MissingMethodClone.qhelp

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


<overview>
<p>
A class that implements <code>Cloneable</code> should override <code>Object.clone</code>. For non-trivial
objects, the <code>Cloneable</code> contract requires a deep copy of the object's internal state. A class
that does not have a <code>clone</code> method indicates that the class is breaking the contract and will have undesired behavior.
</p>

</overview>
<include src="MissingCloneDetails.inc.qhelp" />

<recommendation>
<p>
The necessity of creating a deep copy of an object's internal state means that, for most objects, 
<code>clone</code> must be overridden to satisfy the <code>Cloneable</code> contract. Implement a 
<code>clone</code> method that properly creates the internal state of the cloned object.
</p>

<p>Notable exceptions to this recommendation are:</p>

<ul>
    <li>Classes that contain only primitive types (which will be properly cloned by <code>Object.clone</code>
    as long as its <code>Cloneable</code> supertypes all call <code>super.clone</code>).</li>
    <li>Subclasses of <code>Cloneable</code> classes that do not introduce new state.</li>
</ul>

</recommendation>
<example>

<p>In the following example, <code>WrongStack</code> does not implement <code>clone</code>. This means
that when <code>ws1clone</code> is cloned from <code>ws1</code>, the default <code>clone</code>
implementation is used. This results in operations on the <code>ws1clone</code>
stack affecting the <code>ws1</code> stack.</p>

<sample src="MissingMethodCloneBad.java" />

<p>In the following modified example, <code>RightStack</code> <i>does</i> implement 
<code>clone</code>. This means that when <code>rs1clone</code> is cloned from <code>rs1</code>, 
operations on the <code>rs1clone</code> stack do not affect the <code>rs1</code> stack.</p>

<sample src="MissingMethodCloneGood.java" />

</example>
<references>


<li>
  J. Bloch, <em>Effective Java (second edition)</em>, Item 11. Addison-Wesley, 2008.
</li>
<li>
  Java API Specification: <a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Object.html#clone()">Object.clone()</a>.
</li>


</references>
</qhelp>