Question

I’m attempting implement the add method mentioned in the Generic sparse matrix addition question

class Matrix<T extends Number>
{
  private T add(T left, T right)
  {
   if (left instanceof Integer)
   {
     return new Integer(((Integer)left).intValue() + ((Integer)right).intValue());
   }
}

The compiler errors with found java.lang.Integer Required T at the line where I return a new Integer. I’m not sure what I’m missing since T extends Number and Integer is a subclass of Number.

Answer 1

The compiler doesn’t let you do this because T might be some other class, such as Double.
You know that T is Integer from the instanceof check, but the compiler doesn’t.