code: // from gee.cs.oswego.edu/home/jsr166/jsr166
  
  package jsr166tests.loops;
  
  /*
   * 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
   */
  
  import java.util.*;
  import java.util.concurrent.*;
  
  public class MapLoops {
      static int nkeys       = 1000;
      static int pinsert     = 60;
      static int premove     = 2;
      static int maxThreads  = 100;
      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 = java.util.concurrent.ConcurrentHashMap.class;
  
          if (args.length > 1)
              maxThreads = Integer.parseInt(args[1]);
  
          if (args.length > 2)
              nkeys = Integer.parseInt(args[2]);
  
          if (args.length > 3)
              pinsert = Integer.parseInt(args[3]);
  
          if (args.length > 4)
              premove = Integer.parseInt(args[4]);
  
          if (args.length > 5)
              nops = Integer.parseInt(args[5]);
  
          // normalize probabilities wrt random number generator
          removesPerMaxRandom = (int)(((double)premove/100.0 * 0x7FFFFFFFL));
          insertsPerMaxRandom = (int)(((double)pinsert/100.0 * 0x7FFFFFFFL));
  
          System.out.print("Class: " + mapClass.getName());
          System.out.print(" threads: " + maxThreads);
          System.out.print(" size: " + nkeys);
          System.out.print(" ins: " + pinsert);
          System.out.print(" rem: " + premove);
          System.out.print(" ops: " + nops);
          System.out.println();
  
          int k = 1;
          int warmups = 2;
          for (int i = 1; i <= maxThreads;) {
              Thread.sleep(100);
              test(i, nkeys, mapClass);
              if (warmups > 0)
                  --warmups;
              else if (i == k) {
                  k = i << 1;
                  i = i + (i >>> 1);
              }
              else if (i == 1 && k == 2) {
                  i = k;
                  warmups = 1;
              }
              else
                  i = k;
          }
          pool.shutdown();
      }
  
      static Integer[] makeKeys(int n) {
          LoopHelpers.SimpleRandom rng = new LoopHelpers.SimpleRandom();
          Integer[] key = new Integer[n];
          for (int i = 0; i < key.length; ++i)
              key[i] = new Integer(rng.next());
          return key;
      }
  
      static void shuffleKeys(Integer[] key) {
          Random rng = new Random();
          for (int i = key.length; i > 1; --i) {
              int j = rng.nextInt(i);
              Integer tmp = key[j];
              key[j] = key[i-1];
              key[i-1] = tmp;
          }
      }
  
      static void test(int i, int nkeys, Class mapClass) throws Exception {
          System.out.print("Threads: " + i + "\t:");
          Map<Integer, Integer> map = (Map<Integer,Integer>)mapClass.newInstance();
          Integer[] key = makeKeys(nkeys);
          // Uncomment to start with a non-empty table
          //        for (int j = 0; j < nkeys; j += 4) // start 1/4 occupied
          //            map.put(key[j], key[j]);
          shuffleKeys(key);
          LoopHelpers.BarrierTimer timer = new LoopHelpers.BarrierTimer();
          CyclicBarrier barrier = new CyclicBarrier(i+1, timer);
          for (int t = 0; t < i; ++t)
              pool.execute(new Runner(t, 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();
      }
  
      static class Runner implements Runnable {
          final Map<Integer,Integer> map;
          final Integer[] key;
          final LoopHelpers.SimpleRandom rng;
          final CyclicBarrier barrier;
          int position;
          int total;
  
          Runner(int id, Map<Integer,Integer> map, Integer[] key,  CyclicBarrier barrier) {
              this.map = map;
              this.key = key;
              this.barrier = barrier;
              position = key.length / 2;
              rng = new LoopHelpers.SimpleRandom((id + 1) * 8862213513L);
              rng.next();
          }
  
          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) {
                      if (map.remove(k) != null) {
                          position = total % key.length; // move from position
                          return 2;
                      }
                  }
              }
              else if (r < insertsPerMaxRandom) {
                  ++position;
                  map.put(k, k);
                  return 2;
              }
  
              // Uncomment to add a little computation between accesses
              //            total += LoopHelpers.compute1(k.intValue());
              total += r;
              return 1;
          }
  
          public void run() {
              try {
                  barrier.await();
                  int ops = nops;
                  while (ops > 0)
                      ops -= step();
                  barrier.await();
              }
              catch (Exception ex) {
                  ex.printStackTrace();
              }
          }
      }
  }
  