Java Program to Implement Hash Tables Chaining with Doubly Linked Lists

Hash Tables(similar to tables in general) provide a subset of the dynamic set operations. Usually, a set of keys are mapped with some values based on certain relations. However, there might be situations when different keys map to the same position provided by the Hash function, which leads to a collision.

One of the ways to overcome this situation is Hash Table Chaining. 

The chaining approach to resolve collisions deals with it by going ahead and putting all the keys that map to a slot in that slot but representing them as a linked list.

Hash Table chaining in Java is possible with both, Singly Linked List and Doubly Linked List. Though the implementation is same, the only difference is that Doubly Linked List allows two-way traversal i.e., the node contains a pointer to the next as well as the previous node. Thus, the complexity of insertion and deletion at a known position reduces to O(1) as compared to the Singly Linked List (O(n)).

Example :

In Singly Linked List : g -> e -> e -> k -> s
Here, each node contains a pointer to the next node only.

In Doubly Linked List : g <-> e <-> e <-> k <-> s
Each node contains pointer to the next as well as previous node.

Approach :

1. Insertion: To insert, a key is simply hashed to get the position in the table (the list where this new key has to be inserted), and then insert the key at the head of the doubly linked list using the standard linked list procedures (given in the code).
2. Deletion: Just go through the list that the key maps to through the hash function and delete the key from that list using the standard linked list procedures (given in the code).

Below is the implementation of the above approach:

Java

// Java implementation of Hashtable chaining
// using doubly linked list

import java.util.*;

class DoublyLinkedListNode {
    // Declaration of Nodes
    DoublyLinkedListNode next, prev;
    int data;

    // constructor
    DoublyLinkedListNode(int data)
    {
        this.data = data;
        next = null;
        prev = null;
    }
}
class HashTableChainingDoublyLinkedList {
    // Declaration of Hash Table
    DoublyLinkedListNode[] hashTable;

    // stores the size of HashTable
    int size;

    // Constructor
    HashTableChainingDoublyLinkedList(int hashTableSize)
    {
        // Creating an empty Hash Table
        hashTable = new DoublyLinkedListNode[hashTableSize];
        size = 0;
    }

    // Function to check if hash table is empty
    public boolean isEmpty() { return size == 0; }

    // Function to clear/delete all elements from Hash table
    public void clear()
    {
        // Capacity of Hash Table
        int len = hashTable.length;

        // Creating new empty Hash Table
        // of same initial capacity
        hashTable = new DoublyLinkedListNode[len];
        size = 0;
    }

    // Function that returns size of Hash Table
    public int getSize() { return size; }

    // Function to insert a value/element
    public void insert(int value)
    {
        size++;

        // gets the position/index where the value should be
        // stored
        int position = hash(value);

        // creates a node for storing value
        DoublyLinkedListNode node
            = new DoublyLinkedListNode(value);

        DoublyLinkedListNode start = hashTable[position];

        if (hashTable[position] == null)
            hashTable[position] = node;
        else {
            node.next = start;
            start.prev = node;
            hashTable[position] = node;
        }
    }

    // Function to remove an element
    public void remove(int value)
    {
        try {
            // gets the position where the value to
            // be deleted exists
            int position = hash(value);

            DoublyLinkedListNode start
                = hashTable[position];

            DoublyLinkedListNode end = start;

            // if value is found at start
            if (start.data == value) {
                size--;
                if (start.next == null) {
                    // removing the value
                    hashTable[position] = null;
                    return;
                }

                start = start.next;
                start.prev = null;
                hashTable[position] = start;

                return;
            }

            // traversing the list
            // until the value is found
            while (end.next != null
                   && end.next.data != value)
                end = end.next;

            // if reached at the end without finding the
            // value
            if (end.next == null) {
                System.out.println("\nElement not found\n");
                return;
            }

            size--;

            if (end.next.next == null) {
                end.next = null;
                return;
            }

            end.next.next.prev = end;
            end.next = end.next.next;

            hashTable[position] = start;
        }
        catch (Exception e) {
            System.out.println("\nElement not found\n");
        }
    }

    // Definition of Hash function
    private int hash(Integer x)
    {
        int hashValue = x.hashCode();

        hashValue %= hashTable.length;

        if (hashValue < 0)
            hashValue += hashTable.length;

        return hashValue;
    }

    // Function to print hash table
    public void printHashTable()
    {
        System.out.println();
        for (int i = 0; i < hashTable.length; i++) {
            System.out.print("At " + i + ":  ");

            DoublyLinkedListNode start = hashTable[i];

            while (start != null) {
                System.out.print(start.data + " ");
                start = start.next;
            }

            System.out.println();
        }
    }

    // Driver Code
    public static void main(String[] args)
    {
        HashTableChainingDoublyLinkedList hashTab
            = new HashTableChainingDoublyLinkedList(5);

        hashTab.insert(99);
        hashTab.insert(23);
        hashTab.insert(36);
        hashTab.insert(47);
        hashTab.insert(80);

        hashTab.printHashTable();

        hashTab.insert(92);
        hashTab.insert(49);

        hashTab.printHashTable();

        hashTab.remove(99);

        hashTab.printHashTable();

        hashTab.clear();

        hashTab.printHashTable();

        System.out.println(hashTab.isEmpty());
    }
}

Output

At 0:  80 
At 1:  36 
At 2:  47 
At 3:  23 
At 4:  99 

At 0:  80 
At 1:  36 
At 2:  92 47 
At 3:  23 
At 4:  49 99 

At 0:  80 
At 1:  36 
At 2:  92 47 
At 3:  23 
At 4:  49 

At 0:  
At 1:  
At 2:  
At 3:  
At 4:  
true