Force operations on same money currencies in Java at compile time.

MoneyTest.java

package ro.igstan.playground;

import java.util.ArrayList;
import java.util.List;

public class MoneyTest {

  public static interface Money<E extends Money<E>> {
    public static final class Zero<E extends Money<E>> implements Money<E> {
      @Override
      public Money<E> add(Money<E> a) {
        return a;
      }

      @Override
      public Number amount() {
        return 0;
      }
    }

    Money<E> add(Money<E> a);

    Number amount();
  }

  public static class USD implements Money<USD> {
    private final Integer amount;

    public USD(Integer amount) {
      this.amount = amount;
    }

    @Override
    public Money<USD> add(Money<USD> a) {
      return new USD(amount().intValue() + a.amount().intValue());
    }

    @Override
    public Integer amount() {
      return amount;
    }
  }

  public static class EUR implements Money<EUR> {
    private final Integer amount;

    public EUR(Integer amount) {
      this.amount = amount;
    }

    @Override
    public Money<EUR> add(Money<EUR> other) {
      return new EUR(this.amount().intValue() + other.amount().intValue());
    }

    @Override
    public Integer amount() {
      return amount;
    }
  }

  public static <E extends Money<E>> Money<E> sum(List<E> moneyList) {
    Money<E> total = new Money.Zero<E>();

    for (Money<E> money : moneyList) {
      total = total.add(money);
    }

    return total;
  }

  public static void main(String[] args) {
    USD amount1 = new USD(100);
    EUR amount2 = new EUR(200);
    EUR amount3 = new EUR(300);

    // compile error
    // amount1.add(amount2);

    List<EUR> list = new ArrayList<EUR>();
    list.add(amount2);
    list.add(amount3);

    System.out.println(sum(list).amount());
  }
}