title: How to use dates as keys in a hashmap
body: <p>I wonder if it is possible to read a text file with the following strucutre</p>
  
  <pre><code>1946-01-12;07:00:00;-1.3;G
  1946-01-12;13:00:00;0.3;G
  1946-01-12;18:00:00;-2.8;Y
  1946-01-13;07:00:00;-6.2;G
  1946-01-13;13:00:00;-4.7;G
  1946-01-13;18:00:00;-4.3;Y
  1946-01-14;07:00:00;-1.5;G
  </code></pre>
  
  <p>but with more lines to a hashmap where the dates are keys and and the values are <code>-1.3, 0.3, -2.8, -6.2...</code> and so on? Is it possible even though there are multiples of the same date to store many values in the same date/key?</p>