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 Using a null seed value may prevent such problems. Java's default seed uses system's time in milliseconds. However, you should neither use two different generators with a null seed value nor "resetting" generator's instance more than once (using new Random() multiple times) as you may get identical numbers in the former case and number not actually random in the latter one.

import java.util.Random;
// ...

Random number = new Random();
//...
for (int i=0; i<20; i++)
{
   // re-seed generator
   number = new Random();
   // generate another random integer in the range [0,20]
   int n = number.nextInt(21);
   System.out.println(n);
}

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Automated Detection

TODO

Related Vulnerabilities

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