Object Reference Conversion

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

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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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. /** The Printable interface should be supported
 2. by objects that can be output on the screen
 3. in a textual way.
 4. */
 5. interface Printable
 6. {
 7. void print();
 8. }
 9. 
 10. class Banana implements Printable
 11. {
 12. private float weigth;
 13. 
 14. // constructor
 15. public Banana(float weigth)
 16. {
 17. this.weigth = weigth;
 18. }
 19. 
 20. 
 21. // implements the Printable interface
 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. // constructor
 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. // implements the Printable interface
 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. // create several printable objects
 54. // and convert the objects into interface
 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. // create a non printable object
 61. Integer i = new Integer(111);
 62. 
 63. // convert to interface references to Object reference
 64. Object[] things = {laguna, i, safrane, smallBanana, bigBanana};
 65. 
 66. // prints all objects
 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. // if o is a Printable object, i.e. implements this interface
 77. if (o instanceof Printable)
 78. {
 79. // convert it to the interface
 80. Printable p = (Printable)o;
 81. 
 82. // call the interface method
 83. p.print();
 84. }
 85. else
 86. {
 87. // displayed for non Printable objects
 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).]

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

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

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.

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