Hash function for strings

Question

Implementation of a hash function in java, haven't got round to dealing with collisions yet. I tried to use good code, refactored, descriptive variable names, nice syntax.

My code must be efficient and extensible.

Would be interested to know how you guys would improve. Three classes:

1. TestHash - main method is here, hash is initialised and data inserted and read
2. HashTable -object were the hash table is held
3. HashEntry - each entry in the array is a 'HashEntry' object

HashTable

import java.io.*;

public class HashTable {
    // number of indexes in hash table
    private final static int TABLE_SIZE = 100;
    // initialise array of HashEntry (table)
    HashEntry[] table;

    // constructor, create array, assign all values to null
    HashTable() {
        // create array of HashEntry, predefined size
        table = new HashEntry[TABLE_SIZE];
        // assign all values to null
        for (int i = 0; i < TABLE_SIZE; i++) {
            table[i] = null;
        }
    }

    // from key, find the correct index, load the HashEntry object and return the value
    public String getValue(String key) {
        int index = this.getKeyIndex(key);
        HashEntry chosen_entry = table[index];
        String chosen_entry_value = chosen_entry.getValue(key);
        return chosen_entry_value;
    }

    // creates a new HashEntry object, inserts that into the table array at the correct index
    // determined by getKeyIndex(key)
    public void insert(String key, String value) {
        int index = this.getKeyIndex(key);
        HashEntry keyValuePair;
        keyValuePair = new HashEntry(key, value);
        table[index] = keyValuePair;
        return;
    }

    // ** PRIVATE FUNCTIONS ** //
    // get index from key
    private Integer getKeyIndex(String key) {
        int hash = 7;
        for (int i = 0; i < key.length(); i++) {
            hash = (hash*31 + key.charAt(i))%100;
        }
        return hash;
    }
}

HashEntry

public class HashEntry {
    private String key;
    private String value;

    HashEntry(String key, String value) {
        this.key = key;
        this.value = value;
    }

    public String getKey() {
        return this.key;
    }

    public String getValue(String key) {
        // use string key to move down linked list in case of collision
        // haven't implemented this yet
        return this.value;
    }
}

TestHash

public class TestHash {
    public static void main(String[] args)
    {
        HashTable tableHash = new HashTable();
        tableHash.insert("david", "england");
        tableHash.insert("joe", "united states");
        tableHash.insert("maria", "china");
        System.out.println(tableHash.getValue("joe"));
    }
}

Answer 1

Very nifty-looking code right there. However, you should declare table as private final. Also, you don't need to initialize each array component of table to null, Java already does this for you. What comes to insert, you could write simply

table[index] = new HashEntry(key, value);

omitting the return statement. Point is to write only necessary statements. In getValue, you might want to check whether table[index] is null, since if that is the case, your implementation will throw NullPointerException.