Java Generics and adding numbers together

Question

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

Answer 1

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 zero(); // Adding zero items
    T add(T lhs, T rhs); // Adding two items
}

class CalcSum {
    // This is your method; it takes an adder now
    public T sumValue(List list, adder 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 list = new ArrayList();
        list.add(1);
        list.add(2);
        list.add(4);
        list.add(8);
        CalcSum calc = new CalcSum();
        // This is how you supply an implementation for integers
        // through an anonymous implementation of an interface:
        Integer total = calc.sumValue(list, new adder() {
            public Integer add(Integer a, Integer b) {
                return a+b;
            }
            public Integer zero() {
                return 0;
            }
        });
        System.out.println(total);
    }
}