NullObjectIsDead.java

import java.util.Arrays;
import java.util.List;

interface Integer {
    int getValue();
}

final class ConcreteInteger implements Integer {
    private final int value;
    ConcreteInteger(int value) { this.value = value; }
    @Override public int getValue() { return value; }
}

final class NullInteger implements Integer {
    private static final NullInteger INSTANCE = new NullInteger();
    private NullInteger() {}
    static NullInteger getInstance() { return INSTANCE; }
    @Override public int getValue() { return 0; }
}

interface Reducer {
    Integer reduce(Integer initial, List<Integer> numbers);
}

final class SumReducer implements Reducer {
    @Override public Integer reduce(Integer initial, List<Integer> numbers) {
        Integer sum = initial;
        for (final Integer number : numbers) {
            sum = new ConcreteInteger(sum.getValue() + number.getValue());
        }
        return sum;
    }
}

final class ProductReducer implements Reducer {
    @Override public Integer reduce(Integer initial, List<Integer> numbers) {
        Integer product = initial;
        for (final Integer number : numbers) {
            product = new ConcreteInteger(product.getValue() * number.getValue());
        }
        return product;
    }
}

public class NullObjectIsDead {
    public static void main(String[] args) {
        final List<Integer> numbers = Arrays.asList(
            new ConcreteInteger(2),
            new ConcreteInteger(3),
            new ConcreteInteger(5)
        );

        final List<Reducer> reducers = Arrays.asList(
            new SumReducer(),
            new ProductReducer()
        );
        for (final Reducer reducer : reducers) {
            System.out.println(reducer.reduce(NullInteger.getInstance(), numbers).getValue());
        }
    }
}