Generic function in Java [требует правки]

I want to write function for getting number of object in List(index notEl) that extends abstract classNotationElement.

For example :

List(Classes) : Bracket, UnaryOperator, Operand, Bracket

(Every class in the list extends NotationElement)

private <T extends NotationElement> Integer getNumberInType(int notEl) {
    if (expression != null) {
        if ((notEl < expression.size()) && (expression.size() > 0)) {
            int num = 0;
            for (int i = 0; i < notEl; i++) {
                if ((NotationElement)expression.get(i) instanceof T)
                    num++;
            }
            return (Integer)num;
        }
        return null;
    }
    return null;
}

When funnction called

getNumberOfElement<Bracket>(3);

it returns 2: it means that in the list expression there are 2 Brackets.