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Help on Hashcode

dhwani mathur
Ranch Hand

Joined: May 08, 2007
Posts: 621
Hi! all


I have read some topics relevant to Hashcode but still
i am not getting the basic idea behind the method
Hashcode,i know it returns an integer value....but
still if anyone could explain me it will be realy
kinda of you....

1)What is Hashcode?

2)What is the use of HashCode?actualy can say why we
use hashcode in our program?


Thanks in Advance

Preparing SCJP 1.5
Maurizio Nagni
Ranch Hand

Joined: May 29, 2004
Posts: 75
Hashcode is a technique used to improve the search or an object (and obviously unce you have found you can modify or delete or just verify if it exist).

As you already understood the basic idea is that each object could be identified from a unique integer.

Lets make it easy.
You hava a collection of Dog class instances with three properties:

String name;
String owner;
String color;

You already (must) know that to compare one object with another you can override the equals(Object a) method.

So for the Dog you could define that
dogFirst.equals(dogSecond) == TRUE

If they have the same name and the same owner, right?

Now suppose that you have a smart function (ok..ok... it is our hash function) that generate a different number for each two unique pair (name, owner).

In this case doing
dogFirst.hashcode == dogSecond.hashcode

should give the same result than the equal method.

Thats the basic idea. After this the things could became complex (the hashfunction is not so easy to write); but the basic idea is this:
you have a very complicated object but if you can hash it you can uniquely identifying in the middle of thousand of (same kind) of objects and you can easily create an index of int without looking everytime inside its properties.

ciao
Maurizio
dhwani mathur
Ranch Hand

Joined: May 08, 2007
Posts: 621
Thanks a lot for such a great explanation Maurizio Nagni



,now i am clear with hashcode,but still
have some doubt if you could help me?

As you said it is used to compare objects,since each object
is now identified with a particular integer,in case of
wrapper classes value of equals method is TRUE bcoz
equals method is overriden in wrapper class....but
what if i try to compare objects of user class say A
it will return false since i have to override the equals method..
but why it is said that if we want to override equals method
we have to write hashcode method for it?


what i think is we have to write hashcode so as
to assign integer value to objects isnt it?

but what the use of this integer value in equals method
as it returns boolean value


if you can explain it will be realy helpful....

Thanks in advance............


Preparing SCJP 1.5
Chandra Bhatt
Ranch Hand

Joined: Feb 28, 2007
Posts: 1707
[dhwani mathur]
why it is said that if we want to override equals method
we have to write hashcode method for it?


The use of hashCode is to improve the performance in placing and retrieving
objects from the collection. It is ok to override equals(Object o) method for
your class so that you could compare two objects whether they are meaningfully
equal or not.

What if you use object of your class as a key to Map. Storing or retrieving
the object from the Map is two step process. We imagine the use of hashCode
as determining which bucket to place the object in. Your hashCode tells
that. As the unique distribution of hashCode is, as much efficient and
fast your search process will be. Before an object corresponding to the key
is stored in the map, hashCode is computed to find the bucket to place the
object in.

Let us talk about retrieving the object from the bucket given the key that
is object of your class. Again hashCode is computed, and bucket is found,
now the role of equals() method start comparing whether the object that is
in the bucket is the one that you are looking for. If not search fails.

If there are more than one object in the map, (that is the case when more
than one object's hashCode are same therefore they had been assigned the same bucket), now equals() starts the comparison in sequential order (not
known, it may apply any other method also, not of our concern).

You think, even if there is no unique distribution of the hashCode to the
objects, our search is limited to the given bucket instead of going through
all the objects in the Map and applying equals() method to compare two
objects.

hashCode is there to improve performance. If you don't override the
hashCode() method for the class, the objects you are using as key to the
Map, the default implementation of the hashCode works.
Let's see the scenario:


Line #1: Why the second get() method returned null, we see both
object emp1 and emp2 are meaningfully equal because they are having same "Amit", then what heck went wrong in retrieval of object from the Map using
another same object reference emp2.

Reason : That is because Map is using hashCode() version of Object
class. You may try this:

System.out.println((emp1.hashCode());
System.out.println(emp2.hashCode());

You will see the hashCodes for both are different. So the search process
fails at first round, before our equals() method could work. That is why
it is said you override the hashCode also.
After you override the hashCode, you should see that emp1.hashCode() and
emp2.hashCode() return the same value. It should be like that, I mean
equals() and hashCode() contract.



Thanks,


cmbhatt
Maurizio Nagni
Ranch Hand

Joined: May 29, 2004
Posts: 75
it is always a pleasure when people took so much of their time to help the others. thanks Chandra
dhwani mathur
Ranch Hand

Joined: May 08, 2007
Posts: 621
Hi!!Chandra

Superb !!!Explanation

I am now clear with the concept.
Thanks a lot for helping me soo much.
vin Hari
Ranch Hand

Joined: Nov 16, 2006
Posts: 161
Hi chandra,
I saw your answers it was very good explanation my problem is also something related to this iam creating menu driven program for some keys iam getting there values for some it is null even i tried to write overriding equals and hashcode it did not work have a patience and check this code and please reply me
This class does not contain any main method because iam calling from other class.please help me.
 
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