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I am writing a java program which uses hashmap. I know how a hashmap works. If I add(key,object), then the java finds the hashcode of the key and uses that to find a bucket to store the object.
Now I have my own hashcode implementation for the object. And I want to give this as the key - something like add(object.hashcode(),object).
Is it possible to prevent java from again hashing object.hashcode()? Because I am already implementing hashcode() calling hashcode() on hashcode will be a waste of time.

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    I assume there is a good reason why you don't simply want to override the hashCode method of the object in question? Commented Nov 15, 2012 at 16:49
  • @Perception : But hashmap is going to consider the hashcode of the key, right? Commented Nov 15, 2012 at 16:50
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    Well, yes, but assuming you aren't changing any of the 'key' attributes of the object after adding it to the map, you would simply call map.put(object, object). Commented Nov 15, 2012 at 16:53
  • I think you really need to ponder why you're trying to do this. More than likely it's not needed. Commented Nov 15, 2012 at 16:58
  • @Perception - if he uses the object as they key, when he needs to get the object out he needs the key, ie the object to retrieve it. If he has the object then he doesn't need to retrieve it, meaning he has no need for a map in the first place. Commented Nov 15, 2012 at 17:01

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The way to do it is to implement hashCode() to cache the hash value once it is calculated. Do note that this implies your object is immutable, or at least the fields contributing to hashCode and equals don't change after putting the object into the map.

You don't need to use the hashcode as the key. Moreover, that would almost certainly be the wrong way to do it because it is actually not how hashtables are supposed to work. Hash collisions are the name of the game, so the hashcode is only used to address a bucket, but (the name says it all) a bucket contains not one, but arbitrarily many objects. These must be checked by equals to find the exact one you were looking for.

Given your initial idea to use hashcode as the key, it looks like you are not really after a map, but after HashSet. You are just adding objects into a collection and will later want to check an object's presence in it. That is a set.

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6 Comments

"I don't understand" is not a helpful comment.
okay. If I use my own implementation for the hashcode and if there is a collision, then the objects in that bucket will be put in a linked list. I can provide my implementation of equals for the "object" also.
You must provide an implementation for equals. These two methods must always be implemented together, and consistently.
@Ashwin Marko is right. Think of the hashcode as a first check and the equals as a second check, before finding the corect reference. hashcode should return the same value for two objects that are equal. A better implementation of hashcode (by better I mean an implementation that returns a trully different value for each object) improves performance by reducing the amount of objects on the second level (same hashcode) check that is made with equals.
@Gamb : In my class two objects are equals of their array components are equals and for that I am using Array.hashcode(). So I think it can be reliable to give as unique values as possible
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Unless you have your own implementation, you can't do that. The reason is that the hashing is used to pick your objects when you use the get method.

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"The reason is that the hashing is used to pick your objects when you use the get method" - yes and I am providing my own hashing.
You are doing like map.put(object.hashCode(), ...). Right?, when doing so, the map is going to get the hashCode for the object that is being passed as the key (here the key is your hashCode).

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