codeql/java/ql/src/Language Abuse/WrappedIterator.qhelp

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

<include src="IterableOverview.inc.qhelp" />

<recommendation>
<p>
When writing the <code>iterator()</code> method in an
<code>Iterable&lt;T&gt;</code> then it is important to make sure that each call
will result in a fresh <code>Iterator&lt;T&gt;</code> instance containing all
the necessary state for keeping track of the iteration.  If the iterator is
stored in the <code>Iterable&lt;T&gt;</code>, or somehow refers to iteration
state stored in the <code>Iterable&lt;T&gt;</code>, then subsequent calls to
<code>iterator()</code> can result in loops that only traverse a subset of the
elements or have no effect at all.
</p>

</recommendation>

<example>
<p>
The following example returns the same iterator on every call, and therefore
causes the second loop to terminate immediately without any effect.
</p>
<sample src="WrappedIteratorBad1.java" />

<p>
This second example returns a newly created iterator each time,
but still relies on iteration state stored
in the surrounding class, and therefore also causes the second loop to
terminate immediately.
</p>
<sample src="WrappedIteratorBad2.java" />

<p>
The code should instead be written like this, such that each call to
<code>iterator()</code> correctly gives a fresh iterator that starts at the
beginning.
</p>
<sample src="WrappedIteratorGood.java" />
</example>

<references>

<li>
Java Language Specification:
<a href="https://docs.oracle.com/javase/specs/jls/se11/html/jls-14.html#jls-14.14.2">The enhanced for statement</a>.
</li>
<li>
Java API Specification:
<a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Iterable.html">Interface Iterable&lt;T&gt;</a>,
<a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/util/Iterator.html">Interface Iterator&lt;T&gt;</a>,
<a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/nio/file/DirectoryStream.html">Interface DirectoryStream&lt;T&gt;</a>.
</li>

</references>

</qhelp>