title: what is the best way to get a sub HashMap based on a list of Keys?
body: <p>I have a HashMap and I would like to get a new HashMap that contains only the elements from the first HashMap where K belongs to a specific List.</p>
  
  <p>I could look through all the keys and fillup a new HashMap but I was wondering if there is a more efficient way to do it?</p>
  
  <p>thanks</p>