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Exercise Move the declaration of equals to class BParticle , changing the code as needed. What now is the value of c1.equals(b1) ? Explain.

Clone

Concept Assigning a variable containing a reference to another of the same type merely copies thereference so that two fields refer to the same object. The method clone is used to copy the content of an object into a new one. clone is defined in class Object and can be overridden in any class definition.

Program: Inheritance08.java

// Learning Object Inheritance08 //    cloneclass Particle implements Cloneable {     int position;      Particle(int p) {        position = p;     }      void newPosition(int delta) {        position = position + delta;     }      protected Object clone() {        try {             Particle p = (Particle) super.clone();            p.newPosition(10);             return p;        }         catch (CloneNotSupportedException e) {            e.printStackTrace();             throw new Error();        }     }}  class Inheritance08 {     public static void main(String[] args) {         Particle p1 = new Particle(20);        Particle p2 = p1;         p1.newPosition(10);        System.out.println(p1.position);         System.out.println(p2.position);          Particle p3 = (Particle) p1.clone();        System.out.println(p1.position);         System.out.println(p3.position);          p3.newPosition(10);        System.out.println(p1.position);         System.out.println(p3.position);    } }

clone is overridden in class Particle . The class must implement the interface Cloneable , the method of the superclass should be called, and we have to take intoaccount that the method might raise an exception. The method returns the object returned by superclass method after calling newPosition .

  • An object of class Particle is allocated and its reference assigned to the field p1 .
  • An assignment statement copies this reference to the field p2 . Check that they have the same value.
  • The method newPosition is called on p1 , but the value of p2.position is also changed, showing that the two fields point to the same object.
  • An object of class Particle is obtained by calling p1.clone() and its reference assigned to the field p3 . Since clone returns a value of type Object , it must be cast to type Particle before the assignment. Check that the objects referenced by p1 and p3 have different values.
  • Calling p3.newPosition changes only the field in the object referenced by p3 and not the separate object referenced by p1 .

Exercise The method clone can perform arbitrary computation. Modify the program so that new objects are initialized withthe absolute value of the field of the object that is being cloned.

Overloading vs. overriding

Concept Overloading is the use of the same method name with a different parameter signature. Overriding is the use in a subclass of the same method name with the same parameter signature as a method of the superclass.

Program: Inheritance09.java

// Learning Object Inheritance09 //    overloading vs. overridingclass 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;    }      void newPosition(double delta) {     if (position < delta)             spin = spin + delta;    } }  class Inheritance09 {    public static void main(/*String[] args*/) {        Particle  p = new Particle(10);         AParticle a1 = new AParticle(20, -1.0);        AParticle a2 = new AParticle(20, -1.0);          p.newPosition(10);         int pPosition = p.position;        a1.newPosition(10);         int a1Position = a1.position;        a2.newPosition(10.0);         int a2Position = a2.position;    } }

The method newPosition(int delta) is declared in Particle and overridden in AParticle . It is also overloaded by a method with the same name takes a parameter of type double .

  • After allocating three objects p , a1 and a2 , newPosition is called on each one.
  • p.newPosition calls the method declared in class Particle .
  • a1.newPosition calls the method declared in class AParticle that overrides the method in Particle .
  • a2.newPosition calls the overloaded method because the actual parameter is of type double .

Exercise At the end of the program add an assignment p = a1 . Add the method invocations p.newPosition(10) and p.newPosition(10.0) in the main method. Explain what happens.

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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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