title: Convert double of null value to String Error
body: <p>How to solve this problem</p>
  
  <pre><code>public class DoubleTest {
  
      public static void main(String[] args) {
          double dbl = (Double) null;
          String str = Double.toString(dbl);
          System.out.println(str);
  
      }
  
  } 
  </code></pre>
  
  <p>With Error:</p>
  
  <pre><code>Exception in thread "main" java.lang.NullPointerException
      at test.java.DoubleTest.main(DoubleTest.java:6)
  </code></pre>
  
  <p>Any idea please?</p>
  
  <p>I am asking this because I have a method that return Double which I have to convert to String again.</p>
  
  <p>If my method returns <strong>null</strong>, then this problem happens.</p>