public String toString() {
        StringBuffer buffer = new StringBuffer();
        ListElement<T> tmp = first;  
        for(int i=0; i<size; i++) {
            buffer.append("["+tmp.data.toString()+"]");
            tmp=tmp.next;
            if(i!=size-1) {
                buffer.append(", ");
            }
        }
        return buffer.toString();
    }