Adding a List Java.Lang.Numbers

Question

I have a list that has to accept either a list of Integer/Double/Float/Long and return the sum. I tried this

public static Number getSum(List< ? extends Number> lst) {
    return lst.stream().reduce(0.0, (a, b) -> a+b);
}

But I am getting this error in compile time

The operator + is undefined for the argument type(s) capture#3-of ? extends java.lang.Number, capture#3-of ? extends java.lang.Number

You cant add two Numbers without knowing if they are integer, long, or double etc. If they are all real numbers, use doubleValue() — SwiftMango, Nov 26 '20 at 05:44
You can try to use reflection in order to get the static `sum` method. You can then call it using the parameters. However, this only works if both have the same type and the type has the `sum` method. — dan1st, Nov 26 '20 at 06:52
Does this answer your question? [How to add two java.lang.Numbers?](https://stackoverflow.com/questions/2721390/how-to-add-two-java-lang-numbers) — dan1st, Nov 26 '20 at 08:52

score 0 · Answer 1 · answered Nov 26 '20 at 16:40

There can be a custom subclass of Number, that cannot be applied to +. E.g. you declared a class Natural

class Natural extends Number {
    ...
}

You can't add it using plus operator.

So I think the best you can to is to cast them to double first.

public static Number getSum(List< ? extends Number> lst) {
    return lst.stream().mapToDouble(Number::doubleValue).sum();
}

1 Answers1