title: How to evaluate whether a string is null?
body: <p>I have a string, which is read from a database. The string can be just "null".  I need to decide whether it is null or not?  Among the following ones, what is the appropriate way to do it?</p>
  
  <pre><code>String a = …;
  If  (a == null)
  
  If ( a.length == 0)
  </code></pre>
  
  <p>I also see something like</p>
  
  <pre><code>If  a.equals(“ “)
  </code></pre>
  
  <p>How does this work?</p>