class ListMap extends HashMap<T, List<V>> {

    public List<V> putValue( T key, V value )
    {
        List<V> list = this.get( key );
        list = list == null ? new ArrayList<>() : list;
        list.add( value );
        super.put( key, list );
        return null;
    }

    public  ListMap();
    public  ListMap( ListMap<T, V> listMap );

}