Java Generics and adding numbers together

As Number class does not expose interface for performing calculations, the only way to solve this problem is to create classes which encapsulates required operations for each supported numeric type. Than in your class you will need to use specific type.

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);
    }
}