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Polymorphism: request behaviour consistently without knowing the full class of the object

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I am trying to understand the concept of Polymorphism. I can't understand the following sentence:

we can request behaviour consistently without knowing the full class of the object

The book shows an example code showing three classes and then creates a reference of superclass and then use the reference to invoke derive class methods which is a usual technique for implementing polymorphism as shown below:

class B extends A
void callme()
System.out.println(“Inside B’s callme method”);
 
 //Q. What can you say about callme( )?
class C extends A 
void callme()
System.out.println(“Inside C’s callme method”);
 

class Dispatch 
public static void main(String args[ ]) 
A a = new A(); //obj of Type A
B b = new B(); //obj of Type B
C c = new C(); //obj of Type C
A r;
r = a; // r refers to an A object
r.callme();

r = b;//r refers to a B object
r.callme(); 
r = c;// r refers to a C object
r.callme();
 

Some body please guide me "Why are we saying we don't have information about the full class of object"? Constructor can tell us about the class. Also all the classes are listed here. What can be a real scenario?

polymorphism

share|improve this question

edited May 26 at 10:25

halfer

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asked Mar 28 at 16:34

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0

I am trying to understand the concept of Polymorphism. I can't understand the following sentence:

we can request behaviour consistently without knowing the full class of the object

The book shows an example code showing three classes and then creates a reference of superclass and then use the reference to invoke derive class methods which is a usual technique for implementing polymorphism as shown below:

class B extends A
void callme()
System.out.println(“Inside B’s callme method”);
 
 //Q. What can you say about callme( )?
class C extends A 
void callme()
System.out.println(“Inside C’s callme method”);
 

class Dispatch 
public static void main(String args[ ]) 
A a = new A(); //obj of Type A
B b = new B(); //obj of Type B
C c = new C(); //obj of Type C
A r;
r = a; // r refers to an A object
r.callme();

r = b;//r refers to a B object
r.callme(); 
r = c;// r refers to a C object
r.callme();
 

Some body please guide me "Why are we saying we don't have information about the full class of object"? Constructor can tell us about the class. Also all the classes are listed here. What can be a real scenario?

polymorphism

share|improve this question

edited May 26 at 10:25

halfer

15.3k77 gold badges6363 silver badges129129 bronze badges

asked Mar 28 at 16:34

user2994783user2994783

533 bronze badges

add a comment
 | 

I am trying to understand the concept of Polymorphism. I can't understand the following sentence:

we can request behaviour consistently without knowing the full class of the object

The book shows an example code showing three classes and then creates a reference of superclass and then use the reference to invoke derive class methods which is a usual technique for implementing polymorphism as shown below:

class B extends A
void callme()
System.out.println(“Inside B’s callme method”);
 
 //Q. What can you say about callme( )?
class C extends A 
void callme()
System.out.println(“Inside C’s callme method”);
 

class Dispatch 
public static void main(String args[ ]) 
A a = new A(); //obj of Type A
B b = new B(); //obj of Type B
C c = new C(); //obj of Type C
A r;
r = a; // r refers to an A object
r.callme();

r = b;//r refers to a B object
r.callme(); 
r = c;// r refers to a C object
r.callme();
 

Some body please guide me "Why are we saying we don't have information about the full class of object"? Constructor can tell us about the class. Also all the classes are listed here. What can be a real scenario?

polymorphism

share|improve this question

edited May 26 at 10:25

halfer

15.3k77 gold badges6363 silver badges129129 bronze badges

asked Mar 28 at 16:34

user2994783user2994783

533 bronze badges

class B extends A
void callme()
System.out.println(“Inside B’s callme method”);
 
 //Q. What can you say about callme( )?
class C extends A 
void callme()
System.out.println(“Inside C’s callme method”);
 

class Dispatch 
public static void main(String args[ ]) 
A a = new A(); //obj of Type A
B b = new B(); //obj of Type B
C c = new C(); //obj of Type C
A r;
r = a; // r refers to an A object
r.callme();

r = b;//r refers to a B object
r.callme(); 
r = c;// r refers to a C object
r.callme();
 

share|improve this question

edited May 26 at 10:25

halfer

15.3k77 gold badges6363 silver badges129129 bronze badges

asked Mar 28 at 16:34

user2994783user2994783

533 bronze badges

share|improve this question

edited May 26 at 10:25

halfer

15.3k77 gold badges6363 silver badges129129 bronze badges

asked Mar 28 at 16:34

user2994783user2994783

533 bronze badges

edited May 26 at 10:25

halfer

15.3k77 gold badges6363 silver badges129129 bronze badges

edited May 26 at 10:25

halfer

15.3k77 gold badges6363 silver badges129129 bronze badges

asked Mar 28 at 16:34

user2994783user2994783

533 bronze badges

asked Mar 28 at 16:34

user2994783user2994783

533 bronze badges