Object Reference Conversion

[lakshmi nair]

Rules of Object Reference Conversion (RHE) says ...
a) Interface type can only be converted to another interface type or Object class.if it is an interface it must be the super interface. Now how will we test these rules when its not possible to instantiate an interface?

b) Also arrays may be converted to Object, Cloneable and Serializable interfaces? Why these two interfaces?
c) Object o = new Sub[5]; This compiles but how'll we access the array members?
Thanx
Lakshmi

[Udayan Naik]

Hi lakshmi.The RHE states that the rules are for Object REFERENCE conversion.This means that u need only a reference to test those rules , and not a object.You need not create an object by instantiation for testing those rules.Also observe the examples given in RHE regarding those rules.
All classes are sub-classes of Object.Thus an array can be referenced by an Object reference.Arrays also implement the Cloneable interface which means that calling the clone() method on an array is legal.And Arrays also implement the Serializable interface which means that it is possible to store the array object to persistent storage.
Thus array reference can be converted to Object,Cloneable or Serializable references.

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[Anonymous]

Iakshmi,
For question a), try the read and execute the following application which illustrate the usage of interface and cast between interfaces and Object.

  1. <font color="#007F00">/** The Printable interface should be supported
  2.     by objects that can be output on the screen
  3.     in a textual way.
  4. */</font>
  5. interface Printable
  6. {
  7.     void print();
  8. }
  9. 
 10. class Banana implements Printable
 11. {
 12.     private float weigth;
 13. 
 14.     <font color="#007F00">// constructor</font>
 15.     public Banana(float weigth)
 16.     {
 17.         this.weigth = weigth;
 18.     }
 19. 
 20. 
 21.     <font color="#007F00">// implements the Printable interface</font>
 22.     public void print()
 23.     {
 24.         System.out.println("A banana weighting " + weigth + "Kg");
 25.     }
 26. }
 27. 
 28. class Car implements Printable
 29. {
 30.     private String brand;
 31.     private String model;
 32.     private double maxSpeed;
 33. 
 34.     <font color="#007F00">// constructor</font>
 35.     public Car(String brand, String model, double maxSpeed)
 36.     {
 37.         this.brand = brand;
 38.         this.model = model;
 39.         this.maxSpeed = maxSpeed;
 40.     }
 41. 
 42.     <font color="#007F00">// implements the Printable interface</font>
 43.     public void print()
 44.     {
 45.         System.out.println(brand + " " + model + ", maximum speed = " + maxSpeed);
 46.     }
 47. }
 48. 
 49. public class PrintObjects
 50. {
 51.     public static void main(String args[])
 52.     {
 53.         <font color="#007F00">// create several printable objects</font>
 54.         <font color="#007F00">// and convert the objects into interface</font>
 55.         Printable laguna = new Car("Renault", "Laguna", 170);
 56.         Printable safrane = new Car("Renault", "Safrane", 180);
 57.         Printable smallBanana = new Banana(0.050f);
 58.         Printable bigBanana = new Banana(0.150f);
 59. 
 60.         <font color="#007F00">// create a non printable object</font>
 61.         Integer i = new Integer(111);
 62. 
 63.         <font color="#007F00">// convert to interface references to Object reference</font>
 64.         Object[] things = {laguna, i, safrane, smallBanana, bigBanana};
 65. 
 66.         <font color="#007F00">// prints all objects</font>
 67.         printObjects(things);
 68.     }
 69. 
 70.     private static void printObjects(Object[] objects)
 71.     {
 72.         for (int i = 0; i < objects.length; i++)
 73.         {
 74.             Object o = objects[i];
 75. 
 76.             <font color="#007F00">// if o is a Printable object, i.e. implements this interface</font>
 77.             if (o instanceof Printable)
 78.             {
 79.                 <font color="#007F00">// convert it to the interface</font>
 80.                 Printable p = (Printable)o;
 81. 
 82.                 <font color="#007F00">// call the interface method</font>
 83.                 p.print();
 84.             }
 85.             else
 86.             {
 87.                 <font color="#007F00">// displayed for non Printable objects</font>
 88.                 System.out.println("Not a printable object: ");
 89.                 System.out.println("  class = '" + o.getClass().getName() + "'");
 90.                 System.out.println("  value = '" + o.toString() + "'");
 91.             }
 92.         }
 93.     }
 94. }

Hope this clarifies,
Beno�t
[This message has been edited by Beno�t d'Oncieu (edited November 01, 2000).]

[lakshmi nair]

That was a great thing, Beno�t.
I think i got it.
Also Udayan, how'll we know that arrays implement those intefaces. This seem to be stupid but i checked the api before posting the question.I could not find any.
Thanks
lakshmi

[yogesh sood]

also look at http://www.javaranch.com/ubb/Forum24/HTML/005188.html for detail rules

[Udayan Naik]

Hi Lakshmi.For information regarding an array implementing the Cloneable interface , look up the Object.clone() method.
All arrays also implement the Serializable interface.You can verify both these statements as below.

int a[] = {1,2,3};
if(a instanceof Cloneable){System.out.println("Implements Cloneable");}
if(a instanceof java.io.Serializable){System.out.println("Implements Serializable"); 

And please , no question is stupid if it is helping you to clear your doubts and passing the Certification Exam.
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