title: Sort keys which are date entries in a hashmap
body: <p>I have a hashMap which has the following values as key <code>value(sql date , integer)</code> pairs:</p>
  
  <pre><code>a.put("31-05-2011",67);
  a.put("01-06-2011",89);
  a.put("10-06-2011",56);
  a.put("25-05-2011",34);
  </code></pre>
  
  <p>when i try to sort the hashMap based on the keys using :
  Map modified_a=new TreeMap(a);
   and display the keys it is as follows :</p>
  
  <pre><code>01-06-2011,10-06-2011,25-05-2011, 31-05-2011
  </code></pre>
  
  <p>but I want the keys to be sorted as </p>
  
  <pre><code>31-05-2011,25-05-2011,01-06-2011 ,10-06-2011
  </code></pre>
  
  <p>I can see that the values are being sorted based on the first 2 digits( which is the date value) but I need the month value to also be considered and sort based on months first and then for each month sort the corresponding days.
  Any clues ??</p>