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<!DOCTYPE qhelp PUBLIC

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

"qhelp.dtd">

<qhelp>

When checking whether a string <code>s</code> is empty, perhaps the most obvious

solution is to write something like <code>s.equals("")</code> (or <code>"".equals(s)</code>).

However, this actually carries a fairly significant overhead, because <code>String.equals</code>

performs a number of type tests and conversions before starting to compare the content

of the strings.

</overview>

The preferred way of checking whether a string <code>s</code> is empty is to check if its

length is equal to zero. Thus, the condition is <code>s.length() == 0</code>. The <code>length</code>

method is implemented as a simple field access, and so should be noticeably faster than

calling <code>equals</code>.

Note that in Java 6 and later, the <code>String</code> class has an <code>isEmpty</code>

method that checks whether a string is empty. If the codebase does not need to support Java 5, it may

be better to use that method instead.

</recommendation>

In the following example, class <code>InefficientDBClient</code> uses <code>equals</code> to test

whether the strings <code>user</code> and <code>pw</code> are empty. Note that the test

<code>"".equals(pw)</code> guards against <code>NullPointerException</code>, but the test

<code>user.equals("")</code> throws a <code>NullPointerException</code> if <code>user</code> is

<code>null</code>.

In contrast, the class <code>EfficientDBClient</code> uses <code>length</code> instead of

<code>equals</code>. The class preserves the behavior of <code>InefficientDBClient</code> by

guarding <code>pw.length() == 0</code> but not <code>user.length() == 0</code> with an explicit test

for <code>null</code>. Whether or not this guard is desirable depends on the intended behavior of

the program.

</example>

<li>

Java API Specification:

<a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/String.html#length()">String.length()</a>,

<a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/String.html#isEmpty()">String.isEmpty()</a>,

<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>.

</li>

</references>

</qhelp>

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