SimplifyBoolExpr.qhelp

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

<overview>
<p>
There are a number of boolean expression patterns that can easily be rewritten
to make them simpler.
Boolean expressions involving comparisons with boolean literals,
ternary conditionals with a boolean literal as one of the results,
double negations, or negated comparisons can all be changed to
equivalent and simpler expressions.
</p>
</overview>

<recommendation>
<p>
If <code>A</code> and <code>B</code> are expressions of boolean type, you can
simplify them using the rewrites shown below. These rewrites are equally suitable
for the primitive type <code>boolean</code> and the boxed type <code>Boolean</code>.
</p>
<table><tbody>
<tr><th>Expression</th><th></th><th>Simplified expression</th><th></th></tr>
<tr><td><code>A == true</code></td><td></td><td><code>A</code></td><td>(*)</td></tr>
<tr><td><code>A != false</code></td><td></td><td><code>A</code></td><td>(*)</td></tr>
<tr><td><code>A == false</code></td><td></td><td><code>!A</code></td><td></td></tr>
<tr><td><code>A != true</code></td><td></td><td><code>!A</code></td><td></td></tr>
<tr><td><code>A ? true : B</code></td><td></td><td><code>A || B</code></td><td></td></tr>
<tr><td><code>A ? B : false</code></td><td></td><td><code>A &amp;&amp; B</code></td><td></td></tr>
<tr><td><code>A ? B : true</code></td><td></td><td><code>!A || B</code></td><td></td></tr>
<tr><td><code>A ? false : B</code></td><td></td><td><code>!A &amp;&amp; B</code></td><td></td></tr>
<tr><td><code>A ? true : false</code></td><td></td><td><code>A</code></td><td>(*)</td></tr>
<tr><td><code>A ? false : true</code></td><td></td><td><code>!A</code></td><td></td></tr>
<tr><td><code>!!A</code></td><td></td><td><code>A</code></td><td>(*)</td></tr>
</tbody></table>
<p>
Note that all of these rewrites yield completely equivalent code, except
possibly for those marked with (*) when <code>A</code> has type <code>Boolean</code>.
These can depend on the context if <code>null</code>
values are involved.  This is because a comparison or test of a
<code>Boolean</code> will perform an automatic unboxing conversion that throws a
<code>NullPointerException</code> if <code>null</code> is encountered, so the rewrites marked
(*) are only completely equivalent if the surrounding context of the expression unboxes the value, for
example by occurring as the condition in an <code>if</code> statement.
</p>

<p>
In addition to the rewrites above, negated comparisons can also be simplified in the following way:
</p>
<table><tbody>
<tr><th>Expression</th><th></th><th>Simplified expression</th></tr>
<tr><td><code>!(A == B)</code></td><td></td><td><code>A != B</code></td></tr>
<tr><td><code>!(A != B)</code></td><td></td><td><code>A == B</code></td></tr>
<tr><td><code>!(A &lt; B)</code></td><td></td><td><code>A >= B</code></td></tr>
<tr><td><code>!(A > B)</code></td><td></td><td><code>A &lt;= B</code></td></tr>
<tr><td><code>!(A &lt;= B)</code></td><td></td><td><code>A > B</code></td></tr>
<tr><td><code>!(A >= B)</code></td><td></td><td><code>A &lt; B</code></td></tr>
</tbody></table>

</recommendation>

<example>
<p>
In the following example the method <code>f2</code> is a straightforward
simplification of <code>f1</code>; they will both throw a <code>NullPointerException</code> if
a <code>null</code> is encountered.
A similar rewrite of <code>g1</code> would however result in a method with a different meaning,
and a rewrite along the lines of <code>g2</code> might be more appropriate.

In any case, care should be taken to ensure correct behavior for
<code>null</code> values when the boxed type <code>Boolean</code> is used.
</p>

<sample src="SimplifyBoolExpr.java" />
</example>

<references>

<li>
The Java Language Specification:
<a href="https://docs.oracle.com/javase/specs/jls/se8/html/jls-15.html#jls-15.15.6">Logical Complement Operator !</a>,
<a href="https://docs.oracle.com/javase/specs/jls/se7/html/jls-15.html#jls-15.21.2">Boolean Equality Operators == and !=</a>,
<a href="https://docs.oracle.com/javase/specs/jls/se8/html/jls-15.html#jls-15.23">Conditional-And Operator &amp;&amp;</a>,
<a href="https://docs.oracle.com/javase/specs/jls/se8/html/jls-15.html#jls-15.24">Conditional-Or Operator ||</a>,
<a href="https://docs.oracle.com/javase/specs/jls/se8/html/jls-15.html#jls-15.25">Conditional Operator ? :</a>.
</li>

</references>

</qhelp>