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In this simple program objects of all four classes are created and their fields are read.

  • The objects are created.
  • For each field of each object, a variable is declared in the main method and the value of the field is assigned to it. Check that each value originatesfrom the correct field.

Exercise In CParticle , add the declaration int charge = -1; . Compile and run the program. Is the output different? Explain what happens.

Inheriting methods

Concept Subclasses inherit the methods of its superclasses and can add new methods of its own. You can override an inherited method by writing a new methodwith the same signature as the inherited method.

Program: Inheritance02.java

// Learning Object Inheritance02 //    inheriting and overriding methodsclass Particle {     int position;      Particle(int p) {        position = p;     }      void newPosition(int delta) {        position = position + delta;     }}  class AParticle extends Particle {     double spin;      AParticle(int p, double s) {        super(p);         spin = s;    }      void newPosition(int delta) {     if (spin < delta)             position = position + delta;    } }class BParticle extends Particle {     int charge;      BParticle(int p, int c) {        super(p);         charge = c;    } }class CParticle extends BParticle {     boolean strange;      CParticle(int p, int c, boolean s) {        super(p, c);         strange = s;    }      void newPosition(int delta) {         if (strange)            position = position * charge;     }}  class Inheritance02 {     public static void main(/*String[] args*/) {         Particle  p = new Particle(10);        AParticle a = new AParticle(10, 2.0);         BParticle b = new BParticle(10, 3);        CParticle c = new CParticle(10, 4, true);          p.newPosition(10);         int pPosition = p.position;        a.newPosition(10);         int aPosition = a.position;        b.newPosition(10);         int bPosition = b.position;        c.newPosition(10);         int cPosition = c.position;    } }

This program calls the method newPosition , which is overridden in AParticle and CParticle but not in BParticle .

  • The objects are created.
  • Method newPosition is invoked for each object and the modified value of position is assigned to a variable.
  • Check that the call on p calls the method defined in class Particle .
  • Check that the call on a calls the method defined in the class AParticle ; this method overrides the method declared in class Particle .
  • Check that the call on b calls the method defined in the superclass Particle ; since the method was not overridden in BParticle , the method called is the one inherited from the superclass.
  • Check that the call on c calls the method defined in the class BParticle ; this method overrides the method declared in class Particle .

Exercise Remove the method newPosition from CParticle . Which method is invoked for c.newPosition ?

Exercise Remove the method newPosition from CParticle and add a method with the same signature to BParticle . Which method is invoked for c.newPosition ?

Dynamic dispatching

Concept A variable v of type T can contain a reference to an object of type T or of the type of any subclass of T . When invoking v.m for some method m that is overridden in a subclass, it is the type of the object currently referenced by v (not the type of the variable v ) that determines which method is called. This is called dynamic dispatching because the call is dispatched at runtime.

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Source:  OpenStax, Learning objects for java (with jeliot). OpenStax CNX. Dec 28, 2009 Download for free at http://cnx.org/content/col10915/1.2
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