HashMap

[k reeta]

I am really confused about how HashMaps work. According to the code below, two instances of Dog are equal if they share the same name, and the hashcode has been overridden as well.

import java.util.*;

public class Map10{

public static void main(String[] args){

Map<Dog,String> s=new HashMap<Dog,String>();

Dog d1=new Dog("A");
Dog d2=new Dog("B");

s.put(d1,"hello1");
s.put(d2,"hello2");

System.out.println(s.get(d1));
System.out.println(s.get(d2));

Dog d3=new Dog("B");
System.out.println(d3.equals(d2));
System.out.println(d3.hashcode());
System.out.println(d2.hashcode());

System.out.println(s.get(d3));

}
}

class Dog{

String name;

Dog(String n){this.name=n;}

public boolean equals(Object o){

if((o instanceof Dog) && (((Dog)o).name==this.name))
{return true;}

else{

return false;
}

}
public int hashcode(){
return name.length();
}
}

My question is - when I try to use d3 as a key, why do I get null as the value?? Since d3.equals(d2) returns true , and their hashcode are equal -shouldn't the value for d3 be "Dog2" (like it is for d2)?

What am I doing/understanding wrong?

Thank You.

[Sanjeev Ba]

d3.equals(d2) returns true due to the overridden equals() method. This is not related to the HashMap.

s.get(d3) depends on the HashMap implementation. HashMap "s" does not have a key called "d3" (which will be a reference to some object).
So, it does not know which "value" you want to retrieve. It doesn't know how to resolve "d3" key to get you a value.

I guess that explains why you are getting a null.

[Darryl Burke]

Sorry, Sanjeev, that's not correct. Read the API for HashMap.get(...)

reeta, Java is case sensitive. Add the @Override annotation to your overridden methods in the Dog class and you'll spot your mistake in no time.

[Sanjeev Ba]

Nice Gotcha..Thanks for pointing out.

[k reeta]

Thanks for pointing me in the direction! I changed hashcode to hashCode and its working now. Thank you!

[sakthi moorthy]

Darryl Burke wrote:Sorry, Sanjeev, that's not correct. Read the API for HashMap.get(...)

d3 Object is not added to the HashMap Object "s" That's why "null" is getting printed as Sanjeev Says.

Please Correct me if i am wrong or where i went wrong.