Java: Overloading (generic) Methods for java.lang.Number

Question

So I read discussion about Numbers here, because I have a similar Problem. In my case, I wanna be able to allow mathematical operations on Numbers. My idea was to write an immutable ´RealNumber´ class that handles primitive Numbers (Integer, Long, Float and Double), without a bunch of instanceof controls. Someone mentioned to Overload Methods and let the compiler do the work.
This was my first attempt:

simple TestClass:

public class Test {
public static void main(String[] args) {
    RealNumber<Double> d = RealNumber.create(3.4);
    d.add(4.7);
    }
}

RealNumber class: (please mention the Method with the comment)

public class RealNumber<N extends Number> extends Number{

    N number;   

    private RealNumber(N number){
        if (number == null){
            throw new NullPointerException("number is null");
        }
        this.number = number;
    }

    public N get(){
        return number;
    }

    //note this Method
    public RealNumber<N> add(N number){
        return add(number);
    }

    private RealNumber<Integer> add(Integer number){
        return new RealNumber<Integer>(intValue() + number);
    }

    private RealNumber<Long> add(Long number){
        return new RealNumber<Long>(longValue() + number);
    }

    private RealNumber<Float> add(Float number){
        return new RealNumber<Float>(floatValue() + number);
    }

    private RealNumber<Double> add(Double number){
        return new RealNumber<Double>(doubleValue() + number);
    }

    @Override
    public int intValue() {
        return number.intValue();
    }

    @Override
    public long longValue() {
        return number.longValue();
    }

    @Override
    public float floatValue() {
        return number.floatValue();
    }

    @Override
    public double doubleValue() {
        return number.doubleValue();
    }


    public static final RealNumber<Integer> create(Integer number){
        return new RealNumber<Integer>(number);
    }

    public static final RealNumber<Long> create(Long number){
        return new RealNumber<Long>(number);
    }

    public static final RealNumber<Float> create(Float number){
        return new RealNumber<Float>(number);
    }

    public static final RealNumber<Double> create(Double number){
        return new RealNumber<Double>(number);
    }
}

so the first test leads me to a StackOverflowError, because the method ´add´ always calls itself.

second try (only changed methods)

public RealNumber<N> add(Number number){
    return add(number);
}

first wasn't as good, cause it will allow to add BigDecimals, or other things like Boolean, and second leads me to the same StackOverflowError. so I changed:

public RealNumber<N> add(N number){
    return add(number);
}

//note the public here
public RealNumber<Double> add(Double number){
    return new RealNumber<Double>(doubleValue() + number);
}
//... public RealNumber<Integer, Long, Float> add....

which fails to compile in my TestClass -> "The Method add(Double) is ambiguous for the Type RealNumber
finally this worked:

public RealNumber<N> add(Number number){
    return add(number);
}

//note the public here
public RealNumber<Double> add(Double number){
    return new RealNumber<Double>(doubleValue() + number);
}
//... public RealNumber<Integer, Long, Float> add....

but brings another 2 issues: this pattern allows to add Doubles to Ints (which results in RealNumber of Integer ), and results in a StackOverflowError if one passes a BigInteger, Byte or some other Number.

So my main questions:

Why the compiler chooses the right method in Test.class if every add Method is public and fails, if they are private.

What can I do to fix the issues?

Answer 1

[ Sorry, I didn't fully understand the question with my first try. ]

I don't think there is an easy answer here @Rafael because as @Andrei Bodnarescu pointed out, type erasure means that you do not have the type of your N parameter at runtime. I think you have to provide a concrete implementation of your add() method for each subclass of Number.

public RealNumber<Integer> add(Integer number) {
    return new RealNumber<Integer>(intValue() + number);
}
public RealNumber<Long> add(Long number) {
    return new RealNumber<Long>(longValue() + number);
}

If you don't want to add integers to doubles then I guess you will need to do something like:

public RealNumber<Integer> add(Integer number){
    if (!(this instanceof Integer)) {
        throw new IllegalArgumentException("You can't do this...");
    }
    return new RealNumber<Integer>(intValue() + number);
}

I don't see any easy way to work around this.

Answer 2

The :

public RealNumber<N> add(N number){
        return add(number);
    }

method always calls itself because of type erasure: in Java generics are only for compile time, they're no longer present at runtime, so the VM effectively doesn't know what type N is, so it calls the most generic methods available, namely this one. You must pass the type as Class as argument to the method aswell, something like:

public RealNumber<N> add(N number,Class<N> numberClass){
        return add(number);
    }