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                                                                          Hashing in Java

The hashing technique is used to efficiently find or store an item in a large collection of items without the need to go through every item. The key is the hash value, and the object is the value the key maps to. The hash value also tells about the location of the item in the large collection of items. The hashing function in java was created to define and return an object’s value in the form of an integer. The returned value obtained as output is called a hash value. The size of the hash value is 4 bytes. Two objects of the same type will have the same integer value as a hash value, whereas different objects will have different hash values. The hashing function is irreversible as the objects cannot be derived from the hash value.

Hashing in Java 

Hashing Function returns an integer value corresponding to an object. The hashing algorithm manipulates the data to create the hash value. The hash values are used as indices in the hash tables. The array itself is called a hash table. A hash table consists of two major components: a bucket array and a hash function. The hash function is designed to be fast and to yield few hash collisions in the expected input domains. The hash function is the composition of two functions where the hash function is applied first, and then the compression function is applied. If any function produces an integer suitable as an array index, it is called a hash function.

A good hash function is fast to compute. It must be quick enough to hash any sort of data and distributes the entities uniformly throughout the hash table to minimize the collision. It must obey the avalanche effect in which the hash value of the message must change whenever there is a slight change to the message.

The method of hashing helps us find the duplicates. The Hash tables use the hash functions. They stores the data in the form of key and value pairs. The key is given as an input to the hashing function and is used to identify the data. Then the hash code (integer) is mapped to a fixed size. If the two hashes are the same, then the two inputs are also the same. There are three types of hashing algorithms MD5 Algorithm, SHA Algorithm, PBKDF2withHmacSHA1 Algorithm.

Hashing functions in Java are used to check the integrity of a file. They can compare the contents of the two files easily and effectively without opening the files. With the help of the hashing function, the search operation in data structure has become faster. As most security algorithms and protocols use hashing, it plays a vital role in data security. They convert the data into shorter fixed-length values or keys representing the original string sent over the network.

The hashing function has some limitations. They cannot be implemented to sort the data. The hash collision cannot be avoided practically, which leads to inefficiency.

Hashing Techniques in Java 

When two or more objects return the same hash value, the hash collision occurs. Two techniques are used in hashing to handle collision.

Separate Chaining:

Objects having different hash values are stored in different buckets. In contrast, when two or more objects have the same hash value, they are stored in the same bucket location using an additional data structure called a linked list. This mechanism is called chaining. All the same hash value objects are chained together using a linked list, which can be traversed to access the item with a unique search key. The problem of using separate chaining is that the data structure can grow without bounds.

Open Addressing:

In this technique, all the elements are stored in the bucket array. So when a new entry is inserted, the bucket is examined, starting with a hashed-to slot and proceeding in some probe sequence until some unoccupied slot is found. When searching for an entry, the buckets are scanned in the same sequence until the entry is found or an unused array slot is found, indicating no such key in the table. Thus, Finding an unused or open location in the hash table is called open addressing. The process of locating an open location in the hash table is called probing. There are three probing techniques.

Linear Probing is a simple method of handling the collision by placing the colliding item in the next available table cell. This process continues until an empty bucket is found that can accept the new entry. The method of linear probing saves space but complicates the removal. The colliding entries Lump together, causing a future collision to cause a longer sequence of probes. 

The main problem of linear probing is clustering. When there are few collisions, the probe sequence remains short and can be searched rapidly. In case there is a collision within a cluster, it increases the size of the cluster resulting in a bigger cluster hence longer search times. You can avoid the problem of clustering by changing the probe sequence.

Quadratic Probing:
In this probing technique, iterative trying of the bucket is carried out to find an empty bucket creating secondary clustering. The set of filled array cells bounces around the array in a fixed pattern. This method may not find an empty slot when the array is not full.

Double Hashing:
To compute the increments in a key-dependent way, the double hashing uses a second hash function. The second hash function should differ from the first hash function and depend on the search key having a nonzero value. If the size of the table is a prime number, double hashing in Java can reach every location in the hash table.

Since a longer sequence of array indices needed to be tried to find the given element, The open addressing method is quite slower than that of the separate chaining technique when used in conjunction with an array of the bucket for the resolution of collision.