Doubt in Hash Map

[sudipto shekhar]

Hello, I was trying one of the examples in K&B chapter Generics and collection

The code is:

 
import java.util.*;
 
class Dog{
public String name;
 
public Dog(String name){
this.name=name;
}
 
public boolean equals(Object o){
if((o instanceof Dog)&&(((Dog)o).name==name))
{return true; }
else
{return false;}
}
 
public int hashCode(){
return name.length();
}
}
 
class Cat{}
 
public class TestHashMap{
public static void main(String aa[]){
 
Map<Object,Object> m=new HashMap<Object, Object>();
 
Dog d1=new Dog("TOM");
m.put(d1,"DOG");
 

 
System.out.println(m.get(d1));   //this returns DOG
System.out.println(m.get("DOG"));         // this returns null
 

}
}
 

   

The line System.out.println(m.get("DOG")); should return the object but it returns null.
I cannot figure it out why..
Can somebody help? :roll:

[Henry Wong]

The line System.out.println(m.get("DOG")); should return the object but it returns null.
I cannot figure it out why..
Can somebody help?

It would help us, if you tell us... what object do you think it should return?

Henry

[sudipto shekhar]

It should return the dog object d1 .... Isn't it?
[ December 14, 2008: Message edited by: Sudipto Shekhar ]

[Harvinder Thakur]

Well i don't know how you missed it
If you are clear on what put(key, value) takes as an argument and
what get(key) takes as an argument then I bet you'll be 100% clear.

Let me try and explain:
m.put(d1,"DOG");
| |
| |
\/ \/
key value

m.get(d1));
//this returns DOG because it searched for the value put against the key d1
//which you did put in earlier.

m.get("DOG");
//this returns null because it searched for the value put against the key
//"DOG" which you never put in earlier.

[sudipto shekhar]

Ok i got it..
Thank you very much...