and Design Patterns/Software Engineering and Design/ /Prof. Dr. E. Dubuis Page 9 of 14 A First Improvement Who knows best how to move a shape? ==> The shapes themselves! public class Client {... private static void moveline(line l, int dx, int dy) {... private static void moverectangle(rectangle r, int dx, int dy) {... public class Rectangle { public class Line { BFH/TI/UML and Design Patterns/Software Engineering and Design/ /Prof. Dr. E. Dubuis Page 10 of 14

6 A First Improvement (cont'd) However, the case distinction in the Client moveallshapes(shapes) method does not disappear: 1 private static void drawallshapes(object[] shapes) { 2 for (int i = 0; i < shapes.length; i++) { 3 int dx =...; int dy =...; 4 if (shapes[i] instanceof Rectangle) { 5 Rectangle r = (Rectangle) shapes[i]; 6 r.move(dx, dy); 7 else if (shapes[i] instanceof Line) { 8 Line l = (Line) shapes[i]; 9 l.moveline(dx, dy); 10 else { 11 System.err.println("Error: Cannot handle shape type: " + shapes[i]); 12 BFH/TI/UML and Design Patterns/Software Engineering and Design/ /Prof. Dr. E. Dubuis Page 11 of 14 Introducing Inheritance to Solve the Shapes Problem Let's introduce the abstract base class Shape: 1 public abstract class Shape { 2 public abstract void move(int dx, int dy); 3 The class hierarchy for shapes now looks like: BFH/TI/UML and Design Patterns/Software Engineering and Design/ /Prof. Dr. E. Dubuis Page 12 of 14

7 Adapting Line and Rectangle public class Rectangle { private int x0, y0, x1, y1; this.x0 = this.x0 + dx; this.y0 = this.y0 + dy; public class Line { this.x0 = this.x0 + dx; this.y0 = this.y0 + dy; this.x1 = this.x1 + dx; this.y1 = this.y1 + dy; BFH/TI/UML and Design Patterns/Software Engineering and Design/ /Prof. Dr. E. Dubuis Page 13 of 14 Method moveallshapes(shapes) of the Client The method moveallshapes(shapes) now looks like: 1 private static void drawallshapes(object[] shapes) { 2 for (int i = 0; i < shapes.length; i++) { 3 int dx =...; int dy =...; 4 Shape s = shapes[i]; 5 //// //// The method is now closed regarding the adding of any new shapes. BFH/TI/UML and Design Patterns/Software Engineering and Design/ /Prof. Dr. E. Dubuis Page 14 of 14