How to create a create a generic class in Java but limit what types you can use?

Question

I want a Fraction class that I can use to encapsulate data and logic regarding fractions, but I want to give the user the choice of what size integer they would like to use. Unfortunately, there is no common super class that covers only these classes: BigInteger, Long, Integer, Short

I would use Number , but I don't want to allow non-integer numbers like floats or decimals.

Can I do something like this?

import java.math.BigInteger;

public class Fraction<T> {

    private T numerator;
    private T denominator;

    Fraction(T numerator, T denominator) {
        if (!(T == BigInteger.class || T == Long.class || T == Integer.class || T == Short.class)) {
            throw new Exception();
        }
    }

}

The error I'm currently getting is T cannot be resolved to a variable

Answer 1

What I would do is just have Fraction<T extends Number> , but have only private constructors and all public methods that return an instance be specific to one of those types. (So, in theory, you could refer to Fraction<Float> , but it's only actually possible to get instances of Fraction that refer to integer-based types.)

Answer 2

You could have a check in your constructor like

if (numerator.doubleValue() / numerator.longValue() != 1)
{

    throw new IllegalArgumentException();
}

check both passed in numbers this way. this however would allow you to pass in something like 10.0, but IMHO, that's ok.