title: Delete all the entries but the specified Key set from Hash Map
body: <p>I am trying to delete all the entries in HashMap apart from the specified Set of Keys.
  For example say HashMap numToalphaMap has entries 1-->a, 2-->b, 3-->c, 4-->d. Given KeySet is {1, 2}. I want to delete other entries ie.., (3-->c, 4-->d) from the numToalphaMap. Could anyone help me with this?</p>