ReadOnlyContainer.qhelp

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

<overview>
<p>
A method that queries the contents of a collection or map (such as <code>containsKey</code>
or <code>isEmpty</code>) is invoked on an object that is known to be empty. Such method
calls do not return interesting results, and may indicate missing code or a logic error. 
</p>

</overview>
<recommendation>

<p>
Either remove the collection/map if it is unnecessary, or ensure that it contains the
elements it was meant to contain.
</p>

</recommendation>
<example>

<p>
The following example code iterates over an array of objects to determine whether it contains
duplicate elements. It maintains a collection <code>seen</code>, which is intended to contain
all the elements seen so far in traversing the array. If the current element is already
contained in that collection then the method returns <code>true</code>, indicating that a
duplicate has been found.
</p> 

<p>
Note, however, that no elements are ever actually added to <code>seen</code>, so the method always
returns <code>false</code>.
</p>

<sample src="ReadOnlyContainer.java" />

<p>
To fix this problem, a statement <code>seen.add(o);</code> should be added to the end of the loop
body to ensure that <code>seen</code> is correctly maintained.
</p>

</example>
<references>


<li>
  Java API Specification:
  <a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/util/Collection.html">Collection</a>,
  <a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/util/Map.html">Map</a>.
</li>


</references>
</qhelp>