Generalise a method

Question

I got this method

public static Integer addCollectionElements(List<Integer> list) {
    return list.stream().reduce(0, Integer::sum).intValue();
} 

which I would like to generalise to any class that implements the method sum (e.g. Float).

Is it possible to achieve this by using Java Generics?

Answer 1

Unfortunately, the Number base class doesn't have a sum method, so you can't do this directly. What you could do, however, is call doubleValue() on each Number you receive, and sum them as a doubles:

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