Every data structure has its own special characteristics for example a BST is used when quick searching of an element (in log(n)) is required. A heap or a priority queue is used when the minimum or maximum element needs to be fetched in constant time. Similarly a hash table is used to fetch, add and remove an element in constant time. It is necessary for anyone to be clear with the working of a hash table before moving on to the implementation aspect. So here is a brief background on the working of hash table and also it should be noted that we will be using Hash Map and Hash Table terminology interchangeably though in Java HashTables are thread safe while HashMaps are not.

The code we are going to implement is available at Link 1 and Link2

But it is strongly recommended that one must read this blog completely and try and decipher the nitty gritty of what goes into implementing a hash map and then try to write the code yourself.

**Background**

Every hash-table stores data in the form of (key, value) combination. Interestingly every key is unique in a Hash Table but values can repeat which means values can be same for different keys present in it. Now as we observe in an array to fetch a value we provide the position/index corresponding to the value in that array. In a Hash Table, instead of an index we use a key to fetch the value corresponding to that key. Now the entire process is described below

Every time a key is generated. The key is passed to a hash function. Every hash function has two parts a **Hash code and a Compressor**.

*Hash code is an Integer number* (random or nonrandom). In Java every Object has its own hash code. We will use the hash code generated by JVM in our hash function and to compress the hash code we modulo(%) the hash code by size of the hash table. *So modulo operator is compressor in our implementation.*

The entire process ensures that for any key, we get an integer position within the size of the Hash Table to insert the corresponding value.

So the process is simple, user gives a (key, value) pair set as input and based on the value generated by hash function an index is generated to where the value corresponding to the particular key is stored. So whenever we need to fetch a value corresponding to a key that is just O(1).

This picture stops being so rosy and perfect when the concept of hash collision is introduced. Imagine for different key values same block of hash table is allocated now where do the previously stored values corresponding to some other previous key go. We certainly can’t replace it .That will be disastrous! To resolve this issue we will use Separate Chaining Technique, Please note there are other open addressing techniques like double hashing and linear probing whose efficiency is almost same as to that of separate chaining and you can read more about them at Link 1 Link 2 Link3

Now what we do is make a linked list corresponding to the particular bucket of the Hash Table, to accommodate all the values corresponding to different keys who map to the same bucket.

Now there may be a scenario that all the keys get mapped to the same bucket and we have a linked list of n(size of hash table) size from one single bucket, with all the other buckets empty and this is the worst case where a hash table acts a linked list and searching is O(n).So what do we do ?

**Load Factor**

If n be the total number of buckets we decided to fill initially say 10 and let’s say 7 of them got filled now, so the load factor is 7/10=0.7.

**In our implementation** whenever we add a key value pair to the Hash Table we check the load factor if it is greater than 0.7 we double the size of our hash table.

**Implementation**

**Hash Node Data Type**

We will try to make a generic map without putting any restrictions on the data type of the key and the value . Also every hash node needs to know the next node it is pointing to in the linked list so a next pointer is also required.

The functions we plan to keep in our hash map are

**get(K key)**: returns the value corresponding to the key if the key is present in**HT**(**H**ast**T**able)**getSize()**: return the size of the HT**add()**: adds new valid key, value pair to the HT, if already present updates the value**remove()**: removes the key, value pair**isEmpty()**: returns true if size is zero

Every Hash Map must have an array list/linked list with an initial size and a bucket size which gets increased by unity every time a key, value pair is added and decreased by unity every time a node is deleted

ArrayList<HashNode<K, V>> bucket =newArrayList<>();

A** Helper Function **is implemented to get the index of the key, to avoid redundancy in other functions like get, add and remove. This function uses the in built java function to generate a hash code and we compress the hash code by the size of the HT so that the index is within the range of the size of the HT

**get()**

The get function just takes a key as an input and returns the corresponding value if the key is present in the table otherwise returns null. Steps are:

- Retrieve the input key to find the index in the HT
- Traverse the liked list corresponding to the HT, if you find the value then return it else if you fully traverse the list without returning it means the value is not present in the table and can’t be fetched so return null

**remove()**

- Fetch the index corresponding to the input key using the helper function
- The traversal of linked list similar like in get() but what is special here is that one needs to remove the key along with finding it and two cases arise
- If the key to be removed is present at the head of the linked list
- If the key to be removed is not present at head but somewhere else

**add()**

Now to the most interesting and challenging function of this entire implementation.It is interesting because we need to dynamically increase the size of our list when load factor is above the value we specified.

- Just like remove steps till traversal and adding and two cases (addition at head spot or non-head spot) remain the same.
- Towards the end if load factor is greater than 0.7
- We double the size of the array list and then recursively call add function on existing keys because in our case hash value generated uses the size of the array to compress the inbuilt JVM hash code we use ,so we need to fetch new indices for the existing keys. This is very important to understand please re read this paragraph till you get a hang of what is happening in the add function.

Java does in its own implementation of Hash Table uses Binary Search Tree if linked list corresponding to a particular bucket tend to get too long.

// Java program to demonstrate implementation of our // own hash table with chaining for collision detection import java.util.ArrayList; // A node of chains class HashNode<K, V> { K key; V value; // Reference to next node HashNode<K, V> next; // Constructor public HashNode(K key, V value) { this.key = key; this.value = value; } } // Class to represent entire hash table class Map<K, V> { // bucketArray is used to store array of chains private ArrayList<HashNode<K, V>> bucketArray; // Current capacity of array list private int numBuckets; // Current size of array list private int size; // Constructor (Initializes capacity, size and // empty chains. public Map() { bucketArray = new ArrayList<>(); numBuckets = 10; size = 0; // Create empty chains for (int i = 0; i < numBuckets; i++) bucketArray.add(null); } public int size() { return size; } public boolean isEmpty() { return size() == 0; } // This implements hash function to find index // for a key private int getBucketIndex(K key) { int hashCode = key.hashCode(); int index = hashCode % numBuckets; return index; } // Method to remove a given key public V remove(K key) { // Apply hash function to find index for given key int bucketIndex = getBucketIndex(key); // Get head of chain HashNode<K, V> head = bucketArray.get(bucketIndex); // Search for key in its chain HashNode<K, V> prev = null; while (head != null) { // If Key found if (head.key.equals(key)) break; // Else keep moving in chain prev = head; head = head.next; } // If key was not there if (head == null) return null; // Reduce size size--; // Remove key if (prev != null) prev.next = head.next; else bucketArray.set(bucketIndex, head.next); return head.value; } // Returns value for a key public V get(K key) { // Find head of chain for given key int bucketIndex = getBucketIndex(key); HashNode<K, V> head = bucketArray.get(bucketIndex); // Search key in chain while (head != null) { if (head.key.equals(key)) return head.value; head = head.next; } // If key not found return null; } // Adds a key value pair to hash public void add(K key, V value) { // Find head of chain for given key int bucketIndex = getBucketIndex(key); HashNode<K, V> head = bucketArray.get(bucketIndex); // Check if key is already present while (head != null) { if (head.key.equals(key)) { head.value = value; return; } head = head.next; } // Insert key in chain size++; head = bucketArray.get(bucketIndex); HashNode<K, V> newNode = new HashNode<K, V>(key, value); newNode.next = head; bucketArray.set(bucketIndex, newNode); // If load factor goes beyond threshold, then // double hash table size if ((1.0*size)/numBuckets >= 0.7) { ArrayList<HashNode<K, V>> temp = bucketArray; bucketArray = new ArrayList<>(); numBuckets = 2 * numBuckets; size = 0; for (int i = 0; i < numBuckets; i++) bucketArray.add(null); for (HashNode<K, V> headNode : temp) { while (headNode != null) { add(headNode.key, headNode.value); headNode = headNode.next; } } } } // Driver method to test Map class public static void main(String[] args) { Map<String, Integer>map = new Map<>(); map.add("this",1 ); map.add("coder",2 ); map.add("this",4 ); map.add("hi",5 ); System.out.println(map.size()); System.out.println(map.remove("this")); System.out.println(map.remove("this")); System.out.println(map.size()); System.out.println(map.isEmpty()); } }

Output :

3 4 null 2 false

The entire code is available at https://github.com/ishaan007/Data-Structures/blob/master/HashMaps/Map.java

This article is contributed by **Ishaan Arora.** If you like GeeksforGeeks and would like to contribute, you can also write an article and mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

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