code: // from gee.cs.oswego.edu/home/jsr166/jsr166
  
  package jsr166tests.jtreg.util.concurrent.locks.ReentrantReadWriteLock;
  
  /*
   * 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/licenses/publicdomain
   */
  
  /*
   * @test
   * @bug 4486658
   * @compile -source 1.5 MapLoops.java
   * @run main/timeout=4700 MapLoops
   * @summary Exercise multithreaded maps, by default ConcurrentHashMap.
   * Multithreaded hash table test.  Each thread does a random walk
   * though elements of "key" array. On each iteration, it checks if
   * table includes key.  If absent, with probability pinsert it
   * inserts it, and if present, with probability premove it removes
   * it.  (pinsert and premove are expressed as percentages to simplify
   * parsing from command line.)
   */
  
  import java.util.*;
  import java.util.concurrent.*;
  
  public class MapLoops {
      static final int NKEYS = 100000;
      static int pinsert     = 60;
      static int premove     = 2;
      static int maxThreads  = 5;
      static int nops        = 1000000;
      static int removesPerMaxRandom;
      static int insertsPerMaxRandom;
  
      static final ExecutorService pool = Executors.newCachedThreadPool();
  
      public static void main(String[] args) throws Exception {
  
          Class mapClass = null;
          if (args.length > 0) {
              try {
                  mapClass = Class.forName(args[0]);
              } catch(ClassNotFoundException e) {
                  throw new RuntimeException("Class " + args[0] + " not found.");
              }
          }
          else
              mapClass = RWMap.class;
  
          if (args.length > 1)
              maxThreads = Integer.parseInt(args[1]);
  
          if (args.length > 2)
              nops = Integer.parseInt(args[2]);
  
          if (args.length > 3)
              pinsert = Integer.parseInt(args[3]);
  
          if (args.length > 4)
              premove = Integer.parseInt(args[4]);
  
          // normalize probabilities wrt random number generator
          removesPerMaxRandom = (int)(((double)premove/100.0 * 0x7FFFFFFFL));
          insertsPerMaxRandom = (int)(((double)pinsert/100.0 * 0x7FFFFFFFL));
  
          System.out.println("Using " + mapClass.getName());
  
          Random rng = new Random(315312);
          Integer[] key = new Integer[NKEYS];
          for (int i = 0; i < key.length; ++i)
              key[i] = new Integer(rng.nextInt());
  
          // warmup
          System.out.println("Warmup...");
          for (int k = 0; k < 2; ++k) {
              Map<Integer, Integer> map = (Map<Integer,Integer>)mapClass.newInstance();
              LoopHelpers.BarrierTimer timer = new LoopHelpers.BarrierTimer();
              CyclicBarrier barrier = new CyclicBarrier(1, timer);
              new Runner(map, key, barrier).run();
              map.clear();
              Thread.sleep(100);
          }
  
          for (int i = 1; i <= maxThreads; i += (i+1) >>> 1) {
              System.out.print("Threads: " + i + "\t:");
              Map<Integer, Integer> map = (Map<Integer,Integer>)mapClass.newInstance();
              LoopHelpers.BarrierTimer timer = new LoopHelpers.BarrierTimer();
              CyclicBarrier barrier = new CyclicBarrier(i+1, timer);
              for (int k = 0; k < i; ++k)
                  pool.execute(new Runner(map, key, barrier));
              barrier.await();
              barrier.await();
              long time = timer.getTime();
              long tpo = time / (i * (long)nops);
              System.out.print(LoopHelpers.rightJustify(tpo) + " ns per op");
              double secs = (double)(time) / 1000000000.0;
              System.out.println("\t " + secs + "s run time");
              map.clear();
          }
          pool.shutdown();
          if (! pool.awaitTermination(Long.MAX_VALUE, TimeUnit.NANOSECONDS))
              throw new Error();
      }
  
      static class Runner implements Runnable {
          final Map<Integer,Integer> map;
          final Integer[] key;
          final LoopHelpers.SimpleRandom rng = new LoopHelpers.SimpleRandom();
          final CyclicBarrier barrier;
          int position;
          int total;
  
          Runner(Map<Integer,Integer> map, Integer[] key,  CyclicBarrier barrier) {
              this.map = map;
              this.key = key;
              this.barrier = barrier;
              position = key.length / 2;
          }
  
          int step() {
              // random-walk around key positions,  bunching accesses
              int r = rng.next();
              position += (r & 7) - 3;
              while (position >= key.length) position -= key.length;
              while (position < 0) position += key.length;
  
              Integer k = key[position];
              Integer x = map.get(k);
  
              if (x != null) {
                  if (x.intValue() != k.intValue())
                      throw new Error("bad mapping: " + x + " to " + k);
  
                  if (r < removesPerMaxRandom) {
                      // get awy from this position
                      position = r % key.length;
                      map.remove(k);
                      return 2;
                  }
                  else
                      total += LoopHelpers.compute2(LoopHelpers.compute1(x.intValue()));
              }
              else {
                  if (r < insertsPerMaxRandom) {
                      map.put(k, k);
                      return 2;
                  }
              }
              return 1;
          }
  
          public void run() {
              try {
                  barrier.await();
                  int ops = nops;
                  while (ops > 0)
                      ops -= step();
                  barrier.await();
              }
              catch (Exception ex) {
                  ex.printStackTrace();
              }
          }
      }
  }
  