NonSerializableComparator.qhelp

<!DOCTYPE qhelp PUBLIC
  "-//Semmle//qhelp//EN"
  "qhelp.dtd">
<qhelp>


<overview>
<p>
A class that implements <code>java.util.Comparator</code> and is used to
construct a sorted collection needs to be serializable.
An ordered collection (such as a <code>java.util.TreeMap</code>) that is constructed
using a comparator serializes successfully only if the comparator is serializable.
</p>

<p>
The <code>Collection</code>s in the Java Standard Library that require a comparator
(<code>TreeSet</code>, <code>TreeMap</code>, <code>PriorityQueue</code>) all call
<code>ObjectOutputStream.defaultWriteObject</code>, which tries to serialize every 
non-static, non-transient field in the class. As the comparator 
is stored in a field in these collections, the attempt to serialize a non-serializable 
comparator throws a <code>java.io.NotSerializableException</code>.
</p>

</overview>
<recommendation>
<p>
Comparators should be serializable if they are used in sorted collections that 
may be serialized. In most cases, simply changing the comparator so it also implements
<code>Serializable</code> is enough. Comparators that have 
internal state may require additional changes (for example, custom <code>writeObject</code> and 
<code>readObject</code> methods). In these cases, it is best to follow general best practices
for serializable objects (see references below).
</p>

</recommendation>
<example>

<p>In the following example, <code>WrongComparator</code> is not serializable because it does not 
implement <code>Serializable</code>. However, <code>StringComparator</code> is serializable because 
it does implement <code>Serializable</code>.

</p><sample src="NonSerializableComparator.java" />

</example>
<references>


<li>
Java API Documentation:
<a href="http://docs.oracle.com/javase/6/docs/api/java/util/Comparator.html">Comparator</a>,
<a href="http://docs.oracle.com/javase/6/docs/api/java/io/ObjectOutputStream.html">ObjectOutputStream</a>,
<a href="http://docs.oracle.com/javase/6/docs/api/java/io/Serializable.html">Serializable</a>.
</li>


</references>
</qhelp>