hashtable tutorial

HelloHash.java

import java.lang.NullPointerException;
import java.util.NoSuchElementException;
import java.lang.StringBuilder;
import java.util.LinkedList;

/**
 * Hashtable tutorial class.
 *
 * Adapted from <a href="http://www.danielacton.com/Data-Structures/Hashtable/Java/>">Daniel Action</a>.
 * November 2014.
 */

@SuppressWarnings({"unchecked"})
public class Hashtable {
    /* 
     * MEMBER VARIABLES
     */

    private final int tableSize = 20;
    private int numElements;

    /* 
     * ugly cast is needed to create a linkedlist array:
     * http://stackoverflow.com/questions/217065/cannot-create-an-array-of-linkedlists-in-java
     */

    private LinkedList<HashtableNode>[] table = (LinkedList<HashtableNode>[]) new LinkedList<?>[20];

    /* 
     * PUBLIC METHODS
     */

    /* tester class*/
    public static void main(String[] args)
    {
        Hashtable hashtable = new Hashtable();

        hashtable.add("XPC", "123 456 7890");
        hashtable.add("XPC DEV", "987 6543");

        System.out.println("Hashtable:");
        System.out.println(hashtable);

        System.out.println("get(\"Bosch family\"): " + hashtable.get("Bosch family"));
    }

    /* adding */
    public void add(Object key, Object data)
    {
        /* no null keys or elements allowed */
        if ((data == null) || (key == null)) 
            throw new NullPointerException("Cannot insert null key or data into Hashtable.");

        /* no duplicates allowed */
        if (this.contains(key)) return;

        int hash = this.hash(key);

        /* if there is nothing at the position, create new linked list */
        if(this.table[hash] == null)
            this.table[hash] = new LinkedList<HashtableNode>();

        (this.table[hash]).add(new HashtableNode(key, data));
        this.numElements++;
    }

    public void add(Object[] keys, Object[] data)
    {
        if(keys.length != data.length)
            throw new ArrayIndexOutOfBoundsException
                ("Array of keys must be the same length as array of data objects when adding to Hashtable.");

        for(int i = 0; i < keys.length; i++)
            this.add(keys[i], data[i]);
    }

    /* removing */
    public void remove(Object key)
    {
        int hash = this.hash(key);
        if (this.table[hash] == null) return;

        HashtableNode node = new HashtableNode();
        node.setKey(key);

        if ((this.table[hash]).indexOf(node) < 0)
            return;

        (this.table[hash]).remove(node);
        numElements--;
    }

    public void remove(Object[] keys)
    {
        for (int i = 0; i < keys.length; i++)
            this.remove(keys[i]);
    }

    /* fetching */
    public boolean contains(Object key)
    {
        if(numElements == 0) return false;

        int hash = this.hash(key);

        HashtableNode node = new HashtableNode();
        node.setKey(key);

        if(this.table[hash] != null)
            if((this.table[hash]).indexOf(node) > -1)
                return true;
        return false;
    }

    public Object get(Object key)
    {
        if(numElements == 0)
            throw new NoSuchElementException("Hashtable is empty.");

        int hash = this.hash(key);

        HashtableNode node = new HashtableNode();
        node.setKey(key);

        if(this.table[hash] == null)
            throw new NoSuchElementException("Key-value pair not found");

        int index = (this.table[hash]).indexOf(node);

        if(index < 0)
            throw new NoSuchElementException("Key-value pair not found");

        return (this.table[hash]).get(index).getData();
    }

    /* outputting */
    public String toString()
    {
        StringBuilder sb = new StringBuilder();

        sb.append("{\n");
        for (int i = 0; i < tableSize; i++)
        {
            if(table[i] != null)
                sb.append(table[i] + "\n");
        }
        sb.append("}");

        return sb.toString();
    }

    /*
     * PRIVATE METHODS
     */

    /* generic hashCode */
    private int hashCode(Object key)
    {
        /* initialize empty strings to be non-zero | 42 chosen at random */
        int result = 42;

        /* 
         * convert object into string | all objects in java have a toString method 
         * the default method returns string along the lines of: 
         * "path.to.lineofcode.ClassType@12345"
         */

        String inString = key.toString().toLowerCase();

        /* add up values of each character in string*/
        for (char c : inString.toCharArray()) {
            /* use hex numbers for a-f, else use ascii value of the character */
            if ((c == 'a')
                || (c == 'b')
                || (c == 'c')
                || (c == 'd')
                || (c == 'e')
                || (c == 'f'))
            { result += Integer.parseInt("" + c, 16); }

            else 
            { result += (int) c; }
        }

        return result;
    }

    private int hash(Object key)
    {
        /* modulo a hash code with the tableSize to map the code to the backing array */
        return this.hashCode(key) % tableSize;
    }

    /*
     * PRIVATE CLASS
     */

    /* internal node class */
    private class HashtableNode 
    {
        private Object key;
        private Object data;

        public HashtableNode() 
        {
            this.key = null;
            this.data = null;
        }

        public HashtableNode(Object key, Object data) 
        {
            this.key = key;
            this.data = data;
        }

        public Object getData()
        { return data; }

        public void setData(Object data)
        { this.data = data; }

        public Object getKey()
        { return key; }

        public void setKey(Object key)
        { this.key = key; }

        public boolean equals(Object obj)
        {
            if (obj == this) return true;
            if (obj == null) return false;
            if (this.getClass() != obj.getClass()) return false;

            HashtableNode node = (HashtableNode) obj;
            return this.key.equals(node.key);
        }

        public String toString()
        { return "Key: [" + this.getKey() + "] Data: [" + this.getData() + "]"; }
    }
}