*         key.
     *
     * @throws NullPointerException if key is null
     * @throws ClassCastException if key is not String
     * @throws IllegalArgumentException if key is empty String
     */
    public boolean containsKey(Object key) {
        checkKey(key);
        return map.containsKey(key);
    }    
    
    /** {@inheritDoc} */
    public boolean containsValue(Object value) {
        return map.containsValue(value);
    }    
    
    /** {@inheritDoc} */
    public Set<Map.Entry<String, Object>> entrySet() {
        return map.entrySet();
    }
    
    /**
     * Returns the value to which this map maps the specified key.  Returns
     * <tt>null</tt> if the map contains no mapping for this key.  A return
     * value of <tt>null</tt> does not <i>necessarily</i> indicate that the
     * map contains no mapping for the key; it's also possible that the map
     * explicitly maps the key to <tt>null</tt>.  The <tt>containsKey</tt>
     * operation may be used to distinguish these two cases.
     *
     * <p>More formally, if this map contains a mapping from a key
     * <tt>k</tt> to a value <tt>v</tt> such that <tt>(key==null ? k==null :
     * key.equals(k))</tt>, then this method returns <tt>v</tt>; otherwise
     * it returns <tt>null</tt>.  (There can be at most one such mapping.)
     *
     * @param key key whose associated value is to be returned.
     * @return the value to which this map maps the specified key, or
     *         <tt>null</tt> if the map contains no mapping for this key.
     *
     * @throws NullPointerException if key is null
     * @throws ClassCastException if key is not String
     * @throws IllegalArgumentException if key is empty String
     */  
    public Object get(Object key) {
        checkKey(key);
        return map.get(key);
    }
    
    /** {@inheritDoc} */
    public boolean isEmpty() {
        return map.isEmpty();
    }
    
    /** {@inheritDoc} */
    public Set<String> keySet() {
        return map.keySet();
    }
 
    /**
     * Removes the mapping for this key from this map if it is present
     * (optional operation).   More formally, if this map contains a mapping
     * from key <tt>k</tt> to value <tt>v</tt> such that
     * <code>(key==null ?  k==null : key.equals(k))</code>, that mapping
     * is removed.  (The map can contain at most one such mapping.)
     *
     * <p>Returns the value to which the map previously associated the key, or
     * <tt>null</tt> if the map contained no mapping for this key.  (A
     * <tt>null</tt> return can also indicate that the map previously
     * associated <tt>null</tt> with the specified key if the implementation
     * supports <tt>null</tt> values.)  The map will not contain a mapping for
     * the specified  key once the call returns.
     *
     * @param key key whose mapping is to be removed from the map.
     * @return previous value associated with specified key, or <tt>null</tt>
     *         if there was no mapping for key.
     *
     * @throws NullPointerException if key is null
     * @throws ClassCastException if key is not String
     * @throws IllegalArgumentException if key is empty String
     */   
    public Object remove(Object key) {
        checkKey(key);
        return map.remove(key);
    }
    
    /** {@inheritDoc} */
    public int size() {
        return map.size();
    }
    
    /** {@inheritDoc} */
    public Collection<Object> values() {
        return map.values();
    }

    private void checkKey(Object key) {
        if (key == null) {
            throw new NullPointerException("key can not be null");
        }
        if (!(key instanceof String)) {
            throw new ClassCastException("key should be a String");