Here is a HashMap example where we have stored the date and its corresponding day of week.

import java.util.*;
import java.text.*;

class HashMapExample 
{
    public static void main(String[] args) throws Exception 
    {
        Map<Date, String> m = new HashMap<Date, String>(); 

    DateFormat dateFormat = new SimpleDateFormat("dd-MM-yyyy");

    Date d1=dateFormat.parse("5-05-2012");
    Date d2=dateFormat.parse("10-06-2012");
    Date d3=dateFormat.parse("15-05-2012");
    Date d4=dateFormat.parse("20-06-2012");
    SimpleDateFormat formatter = new SimpleDateFormat("EEEE");
    String day1=formatter.format(d1);
    String day2=formatter.format(d2);
    String day3=formatter.format(d3);
    String day4=formatter.format(d4);
    m.put(d1,day1);
    m.put(d2,day2);
    m.put(d3,day3);
    m.put(d4,day4);
    Set set = m.entrySet();
    Iterator i = set.iterator();
    while(i.hasNext()) {
    Map.Entry me = (Map.Entry)i.next();
    System.out.println(me.getKey() + ": \t "+me.getValue());
    } 
  }
}