Is it possible to write a generic +1 method for numeric box types in Java?

Question

This is NOT homework.

Part 1

Is it possible to write a generic method, something like this:

Answer 1

Not the prettiest solution ever, but if you rely in the following properties of every known implementation of Number (in the JDK):

• They can all be created from their String representation via a one-argument constructor
• None of them has numbers that can't be represented by BigDecimal

You can implement it using reflection and using Generics to avoid having to cast the result:

public class Test {

    @SuppressWarnings("unchecked")
    public static  T plusOne(T num) {
        try {
            Class c = num.getClass();
            Constructor constr = c.getConstructor(String.class);
            BigDecimal b = new BigDecimal(num.toString());
            b = b.add(java.math.BigDecimal.ONE);
            return (T) constr.newInstance(b.toString());
        } catch (Exception e) {
            throw new RuntimeException(e);
        }
    }

    public static void main(String[] args) {
        System.out.println(plusOne(1));
        System.out.println(plusOne(2.3));
        System.out.println(plusOne(2.4E+120));
        System.out.println(plusOne(2L));
        System.out.println(plusOne(4.5f));
        System.out.println(plusOne(new BigInteger("129481092470147019409174091790")));
        System.out.println(plusOne(new BigDecimal("12948109247014701940917.4091790")));
    }

}

The return is done using an apparently unsafe cast but given that you're using a constructor of the class of some T or child of T you can assure that it will always be a safe cast.