NoNonFinalInConstructor.qhelp

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


<overview>
<p>If a constructor calls a method that is overridden in a subclass, 
it can cause the overriding method in the subclass
to be called before the subclass has been initialized. This can lead to unexpected results.
</p>

</overview>
<recommendation>

<p>Do not call a non-final method from within a constructor if that method
could be overridden in a subclass.</p>

</recommendation>
<example>

<p>
In the following example, executing <code>new Sub("test")</code> results in a <code>NullPointerException</code>.
This is because the subclass constructor implicitly calls the superclass constructor, which in turn
calls the overridden <code>init</code> method before the field <code>s</code> is initialized in the subclass constructor.
</p>

<sample src="NoNonFinalInConstructor.java" />

<p>
To avoid this problem:
</p>

<ul>
    <li>The <code>init</code> method in the super constructor should be made <code>final</code> or <code>private</code>.</li>
    <li>The initialization that is performed in the overridden <code>init</code> method in the subclass 
    can be moved to the subclass constructor itself, or delegated to a separate final 
    or private method that is called from within the subclass constructor.</li>
</ul>

</example>
<references>


<li>
  J. Bloch,
  <em>Effective Java (second edition)</em>, pp. 89&ndash;90.
  Addison-Wesley, 2008.
</li>
<li>
  The Java Tutorials:
  <a href="https://docs.oracle.com/javase/tutorial/java/IandI/final.html">Writing Final Classes and Methods</a>.
</li>


</references>
</qhelp>