understanding of hash functions

[aizen92]

I read about hash table and Hash functions on wikipedia, becuase I have to use HashTable in a code, but I can't understand the Hash functions quite properly, now from what I read, it said its like a map with each key having several values, is it correct?
What about HashSet in the Java Docs, it said its backed by a HashMap, but what's it use?
Thanks

[KevinWorkman]

Do you really need to fully understand hash functions in order to use Hashtable?

[JosAH]

The fact that a HashSet is implemented/backed by a HashMap is an 'implementation detail' and is none of our business as long as the HashSet implements the Set interface.

kind regards,

Jos

[aizen92]

No, but I want to know how it works, to better understand it.

But is this part correct:

its like a map with each key having several values, is it correct?

[JosAH]

One key in a HashMap can only have one value associated with it and all keys have to be unique according to their equals( ... ) method.

kind regards,

Jos

[aizen92]

One key in a HashMap can only have one value associated with it and all keys have to be unique according to their equals( ... ) method.

kind regards,

Jos

But isnt this is how a normal map works, each key has a unique value?

[JosAH]

But isnt this is how a normal map works, each key has a unique value?

Yep, but you mentioned "several values" per key in reply #4, so I replied to that and the values needn't be unique, only the keys need to be unique in the Map.

kind regards,

Jos

[aizen92]

Ohhh, okay
But I'm getting a little confused here lol
Do you mean that in a map values are not unique but in hash map they are unique?
Sorry for the long discussion but I want to grasp a better understanding of it

[JosAH]

Ohhh, okay
But I'm getting a little confused here lol
Do you mean that in a map values are not unique but in hash map they are unique?
Sorry for the long discussion but I want to grasp a better understanding of it

No, according to the contract of the Map interface, each Map can store unique keys only. Values are free to take any value they want.

kind regards,

Jos

[aizen92]

Then what's the use of Hash here?

[JosAH]

Then what's the use of Hash here?

Hashing is a (fast) way to find an integer value for any type of object that is mostly unique. Such a hash value can be used in hash tables/maps to store and find those key values.

kind regards,

Jos

[aizen92]

Oh Okay
Thanks Jos