title: Convert ArrayList<Long> to ArrayList<String> with for loop
body: <p>Why do I get a NullPointerException when I try this:</p>
  
  <pre><code>ArrayList&lt;String&gt; longsToStrings() throws IOException {
      LinkedList&lt;Long&gt; longNumbers = populateArrayWithLongs();
      ArrayList&lt;String&gt; strings = null;
      for (Long l : longNumbers) {
          strings.add(l.toString());
      }
      return strings;
  }
  </code></pre>
  
  <p>When I try to print the results to console, I get NullPointerException.
  populateArrayWithLongs() function returns array with longs: [1488758722000, 1488773070000, 1488787530000, 1488801890000]</p>