Generics don´t cast and add together

Question

I wanted to check, if my Generics are of the type Integer. If they are, they should get converted to double and afterwards should be added together. I get the error "The operator * is undefined for the argument type(s) E, double". There is also the same problem afterwards with adding the two casted values together.

How can I fix this?

Thanks in advance :)

public void add(E value1, E value2) {
            if(value1 instanceof Integer && value2 instanceof Integer) {
                double valueCast1 = value1 * 1.0; 
                double valueCast2 = value2 * 1.0;
                System.out.println(value1 + value2);
            }
        }

I think what Kayaman maybe asking is why would you want to multiply something by 1. — Griffin, Oct 18 '17 at 14:35
Java and JavaScript are two different languages... *1.0 is for javascript... — Alexei Levenkov, Oct 18 '17 at 14:36
Well the body itself is nonsense of course. The code is also in no way generic. Sure, there's the generic type `E`, but it might as well just be `Object`. — Kayaman, Oct 18 '17 at 14:37
The main question would be why you would use two generic parameters for a method that only does something if you supply two integers which are then attempted to cast (incorrectly) to doubles and have their sum printed. — Rob Obdeijn, Oct 18 '17 at 14:39
I think you misunderstand how generics are intended to work. This doesn't work because there's no guarantee that your generic type `E` can be successfully cast to Integer, Double or anything else. You have to explicitly, not implicitly cast, even after you've checked the `instanceof`. — PaulProgrammer, Oct 18 '17 at 19:41

score 1 · Answer 1 · answered Oct 18 '17 at 14:39

You must add a cast for your Integer values. You've previously checked if they were Integer, but in sentences

double valueCast1 = value1 * 1.0; 
double valueCast2 = value2 * 1.0;

they still be just generic E classes. You must specify that they should be considered integer so

double valueCast1 = (Integer)value1 * 1.0; 
double valueCast2 = (Integer)value2 * 1.0;

That's it.

Generics don´t cast and add together

1 Answers1