DangerousNonCircuitLogic.qhelp

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<overview>
<p>Using a bitwise logical operator (<code>&amp;</code> or
<code>|</code>) on a boolean where a conditional-and or conditional-or operator
(<code>&amp;&amp;</code> or <code>||</code>) is intended may yield the wrong result or
cause an exception. This is especially true if the left-hand operand is a guard for the
right-hand operand.</p>

<p>Typically, as in the example below, this kind of defect is introduced by
simply mistyping the intended logical operator rather than any conceptual
mistake by the programmer.</p>

</overview>
<recommendation>

<p>If the right-hand side of an expression is only intended to be
evaluated if the left-hand side evaluates to <code>true</code>, use a
conditional-and.</p>

<p>Similarly, if the right-hand side of an expression is only intended
to be evaluated if the left-hand side evaluates to <code>false</code>, use a
conditional-or.</p>

</recommendation>
<example>

<p>In the following example, the <code>hasForename</code> method is implemented correctly. For a forename
to be valid it must be a non-null string with a non-zero length. The method has
two expressions (<code>forename != null</code> and <code>forename.length() &gt;
0</code>) to check these two properties. The second check is 
executed only if the first succeeds, because they are combined using a conditional-and
operator (<code>&amp;&amp;</code>).</p>

<p>In contrast, although <code>hasSurname</code> looks almost the same, it contains
a defect. Again there are two tests (<code>surname != null</code> and
<code>surname.length() &gt; 0</code>), but they are linked by a
bitwise logical operator (<code>&amp;</code>). Both sides of a bitwise logical
operator are <i>always</i> evaluated, so if <code>surname</code> is
<code>null</code> the <code>hasSurname</code> method throws a
<code>NullPointerException</code>. To fix the defect, change <code>&amp;</code> to <code>&amp;&amp;</code>.</p>

<sample src="DangerousNonCircuitLogic.java" />

</example>
<references>


<li>
  J. Bloch and N. Gafter, <em>Java Puzzlers: Traps, Pitfalls, and Corner Cases</em>, Puzzle 42.
  Addison-Wesley, 2005.
</li>
<li>
Java Language Specification:
<a href="https://docs.oracle.com/javase/specs/jls/se11/html/jls-15.html#jls-15.22.2">15.22.2 Boolean Logical Operators &amp;, ^, and |</a>,
<a href="https://docs.oracle.com/javase/specs/jls/se11/html/jls-15.html#jls-15.23">15.23 Conditional-And Operator &amp;&amp;</a>,
<a href="https://docs.oracle.com/javase/specs/jls/se11/html/jls-15.html#jls-15.24">15.24 Conditional-Or Operator ||</a>.
</li>


</references>
</qhelp>