Java Program to Implement Hash Table Chaining with List Heads

A hash table is a data structure that executes a connected array, it is a structure that maps keys to its values. It uses a hash function to calculate an index position for the keys, also called hash code. It will store key and value pair in an array of buckets, and from that bucket, the values can be easily found with the help of keys. To find the value, the key will be hashed and the result of that will give the position where that value resides.

Approach:

INSERTION: To insert a key in a hash table, a key will be simply hashed to find the position in which it will be going to insert and after that, the key will be inserted at the head of the list.

DELETION: Just traverse the list with the help of the hash function until the key is found and then simply delete that key from the list

Implementation:

Java

// Java program which implement Hash Tables chaining with
// List Heads

import java.util.Scanner;

public class TestHashTables {

    public static void main(String[] args)
    {
        Scanner sc = new Scanner(System.in);
        System.out.println(
            "***** Hash Table Implementation *****");
        System.out.println("Enter size of hash table");

        // create object of hash table
        HashTablesExample table
            = new HashTablesExample(sc.nextInt());
        boolean exit = false;

        //  Perform HashTable operations
        while (!exit) {
            System.out.println(
                "** Hash Table Operations **");
            System.out.println("1. insert ");
            System.out.println("2. remove");
            System.out.println("3. print hash table");
            System.out.println("4. get");
            System.out.println("5. clear");
            System.out.println("6. size");

            int choice = sc.nextInt();
            switch (choice) {
            case 1:
                System.out.println("Enter key and value");
                String key = sc.next();
                int value = sc.nextInt();
                table.insert(key, value);
                break;

            case 2:
                System.out.println("Enter key");
                String key2 = sc.next();
                table.remove(key2);
                System.out.println(
                    "Removed Successfully...");
                break;

            case 3:
                System.out.println(
                    "Key-value pairs in the hash table are : ");
                table.printHashTable();
                break;

            case 4:
                System.out.println("Enter key");
                String key3 = sc.next();
                System.out.println("Key = " + key3
                                   + " Value = "
                                   + table.get(key3));
                break;

            case 5:
                table.makeEmpty();
                System.out.println(
                    "Hash Table Cleared Successfully..");
                break;

            case 6:
                System.out.println("Size of the table is = "
                                   + table.getSize());
                break;

            default:
                exit = true;
                System.out.println(
                    "Exited Successfully...");
            }
        }
        sc.close();
    }
}

class HashTablesExample {
    // class linked hash
    class LinkedHash {
        String key;
        int value;
        LinkedHash next;

        // constructor of linked hash
        LinkedHash(String key, int value)
        {
            this.key = key;
            this.value = value;
            this.next = null;
        }
    }

    private int tableSize;
    private int size;
    private LinkedHash[] table;

    // constructor of HashTableExample
    public HashTablesExample(int ts)
    {
        size = 0;
        tableSize = ts;
        table = new LinkedHash[tableSize];

        // initializing the hash table with null values
        for (int i = 0; i < tableSize; i++)
            table[i] = null;
    }

    // method to get the number of key-value pairs in the
    // hash table
    public int getSize() { return size; }

    // method to clear the hash table entry
    public void makeEmpty()
    {
        for (int i = 0; i < tableSize; i++)
            table[i] = null;
    }

    // method to get value of a key
    public int get(String key)
    {
        int value = (myhash(key) % tableSize);
        if (table[value] == null)
            return -1;
        else {
            LinkedHash current = table[value];
            while (current != null
                   && !current.key.equals(key)) {
                current = current.next;
            }
            if (current == null) {
                return -1;
            }
            else {
                return current.value;
            }
        }
    }

    // method to insert a value in a hash table
    public void insert(String key, int value)
    {
        int hash = (myhash(key) % tableSize);
        if (table[hash] == null) {
            table[hash] = new LinkedHash(key, value);
        }
        else {
            LinkedHash entry = table[hash];
            while (entry.next != null
                   && !entry.key.equals(key)) {
                entry = entry.next;
            }
            if (entry.key.equals(key)) {
                entry.value = value;
            }
            else {
                entry.next = new LinkedHash(key, value);
            }
        }
        size++;
    }

    // method to remove the value with the specified key
    public void remove(String key)
    {
        int value = (myhash(key) % tableSize);
        if (table[value] != null) {
            LinkedHash prev = null;
            LinkedHash current = table[value];
            while (current.next != null
                   && !current.key.equals(key)) {
                prev = current;
                current = current.next;
            }
            if (current.key.equals(key)) {
                if (prev == null) {
                    table[value] = current.next;
                }
                else {
                    prev.next = current.next;
                }
                size--;
            }
        }
    }

    // method which gives a hash value from a given
    // specified string
    private int myhash(String x)
    {
        int value = x.hashCode();
        value %= tableSize;
        if (value < 0) {
            value = value + tableSize;
        }
        return value;
    }

    // method to print the value of hash table
    public void printHashTable()
    {
        for (int i = 0; i < tableSize; i++) {
            LinkedHash current = table[i];
            while (current != null) {
                System.out.println(
                    "Value = " + current.value + " "
                    + "Key = " + current.key);
                current = current.next;
            }
        }
        System.out.println();
    }
}

Output :

insert into hash table

remove from hash table

print the key-value pair of hash table

get the value from specified key

clear the hash table

size of the hash table