1. 1 Inheritance and Subclasses

2. 2 Inheritance Often we create very similar classes –Different types of triangles: equilateral, isosceles, etc. –Different types of quadrilaterals: squares, rectangles, parallelograms, etc. –Different types of animals: dogs, cows, etc. It seems wasteful to write these classes from scratch –These have many features in common Inheritance in Java allows you to extend a base class to another class with similar features

3. 3 Inheritance Usually a class hierarchy is implemented through inheritance –Example: a polygon hierarchy The class that inherits from a base class becomes a subclass of the base class –The constructors are not inherited –Each subclass must offer its own constructor; however it is possible to access the constructors of the base class –All public members and methods are inherited in the subclass –The subclass may define additional members and methods; same methods override those in the base class

4. 4 Polygon hierarchy class Point { // to be used later private double x; private double y; public Point (double x, double y) { this.x = x; this.y = y; } public Point (Point p) { // copy constructor this.x = p.GetX(); this.y = p.GetY(); } // next slide

5. 5 Polygon hierarchy public double GetX () { return x; } public double GetY () { return y; } public void SetX (double x) { this.x = x; } public void SetY (double y) { this.y = y; } // next slide

6. 6 Polygon hierarchy public double Distance (Point p) { return Math.sqrt ((x-p.GetX())*(x- p.GetX()) + (y-p.GetY())*(y-p.GetY())); } } // end class

7. 7 Polygon hierarchy class Polygon { private int numVertices; public Point vertices[]; public Polygon (Point vertices[]) { int i; numVertices = vertices.length; this.vertices = new Point[numVertices]; for (i=0; i<numVertices; i++) { this.vertices[i] = new Point (vertices[i]); } } // next slide

8. 8 Polygon hierarchy public Polygon (double x[], double y[]) { int i; numVertices = x.length; vertices = new Point[numVertices]; for (i=0; i<numVertices; i++) { vertices[i] = new Point (x[i], y[i]); } // next slide

9. 9 Polygon hierarchy public double Perimeter () { int i; double perimeter = 0; // Assume that the vertices are in order for (i=0; i<numVertices-1; i++) { perimeter += vertices[i].Distance (vertices[i+1]); } perimeter += vertices[i].Distance (vertices[0]); System.out.println (“This is perimeter of Polygon class.”); return perimeter; } // next slide

10. 10 Polygon hierarchy public boolean isRegular () { int i; double lastSide = vertices[0].Distance (vertices[1]); double thisSide; for (i=1; i<numVertices-1; i++) { thisSide = vertices[i].Distance (vertices[i+1]); if (lastSide != thisSide) return false; } // next slide

11. 11 Polygon hierarchy thisSide = vertices[i].Distance (vertices[0]); if (lastSide != thisSide) return false; return true; } // next slide

12. 12 Polygon hierarchy public Point Centroid () { int i; double x = 0, y = 0; for (i=0; i<numVertices; i++) { x += vertices[i].GetX(); y += vertices[i].GetY(); } return (new Point (x/numVertices, y/numVertices)); } } // end class

13. 13 Polygon hierarchy class Triangle extends Polygon { private double a, b, c; // new members public Triangle (Point vertices[]) { super (vertices); // base class constr. // Other member initialization must // come after call to super a = vertices[0].Distance (vertices[1]); b = vertices[1].Distance (vertices[2]); c = vertices[2].Distance (vertices[0]); } // next slide

14. 14 Polygon hierarchy public Triangle (double x[], double y[]) { super (x, y); a = vertices[0].Distance (vertices[1]); b = vertices[1].Distance (vertices[2]); c = vertices[2].Distance (vertices[0]); } // next slide

15. 15 Polygon hierarchy // Add a new method public double Area () { double term1 = 0.5*Perimeter () – a; double term2 = 0.5*Perimeter () – b; double term3 = 0.5*Perimeter () – c; return (Math.sqrt (0.5*Perimeter () * term1 * term2 * term3)); } // next slide

16. 16 Polygon hierarchy // One more new method public double[] Angles () { double angles[] = new double[3]; angles[0] = Math.asin (2*Area()/(b*c)); angles[1] = Math.asin (2*Area()/(c*a)); angles[2] = Math.asin (2*Area()/(a*b)); return angles; } // next slide

17. 17 Polygon hierarchy // Override Perimeter with a simpler one public double Perimeter () { System.out.println (“This is perimeter of Triangle class.”); return (a+b+c); } } // end class // next slide

18. 18 Polygon hierarchy class Equilateral extends Triangle { public Equilateral (Point vertices[]) { super (vertices); } public Equilateral (double x[], double y[]) { super (x, y); } public double Median () { return (0.5*vertices[0].Distance(vertices[1])*Math.sq rt (3.0)); } } // end class

19. 19 Polygon hierarchy class PolygonBuilder { public static void main (String a[]) { double x[] = {0, 0.5, -0.5}; double y[] = {0.5*Math.sqrt(3.0), 0, 0}; Equilateral eqT = new Equilateral (x, y); System.out.println (“Perimeter: ” + eqT.Perimeter()); System.out.println (“Area: ” + eqT.Area()); System.out.println (“Centroid: (” + eqT.Centroid().GetX() + “, ” + eqT.Centroid().GetY() + “)”); System.out.println (“Median: ” + eqT.Median()); } } // end class

20. A protected data or method in a public can be accessed by any class in the same package or subclass in other package 20 modifierSame classSame package subclassDifferent package publicYYyy protectedyyy n (default)yynn privateynnn

21. Example package p1; public class C1{ protected int y; public int x; private int u; protected void m(){ } 21

22. public class C2{ } public class C3 extends C1{ } package p2; public class C4 extends C1{ } public class C5{ } 22

23. Preventing Extending and Overriding Use Keyword final public final class C{ } //no subclasses public class test{ public final void m(){ } 23

24. Abstract classes A superclass which can not have any specific instances can be declared abstract. Methods which can not be implemented in an abstract class use modifier abstract. 24

25. public abstract class Geometricobject{ private String color = “white”; private boolean filled; private java.util.Date dateCreated; protected Geometricobject(){ dateCreated=new java.util.Date(); } public String getColor(){ return color; } …. public abstract double getArea(); public abstract double getPerimeter(); } 25

26. public class TestGO{ public static void main(String[] args){ GeometricObject o1= new Circle(5); GeometricObject o2= new rect(5,3); System.out.println (“Equal area?”+ equalArea (o1,o2); displayGO(o1); displayGO(o2); } public static boolean equalArea (GeometricObject o1, GeometricObject o2){ return o1.getArea()==o2.getArea(); } public static void displayGO(GeometricObject o){ System.out.println (); System.out.println (“Area” + o.getArea()); System.out.println (“Perimeter” + o.getPerimeter()); } 26

27. 27 Multiple inheritance Java does not allow inheritance from more than one base classes –At most one extension –However, one can “implement” multiple interfaces –Interface is a collection of abstract methods i.e. no method body is specified –A class implementing an interface must provide the suitable method bodies

28. 28 Multiple inheritance interface RegularPolygon { public double Circumradius (); //abstract } class Equilateral extends Triangle implements RegularPolygon { public Equilateral (Point vertices[]) { super (vertices); } public Equilateral (double x[], double y[]) { super (x, y); } // next slide

29. 29 Multiple inheritance public double Median () { return (0.5*vertices[0].Distance(vertices[1])*M ath.sqrt (3.0)); } public double Circumradius () { return (2*Median()/3); } } // end class // You can now print eqT.Circumradius() in // PolygonBuilder class.

30. Comparable interface package java.lang; public interface Comparable{ public int compareTo(Object o); } public class String extends Object implements Comparable{ } public class Date extends Object implements Comparable{ } Each of these have to define method compareTo in its body. 30