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| | | ' Related to HashMap OR HashTable | ' Related to HashMap OR HashTable 2004-07-28 - By Sanjeev Chakravarty
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Hi Manish,
Though this is not the right forum to discuss about HashMap which is purely
a Java topic, embedded is my explanation on why you don't get the exception.
Sanjeev Chakravarty
Sr. Consultant
Manish Malhotra <manish.mmalhotra@(protected)>
Sent by: "A mailing list for Java(tm) 2 Platform, Enterprise Edition"
<J2EE-INTEREST@(protected)>
07/27/2004 09:08 AM
Please respond to manish.mmalhotra
To: J2EE-INTEREST@(protected)
cc:
Subject: ' Related to HashMap OR HashTable
Hi,
I know its not purely J2EE issue but, I have a doubt regarding the Map's
func. in Java.
HashMap have one constructor as HashMap(int cap, float loadFactor)
which says that create a HashMap with this much capacity and this
loadFactor.
And its threshold value is (cap*loadFactor) after which it will reHash the
Map.
> Javadoc details further :
> The capacity is the number of buckets in the hash table, and the initial
capacity is simply the capacity
>at the time the hash table is created. The load factor is a measure of how
full the hash table is allowed
> to get before its capacity is automatically increased. When the number of
entries in the hash table exceeds
>the product of the load factor and the current capacity, the capacity is
roughly doubled by calling the rehash method.
But actually Map's const code is like this :
public HashMap(int initialCapacity, float loadFactor) {
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal initial capacity: "
+
initialCapacity);
if (initialCapacity > MAXIMUM_CAPACITY)
initialCapacity = MAXIMUM_CAPACITY;
if (loadFactor <= 0 || Float.isNaN(loadFactor))
throw new IllegalArgumentException("Illegal load factor: " +
loadFactor);
// Find a power of 2 >= initialCapacity
int capacity = 1;
while (capacity < initialCapacity)
capacity <<= 1;
this.loadFactor = loadFactor;
threshold = (int)(capacity * loadFactor);
table = new Entry[capacity];
init();
}
So its making capacity as power of 2.
And put method resize only if there is data more then the threshold is try
to add.
Now if I Create HashMap(1, 2.0f) const.
> A load factor of greater than 0.75 is not efficient. Increasing the load
factor
> will increase the number of collisions in inserting/searching an entry. It is
> important to understand the concept of buckets. In the above creation of
HashMap
> there will be a single bucket created and after the second entry is added
to the
> same bucket (entries in the same bucket are represented as a linked list)
then
> there will be resizing of the internal table
Its capacity would be
cap = 1*2 = 2
> capacity will remain 1
Its threshold would be
th = 2.0 * 2 = 4;
> threshold will be 2
And the size of the array of Entry would be 2. (Internally HashMap uses
Entry to maintain the objects key, value)
> The size of the Entry array will be 1
Hence, If I add more then 2 entries in this HashMap then how it worked
because before the threeshold comes it wont resize
and the inti array of Entry class is size of 2, So third should give
exception ...... I know its handeled but I dnt kn why?
> Since the second entry is added to the same bucked as a linked list we
> don't get an exception
Its interesting.... So please explain.......
> I hope you will now be able to comprehend better the way HashMap works.
> You can find more indepth analysis at:
> http://java.sun.com/docs/books/tutorial/collections/TOC.html#implementations
regards,
Manish
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