improper use of ArrayList

here the following code from my user defined class:
        long id;
        ArrayList theArray;
        public classname(long num)
        {
            id = num;
            theArray = new ArrayList();
        }
        public void add(lender newLender)
        {
            theArray.Add(newLender);
        }
        public lender get(int num)
        {
          ...