How to evaluate whether a string is null?
<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>
