CounterConcurrentHashMap.java

import java.util.List;
import java.util.concurrent.*;
import java.util.stream.Collectors;
import java.util.stream.IntStream;

public class Test {
  public static class MutableInteger {
    private int val;

    public MutableInteger() {
      this(0);
    }

    public MutableInteger(int val) {
      this.val = val;
    }


    public int get() {
      return val;
    }

    public void set(int val) {
      this.val = val;
    }
  }

  public static class CounterConcurrentHashMap<K> extends ConcurrentHashMap<K, MutableInteger> {

    public MutableInteger up(K key) {
      MutableInteger initValue = new MutableInteger(1);
      MutableInteger oldValue = super.put(key, initValue);

      if(oldValue != null) {
        initValue.set(oldValue.get() + 1);
      }

      return initValue;
    }
  }

  public static void main(String... argv) throws Exception {
    ExecutorService pool = Executors.newFixedThreadPool(8);
    CounterConcurrentHashMap<String> map = new CounterConcurrentHashMap<>();

    List<Future> futures = IntStream.range(0, 8).mapToObj(i -> pool.submit(() -> {
      for(int j = 0;j < 1000000;j++) {
        map.up("key");
      }
    })).collect(Collectors.toList());

    for(Future f :  futures) {
      f.get();
    }

    System.out.println(map.get("key").get());
  }
}