Map Question

[Chandra Bhatt]

Hi,

    
import java.util.*;
class GFC111 {
  public static void main (String[] args) {
    Object m = new LinkedHashMap();
    System.out.print((m instanceof Collection)+","); //false, OK!
    System.out.print((m instanceof Map)+","); //true , OK!
    System.out.print(m instanceof HashMap); //true, [B]WHY?[/B]
        
}}
 

Source of the question
Please guide me!

Thanks and Regards,
cmbhatt
[ April 04, 2007: Message edited by: Chandra Bhatt ]

[Keith Lynn]

LinkedHashMap is a subclass of HashMap.

[Chandra Bhatt]

Thanks Keith,

One more doubt:

 
import java.util.*;
class Animal {
 
}
 
class GFC114 {
  public static void main (String[] args) {
    Object a = new ArrayList();
    System.out.print((a instanceof Collections)+","); //No error, why?
    System.out.print((a instanceof Arrays)+",");
    System.out.print(a instanceof List);

Animal a1= new Animal();
System.out.println("\n" + (a1 instanceof Collections)); //why error here
/*
Animal extends Object so why error here
*/
}}

It may be silly to ask, but my brain is not responding?

I preferred not to start a new thread and continue with this. Is that OK?

Source of the question

Thanks and Regards,
cmbhatt
[ April 04, 2007: Message edited by: Chandra Bhatt ]

[Ulf Dittmer]

Object a = new ArrayList();
System.out.print((a instanceof Collections)+","); //No error, why?

It doesn't matter that the type of the reference if Object - a is an ArrayList, and thus an instance of Collection. Not of Collections, though.

Animal a1= new Animal();
System.out.println("\n" + (a1 instanceof Collections)); //why error here

Yes, Animal extends Object, so "a1 instanceof Object" would be true, but it does not extend Collection.
[ April 05, 2007: Message edited by: Ulf Dittmer ]

[Chandra Bhatt]

Hi Ulf,

Object a = new ArrayList();
System.out.print((a instanceof Collections)+","); //No error, why?

It is Collections, Collections is a class; runtime type of the a would be ArrayList. What is association between Collections and ArrayList? Please clarify.


Thanks and Regards,
cmbhatt

[Srinivasan thoyyeti]

Hi Chandra,

case 1>
Object a = new ArrayList();
System.out.print((a instanceof Collections)+",");

Compiler : By seeing "a instanceof Collections" thinks, a which is object reference can be convertible to any compatible type. So it will not complain rather it don't know what the object pointing to.

Case 2>
Compiler can only retrict or throw error when it knows type of the LHS operand.
Like ...
ArrayList a = new ArrayList();
System.out.print((a instanceof Collections)+",");

Here copiler throws error bec'ze there is no IS-A correspondence between them.

Thats the situation with compiler (Type place a role here).

JVM(runtime): returns true or false for the statement.

Hope its clear.

[Chandra Bhatt]

Thanks Srini,

Oh Yeah, I was missing the vital part that was IS-A. Thanks for helping me to recall.
what I got:
class Collections extends Object (ofcourse as everyclass do). So compiler can only see the reference variable that is of Object class and LHS Collections
(It is ok because, Collections extends Object). At run time returns false, becaue a's run timetype is ArrayList.

ArrayList al = new ArrayList();
No IS-A between ArrayList and Collections so the compiler error.

Correct me if I still miss something!

Thanks and Regards,
cmbhatt

[gianni ipez]

interesting to know: instanceof operator can just be used to relate objects at runtime.
at compile time you already know if the 2 objects can be ever related, so the compiler prevents you to use if there is no need.

[Ulf Dittmer]

What is association between Collections and ArrayList? Please clarify.

There's no association between ArrayList and Collections whatsoever. There is an association between ArrayList and Collection in the sense that an ArrayList is an instance of Collection.
[ April 05, 2007: Message edited by: Ulf Dittmer ]

[Keith Lynn]

Originally posted by gianni ipez:
interesting to know: instanceof operator can just be used to relate objects at runtime.
at compile time you already know if the 2 objects can be ever related, so the compiler prevents you to use if there is no need.

The form of an instanceof expression is

RelationalExpression instanceof ReferenceType


If the type of the RelationalExpression could not be cast to the ReferenceType without causing a ClassCastException, then the instanceof operator will cause a compile-time error.

Type Comparison Operator instanceof

[sravanthi pulukuri]

Can you explain what you exactly mean that sentence

[Ulf Dittmer]

Originally posted by sravanthi pulukuri:
Can you explain what you exactly mean that sentence

Which sentence?