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Java Generics and adding numbers together

I would like to generically add numbers in java. I’m running into difficulty because the Numbers class doesn’t really support what I want to do. What I’ve tried so far is this:

public class Summer<E extends Number> {


    public E sumValue(List<E> objectsToSum) {
        E total = (E) new Object();
        for (E number : objectsToSum){
            total += number;
        }
        return null;

    }

Obviously this will not work. How can I go about correcting this code so I could be given a list of <int> or <long> or whatever and return the sum?

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Answer

In order to calculate a sum generically, you need to provide two actions:

  • A way to sum zero items
  • A way to sum two items

In Java, you do it through an interface. Here is a complete example:

import java.util.*;

interface adder<T extends Number> {
    T zero(); // Adding zero items
    T add(T lhs, T rhs); // Adding two items
}

class CalcSum<T extends Number> {
    // This is your method; it takes an adder now
    public T sumValue(List<T> list, adder<T> adder) {
        T total = adder.zero();
        for (T n : list){
            total = adder.add(total, n);
        }
        return total;
    }
}

public class sum {
    public static void main(String[] args) {
        List<Integer> list = new ArrayList<Integer>();
        list.add(1);
        list.add(2);
        list.add(4);
        list.add(8);
        CalcSum<Integer> calc = new CalcSum<Integer>();
        // This is how you supply an implementation for integers
        // through an anonymous implementation of an interface:
        Integer total = calc.sumValue(list, new adder<Integer>() {
            public Integer add(Integer a, Integer b) {
                return a+b;
            }
            public Integer zero() {
                return 0;
            }
        });
        System.out.println(total);
    }
}
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