StringComparison.qhelp

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


<overview>
<p>Comparing two <code>String</code> objects using <code>==</code> or
<code>!=</code> compares object identity, which may not be intended.
The same sequence of characters can be represented by two distinct 
<code>String</code> objects.
</p>

</overview>
<recommendation>

<p>To see if two <code>String</code> objects represent
the same sequence of characters, you should usually compare the objects by
using their <code>equals</code> methods.
</p>

</recommendation>
<example>

<p>With the following definition, <code>headerStyle</code> is compared
to the empty string using <code>==</code>. This comparison can yield
<code>false</code> even if <code>headerStyle</code> is the empty string, because
it compares the identity of the two string objects rather than their contents.
For example, if <code>headerStyle</code> was initialized by an XML parser or a JSON
parser, then it might have been created with code like
<code>String.valueOf(buf,start,len)</code>. Such code will produce a new string
object every time it is called.</p>  

<sample src="StringComparison.java" />

<p>With the following definition, <code>headerStyle</code> is tested using
the <code>equals</code> method. This version will reliably detect whenever
<code>headerStyle</code> is the empty string.</p>

<sample src="StringComparisonGood.java" />

</example>
<references>
<li>
  Java API Specification:
  <a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/String.html#equals(java.lang.Object)">String.equals()</a>,
  <a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/String.html#intern()">String.intern()</a>.
</li>
<li>
  Java Language Specification:
  <a href="https://docs.oracle.com/javase/specs/jls/se11/html/jls-15.html#jls-15.21.3">15.21.3 Reference Equality Operators == and !=</a>,
  <a href="https://docs.oracle.com/javase/specs/jls/se11/html/jls-3.html#jls-3.10.5">3.10.5 String Literals </a>,
  <a href="https://docs.oracle.com/javase/specs/jls/se11/html/jls-15.html#jls-15.28">15.28 Constant Expressions</a>.
</li>


</references>
</qhelp>