Java HashMap

• The java HashMap class does not guarantee that the order will remain constant over time.
  
• This implementation provides constant-time performance for the basic operations (get and put), assuming the hash function disperses the elements properly among the buckets.
  
• The HashMap implementation is not synchronized. If multiple threads access this map concurrently, and at least one of the threads modifies the map structurally, it must be synchronized externally.
  

To prevent unsynchronized access to the Map: Map m = Collections.synchronizedMap(new HashMap(...));

Below is a HashMap Example used to store the number of words that begin with a given letter

/*
 *  Using a HashMap to store the number of words that begin with a given letter.
 */

import java.util.HashMap;

public class HashMapExample {

	static String[] names = { "heman", "bob", "hhh", "shawn", "scot","shan", "keeth" };
	private static HashMap counter = new HashMap();
	private static Integer cnt = null;
	public static void main(String args[]) {
		for (int i = 0; i < names.length; i++) {
			cnt = (Integer) (counter.get(new Character(names[i].charAt(0))));
			if (cnt == null) {
				counter.put(new Character(names[i].charAt(0)),new Integer("1"));
			} else {
				counter.put(new Character(names[i].charAt(0)),
						new Integer(cnt.intValue() + 1));
			}
		}
		System.out.println("\nnumber of words beginning with each letter is shown below ");
		System.out.println(counter.toString());
	}
}

Output

number of words beginning with each letter is shown below
{s=3, b=1, k=1, h=2}

Download  HashMapExample.java