RandomUsedOnce.qhelp

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


<overview>
<p>
A program that uses <code>java.util.Random</code> to generate a sequence of pseudo-random numbers <em>should not</em> create a new instance of <code>Random</code> 
every time a new pseudo-random number is required (for example, <code>new Random().nextInt()</code>).
</p>

<p> 
According to the Java API Specification:</p>

<blockquote>
<p>If two instances of <code>Random</code> are created with the same seed, and the same sequence of method calls is made for each, they will generate and return 
identical sequences of numbers.</p>
</blockquote>

<p>The sequence of pseudo-random numbers returned by these calls depends only on the value of the seed. 
If you construct a new <code>Random</code> object each time a pseudo-random number is needed, this does not 
generate a good distribution of pseudo-random numbers, even though the parameterless <code>Random()</code>
constructor tries to initialize itself with a unique seed.
</p>

</overview>
<recommendation>
<p>
Create a <code>Random</code> object once and use the same instance when generating sequences of 
pseudo-random numbers (by calling <code>nextInt</code>, <code>nextLong</code>, and so on).
</p>

</recommendation>
<example>

<p>In the following example, generating a series of pseudo-random numbers, such as <code>notReallyRandom</code> 
and <code>notReallyRandom2</code>, by creating a new instance of <code>Random</code> each time is 
unlikely to result in a good distribution of pseudo-random numbers. In contrast, generating a series
of pseudo-random numbers, such as <code>random1</code> and <code>random2</code>, by calling
<code>nextInt</code> each time <em>is</em> likely to result in a good distribution. This is because 
the numbers are based on only one <code>Random</code> object.</p>

<sample src="RandomUsedOnce.java" />

</example>
<references>


<li>
  Java API Specification:
  <a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/util/Random.html">Random</a>.
</li>


</references>
</qhelp>