Casting problem with LinkedHashMap !!

[Prabhat Gupta]

Hi All,
When i m trying to run this code i am getting exception saying
java.lang.ClassCastException: java.util.HashMap

 
package abc ;
import java.util.*;
public class Demo(){
public static void main(String[] args){
HashMap<Integer,String> hm=new HashMap<Integer,String>();
hm.put(new Integer(1),"abc");
LinkedHashMap<Integer,String>lhm=(LinkedHashMap<Integer,String> :wink: hm;
System.out.println(lhm);
}
}
 

   

As I know LinkedHashMap is direct subclass of HashMap so casting can be done. Please suggest where i am doing mistake.

[Hrishikesh Maluskar]

casting can be done but if you do downcating , it will always give classCastException.

try doing up casting as follows

package abc ;
import java.util.*;

public class H
{
public static void main(String[] args)
{
LinkedHashMap<Integer,String>lhm=new LinkedHashMap<Integer,String>();
lhm.put(new Integer(1),"abc");
System.out.println(lhm);

HashMap<Integer,String> hm= (HashMap<Integer,String> lhm;
System.out.println (lhm);
}
}

[Brij Garg]

class base
{
}
class sub extends base
{
}
public class test 
{
public static void main(String args[])
{
base b = new base();
sub s = (sub)b;
}
}

check this code. This code is also throwing same exception.
If you know the reason why .... then same reason is there with your code

[Brij Garg]

casting can be done but if you do downcating , it will always give classCastException.

I doubt this statement.

check the example given below.

class base
{
}
class sub extends base
{
}
public class test 
{
public static void main(String args[])
{
base b = new sub();
sub s = (sub)b;
}
}

This is downcasting example and it is not throwing any exception.

[Brij Garg]

case 1:

base b = new base();
sub s = (sub)b;

case 2:

base b = new sub();
sub s = (sub)b;

We can not perform downcasting in case one. Reason is very clear superclass object can not be cast to subclass.

In case 2, superclass reference variable refering to an object to subclass.
and subclass object can always cast to another subclass reference variable.

[Seetharaman Venkatasamy]

Originally posted by Prabhat Gupta:

As I know LinkedHashMap is direct subclass of HashMap so casting can be done.

it can not be... downcasting will give ClassCastException.

-----------------------------------------
LinkedHashMap is direct subclass of HashMap
-------------------------------------------
so, why you try to casting?... what is the reason ?


Hope This Helps

[Prabhat Gupta]

Thanks all,
I got the point.if Object is of SuperClass type than it can't be downcasted to sub-class..as here i am doing try to cast HashMap type object to LinkedHashMap type .

[Jesper de Jong]

Indeed. Casting does not magically convert one kind of object (a HashMap in this case) to a different kind of object (a LinkedHashMap).

Casting is only a hint to the compiler where you express something like "Compiler, this object is really a LinkedHashMap, so I want you to treat it as that". At runtime, if the object really is not a LinkedHashMap, you'll get a ClassCastException.