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/* |
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* @test |
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< |
* @bug 4949279 |
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* @bug 4949279 6937857 |
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* @summary Independent instantiations of Random() have distinct seeds. |
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* @key randomness |
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*/ |
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import java.util.ArrayList; |
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import java.util.HashSet; |
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import java.util.List; |
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import java.util.Random; |
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public class DistinctSeeds { |
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public static void main(String[] args) throws Exception { |
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// Strictly speaking, it is possible for these to randomly fail, |
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// but the probability should be *extremely* small (< 2**-63). |
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// but the probability should be small (approximately 2**-48). |
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if (new Random().nextLong() == new Random().nextLong() || |
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new Random().nextLong() == new Random().nextLong()) |
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throw new RuntimeException("Random() seeds not unique."); |
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|
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// Now try generating seeds concurrently |
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class RandomCollector implements Runnable { |
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long[] randoms = new long[1<<17]; |
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public void run() { |
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for (int i = 0; i < randoms.length; i++) |
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randoms[i] = new Random().nextLong(); |
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} |
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} |
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final int threadCount = 2; |
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List<RandomCollector> collectors = new ArrayList<>(); |
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List<Thread> threads = new ArrayList<Thread>(); |
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for (int i = 0; i < threadCount; i++) { |
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RandomCollector r = new RandomCollector(); |
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collectors.add(r); |
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threads.add(new Thread(r)); |
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} |
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for (Thread thread : threads) |
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thread.start(); |
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for (Thread thread : threads) |
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thread.join(); |
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int collisions = 0; |
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HashSet<Long> s = new HashSet<Long>(); |
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for (RandomCollector r : collectors) { |
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for (long x : r.randoms) { |
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if (s.contains(x)) |
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collisions++; |
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s.add(x); |
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} |
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} |
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System.out.printf("collisions=%d%n", collisions); |
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if (collisions > 10) |
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throw new Error("too many collisions"); |
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} |
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} |
