java - Generic type extending Number, calculations

Question

I've made an interface of math operation with one method, calculate, taking various number of arguments

public interface MathOperation {
    public <T extends Number> T calculate(T... args);
}

There's also simple implementation of this class, which does not work:

private class Sum implements MathOperation {
    @Override
    public <T extends Number> T calculate(T... args) {
        return args[0] + args[1];
    }
}

The problem is:

bad operand types for binary operator '+'
  first type:  T
  second type: T
  where T is a type-variable:
    T extends Number declared in method <T>calculate(T...)

What I'm trying to achieve is a simple class, taking for example two Doubles and returning Double as well.

Is there possibility to achieve this?

Answer 1

+ cannot be applied to types that extend Number. new Integer(5) + new Integer(5) works because of autoboxing. You will have to look at the runtime type of args and do the operation accordingly.

Something on the lines of:

private class Sum implements MathOperation {
    @Override
    public <T extends Number> T calculate(Class<T> clazz, T... args) {
         if (clazz.equals(Integer.class))
         {
             return Integer.class.cast(args[0]) + Integer.class.cast(args[1]);
         } else (....) 
    }
}