Original.java

  public class MutableInteger {
    private int val;

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

    public int get() {
        return val;
    }

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

    @Override
    public String toString() {
        return Integer.toString(val);
    }
}

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

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

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