--- jsr166/src/test/jtreg/util/Random/DistinctSeeds.java	2003/11/07 01:35:36	1.1
+++ jsr166/src/test/jtreg/util/Random/DistinctSeeds.java	2015/09/15 07:53:31	1.8
@@ -1,16 +1,60 @@
-/**
- * @test 1.1 03/11/06
- * @bug 4949279
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/publicdomain/zero/1.0/
+ */
+
+/*
+ * @test
+ * @bug 4949279 6937857
  * @summary Independent instantiations of Random() have distinct seeds.
+ * @key randomness
  */
 
+import java.util.ArrayList;
+import java.util.HashSet;
+import java.util.List;
 import java.util.Random;
 
 public class DistinctSeeds {
     public static void main(String[] args) throws Exception {
-	// Strictly speaking, it is possible for these to be equal,
-	// but the likelihood should be *extremely* small.
-	if (new Random().nextLong() == new Random().nextLong())
-            throw new RuntimeException("Random seeds not unique.");
+        // Strictly speaking, it is possible for these to randomly fail,
+        // but the probability should be small (approximately 2**-48).
+        if (new Random().nextLong() == new Random().nextLong() ||
+            new Random().nextLong() == new Random().nextLong())
+            throw new RuntimeException("Random() seeds not unique.");
+
+        // Now try generating seeds concurrently
+        class RandomCollector implements Runnable {
+            long[] randoms = new long[1<<17];
+            public void run() {
+                for (int i = 0; i < randoms.length; i++)
+                    randoms[i] = new Random().nextLong();
+            }
+        }
+        final int threadCount = 2;
+        List<RandomCollector> collectors = new ArrayList<>();
+        List<Thread> threads = new ArrayList<Thread>();
+        for (int i = 0; i < threadCount; i++) {
+            RandomCollector r = new RandomCollector();
+            collectors.add(r);
+            threads.add(new Thread(r));
+        }
+        for (Thread thread : threads)
+            thread.start();
+        for (Thread thread : threads)
+            thread.join();
+        int collisions = 0;
+        HashSet<Long> s = new HashSet<Long>();
+        for (RandomCollector r : collectors) {
+            for (long x : r.randoms) {
+                if (s.contains(x))
+                    collisions++;
+                s.add(x);
+            }
+        }
+        System.out.printf("collisions=%d%n", collisions);
+        if (collisions > 10)
+            throw new Error("too many collisions");
     }
 }