Dear Friends,

            i want to know , how they are Generating Hsah Code .

what is the use of Hsahcode.....

In the Object class, hash codes correspond to 32-bit internal JVM address of the object in memory. In the String class, hash codes are calculated using an algorithm where two Strings with the same contents will always have the same hash code. In both cases, an effort is made to distribute the hash code values across the 32-bit spectrum. I'll explain the importance of this below.

The basic contract of any hash coding algorithm is that if two objects are to be considered equal, they must share the same hash code. Therefore if two objects have different hash codes, you know they are different. Java's built-in Collections classes often use this trick in order to speed up performance. You can also compare hashCode values in your own code.

But the primary use I have for hashCodes is the java.util.HashMap and java.util.HashSet classes. Because hashCodes are evenly distributed, these maps and sets can use the hash code to distribute objects placed in them across an array. Later, when objects need to be looked up, the hash code can be used to find the location without requiring any looping or other iterations. Such algorithms are highly regarded for their performance in larger systems -- so the HashMap is very popular due to its use of hash codes.

For a more technical explanation, you might want to read this: http://mindprod.com/jgloss/hashcode.html

Hope this helps!

Jon Emerson
Adobe Systems, Inc.
http://www.jonemerson.net/

ok,

  if two Strings have same hash code . then, how

they will be stored ?

I am assuming you're asking about how Strings are stored in a HashMap (or other hash-based storage device). If the two Strings are equal (that is, str1.equals(str2) is true), they cannot both be used as keys in a HashMap. One will override the other.

If they are not equal, then the HashMap interface will allow both to act as keys. In a traditional HashMap implementation, allowing both would be enabled through failing-over to the next available key row. Then the getter method would know that if a row is taken by something other than what's asked for, look in the next row and so on. This has some negative performance aspects to it, but in general it's pretty rare for two keys to have the same hash value.

If you're not talking about how Strings are stored with respect to HashMaps, let me know. (Basically they're stored in memory in their own space -- they just happen to have the same HashCodes because their hash code algorithm is String value based, not memory location based.)

Jon Emerson
Adobe Systems, Inc.
http://slashdot.org/~panaceaa