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e21cbe82a9
Where possible update Java documentation links to Java 11. Additionally update some other links to use HTTPS.
58 lines
2.0 KiB
XML
58 lines
2.0 KiB
XML
<!DOCTYPE qhelp PUBLIC
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"-//Semmle//qhelp//EN"
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"qhelp.dtd">
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<qhelp>
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<overview>
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<p>
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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>
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every time a new pseudo-random number is required (for example, <code>new Random().nextInt()</code>).
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</p>
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<p>
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According to the Java API Specification:</p>
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<blockquote>
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<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
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identical sequences of numbers.</p>
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</blockquote>
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<p>The sequence of pseudo-random numbers returned by these calls depends only on the value of the seed.
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If you construct a new <code>Random</code> object each time a pseudo-random number is needed, this does not
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generate a good distribution of pseudo-random numbers, even though the parameterless <code>Random()</code>
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constructor tries to initialize itself with a unique seed.
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</p>
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</overview>
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<recommendation>
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<p>
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Create a <code>Random</code> object once and use the same instance when generating sequences of
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pseudo-random numbers (by calling <code>nextInt</code>, <code>nextLong</code>, and so on).
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</p>
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</recommendation>
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<example>
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<p>In the following example, generating a series of pseudo-random numbers, such as <code>notReallyRandom</code>
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and <code>notReallyRandom2</code>, by creating a new instance of <code>Random</code> each time is
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unlikely to result in a good distribution of pseudo-random numbers. In contrast, generating a series
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of pseudo-random numbers, such as <code>random1</code> and <code>random2</code>, by calling
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<code>nextInt</code> each time <em>is</em> likely to result in a good distribution. This is because
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the numbers are based on only one <code>Random</code> object.</p>
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<sample src="RandomUsedOnce.java" />
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</example>
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<references>
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<li>
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Java API Specification:
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<a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/util/Random.html">Random</a>.
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</li>
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</references>
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</qhelp>
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