What is a biased object? Biased objects appear as a result of Biased Locking. A patented! It was observed that in most applications, the majority of objects are only ever locked by one thread so paying the cost of the atomic operation was often a waste.
The lock bits indicate whether the object is locked. This could explain the additional complexity in FastHashCode. So we need to consider all cases and find where the identity hash resides. The first thing we find is:. A few lines down, this is indeed an invariant:. Even if there is only one thread. To answer this question we must understand which are the possible locations of the mark word that contains the identity hash depending on the lock state of the object.
The transitions are illustrated in this diagram from the HotSpot Wiki :. If we have contended objects e. A similar need for a list of waiters appears if a thread executes object.
Which is more expensive and requires coordination. Lines L to L deal with finding the header and checking for a cached identity hash. From L it needs to bite the bullet:. At this point, if the id. I wrote a simple JMH harness to verify those conclusions. The benchmark source does something equivalent to this:. One configuration withIdHash synchronizes on an object that uses the identity hash, so we expect that biased locking will be disabled as soon as hashCode is invoked.
A second configuration withoutIdHash implements a custom hash code so biased locking should not be disabled. When two threads contend for the lock, biased locking is disabled anyway so there is no significative difference between both hash methods. Note that this is true only for the current execution of an application, and that a different hash code can be returned if the application is run again.
For the best performance, a hash function should generate an even distribution for all input, including input that is heavily clustered. An implication is that small modifications to object state should result in large modifications to the resulting hash code for best hash table performance.
The GetHashCode method should not throw exceptions. For example, the implementation of the GetHashCode method provided by the String class returns identical hash codes for identical string values.
Therefore, two String objects return the same hash code if they represent the same string value. Also, the method uses all the characters in the string to generate reasonably randomly distributed output, even when the input is clustered in certain ranges for example, many users might have strings that contain only the lower ASCII characters, even though a string can contain any of the 65, Unicode characters.
Providing a good hash function on a class can significantly affect the performance of adding those objects to a hash table. In a hash table with keys that provide a good implementation of a hash function, searching for an element takes constant time for example, an O 1 operation.
In a hash table with a poor implementation of a hash function, the performance of a search depends on the number of items in the hash table for example, an O n operation, where n is the number of items in the hash table.
A malicious user can input data that increases the number of collisions, which can significantly degrade the performance of applications that depend on hash tables, under the following conditions:. When a large proportion of objects in a hash table produce hash codes that are equal or approximately equal to one another. Derived classes that override GetHashCode must also override Equals Object to guarantee that two objects considered equal have the same hash code; otherwise, the Hashtable type might not work correctly.
Feedback will be sent to Microsoft: By pressing the submit button, your feedback will be used to improve Microsoft products and services. Privacy policy. Skip to main content. This browser is no longer supported. Download Microsoft Edge More info. Contents Exit focus mode. Serves as the default hash function. Note For information about how hash codes are used in hash tables and for some additional hash code algorithms, see the Hash Function entry in Wikipedia.
Warning A hash code is intended for efficient insertion and lookup in collections that are based on a hash table. For this reason: Do not serialize hash code values or store them in databases. Do not use the hash code as the key to retrieve an object from a keyed collection. Do not send hash codes across application domains or processes.
In some cases, hash codes may be computed on a per-process or per-application domain basis. Do not use the hash code instead of a value returned by a cryptographic hashing function if you need a cryptographically strong hash. For cryptographic hashes, use a class derived from the System. HashAlgorithm or System. KeyedHashAlgorithm class. Do not test for equality of hash codes to determine whether two objects are equal.
Computing the String hash codes of the email contents, I found 50 pairs and even 2 triples of different emails with the same hash code. For half a million strings, this is a pretty good result. But the message here is: if you have many data items, collisions will occur. If you were using the hashCode as a key here, you would not immediately notice your mistake.
But a few people would get the wrong mail. Whenever it is invoked on the same object more than once during an execution of a Java application, the hashCode method must consistently return the same integer, provided no information used in equals comparisons on the object is modified.
This integer need not remain consistent from one execution of an application to another execution of the same application.
This is uncommon, in fact, some classes in the class library even specify the exact formula they use to calculate hash codes e.
For these classes, the hash code will always be the same. But while most of the hashCode implementations provide stable values, you must not rely on it. As this article points out, there are Java libraries that actually return different hashCode values in different processes and this tends to confuse people.
Therefore, you should not use the hash code in distributed applications. A remote object may have a different hash code than a local one, even if the two are equal. Do not use hashCode in distributed applications. Moreover, you should be aware that the implementation of a hashCode function may change from one version to another.
Therefore your code should not depend on any particular hash code values. For example, your should not use the hash code to persist state.
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