<!DOCTYPE qhelp PUBLIC

"-//Semmle//qhelp//EN"

"qhelp.dtd">

<qhelp>

<overview>

<p>

Testing whether <code>this</code> is an instance of a derived type creates a dependency cycle.

Derived types already depend on their parent type and the cast creates a dependency in the other direction.

</p>

<p>Dependency cycles should be avoided as they make code both difficult to read and difficult to test.

In addition, a type should not know about its specific descendants, even though it may impose some

constraints on them as a group (for example, the need for every derived type to implement a method

with a specific signature).</p>

</overview>

<recommendation>

<p>

The base and derived types should be redesigned so that there is no need for the base type to depend

on the types deriving from it.</p>

</recommendation>

<example>

<p>In this example, <code>BadBase</code> introduces a dependency cycle with <code>Derived</code> by

testing the type of <code>this</code>.</p>

<sample src="DubiousTypeTestOfThis.java" />

</example>

</qhelp>