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Inheritance and Subclasses

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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

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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

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Polygon hierarchyclass 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

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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

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Polygon hierarchy public double Distance (Point p) { return Math.sqrt ((x-p.GetX())*(x-

p.GetX()) + (y-p.GetY())*(y-p.GetY())); }} // end class

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Polygon hierarchyclass 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

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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

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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

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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

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Polygon hierarchy thisSide = vertices[i].Distance

(vertices[0]); if (lastSide != thisSide) return false; return true; } // next slide

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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

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Polygon hierarchyclass 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

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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

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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

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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

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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

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Polygon hierarchyclass 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.sqrt (3.0));

}} // end class

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Polygon hierarchyclass 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

• A protected data or method in a public can be accessed by any class in the same package or subclass in other package

•

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modifier Same class

Same package

subclass Different package

public Y Y y yprotected y y y

n(default) y y n nprivate y n n n

Examplepackage p1;public class C1{protected int y;public int x;private int u;protected void m(){}}

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public class C2{}public class C3 extends C1{}

package p2;public class C4 extends C1{}public class C5{}

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Preventing Extending and Overriding

Use Keyword final

public final class C{} //no subclassespublic class test{ public final void m(){}}

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Abstract classesA 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.

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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();}

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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());}}

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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

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Multiple inheritanceinterface 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

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Multiple inheritance public double Median () { return

(0.5*vertices[0].Distance(vertices[1])*Math.sqrt (3.0));

} public double Circumradius () { return (2*Median()/3); }} // end class// You can now print eqT.Circumradius()

in// PolygonBuilder class.

Comparable interfacepackage 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.

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