Unit test :: demo 2

Money.java

public class Money {
    ...

    public Money add(Money money) {
        return new Money(amount() + money.amount(), currency());
    }

    ...

}

Money2.java

public class Money {
    ...

    public Money add(Money money) {
        if (money.currency().equals(currency())) {
            return new Money(amount() + money.amount(), currency());
        }
        return new MoneyBag(this, money);
    }

    ...

}

MoneyBag.java

public class MoneyBag {
    private List<Money> monies = new ArrayList<Money>();
    
    public MoneyBag(Money firstMoney, Money secondMoney) {
        appendMoney(firstMoney);
        appendMoney(secondMoney);
    }
    
    public MoneyBag(Money[] moneyBag) {
        for (Money money : moneyBag) {
            appendMoney(money);
        }
    }
    
    private void appendMoney(Money money) {
        monies.add(money);
    }
}

MoneyBag2.java

public class MoneyBag {

...

   public boolean equals(Object anObject) {
        if (anObject instanceof MoneyBag) {
            List<Money> target = ((MoneyBag) anObject).monies;
            if( target.size() == monies.size()) {
                return target.equals(monies);
            }
        }
        return false;
    }

...

}

MoneyBag3.java

public class MoneyBag implements Money {

    public Money add(Money money) {
        return money.addMoneyBag(this);
    }

    public Money addSimpleMoney(SimpleMoney simpleMoney) {
        return new MoneyBag(simpleMoney, this);
    }

    public Money addMoneyBag(MoneyBag moneyBag) {
        return new MoneyBag(moneyBag, this);
    }

}

MoneyBagTest.java

public class MoneyBagTest {

    private Money money5THB;
    private Money money10THB;
    private Money money3USD;
    private Money money8USD;
    private MoneyBag firstMoneyBag;
    private MoneyBag secondMoneyBag;

    @Before
    public void setupFixture() {
        money5THB = new Money(5, "THB");
        money10THB = new Money(10, "THB");
        money3USD = new Money(3, "USD");
        money8USD = new Money(8, "USD");

        firstMoneyBag = new MoneyBag(money5THB, money3USD);
        secondMoneyBag = new MoneyBag(new Money[] { money10THB, money8USD });
    }

    @Test
    public void equalsMoneyBag() throws Exception {
        assertTrue(!firstMoneyBag.equals(null));
        assertEquals(firstMoneyBag, firstMoneyBag);
        assertEquals(firstMoneyBag, new MoneyBag(money5THB, money3USD));
        assertTrue(!firstMoneyBag.equals(money5THB));
        assertTrue(!money5THB.equals(firstMoneyBag));
        assertTrue(!firstMoneyBag.equals(secondMoneyBag));
    }

}

MoneyBagTest2.java

public class MoneyBagTest {

...

    @Test
    public void mixedSimpleMoneyInBag() throws Exception {
        Money[] bag = {money5THB, money3USD};
        MoneyBag expectedMoneyBag = new MoneyBag(bag);
        assertEquals(expectedMoneyBag, money5THB.add(money3USD));
    }

...

}

MoneyInterface.java

public interface Money {
    Money add(Money money);
}

MoneyInterface2.java

public interface Money {
    Money add(Money money);

    Money addSimpleMoney(SimpleMoney simpleMoney);
    Money addMoneyBag(MoneyBag moneyBag);
}

SimpleMoney.java

public class SimpleMoney implements Money {

    public Money add(Money money) {
        return money.addSimpleMoney(this);
    }

    public Money addSimpleMoney(SimpleMoney simpleMoney) {
        if (simpleMoney.currency().equals(currency())) {
            return new SimpleMoney(amount() + simpleMoney.amount(), currency());
        }
        return new MoneyBag(simpleMoney, this);
    }

    public Money addMoneyBag(MoneyBag moneyBag) {
        return moneyBag.addSimpleMoney(this);
    }

}

Update.java

public class SimpleMoney implements Money {
   ...
}


public class MoneyBag implements Money {
   ...
}

Update2.java

class SimpleMoney implements Money {
    public Money add(Money money) {
        return money.addMoney(this);
    }
}


class MoneyBag implements Money {
    public Money add(Money money) {
        return money.addMoneyBag(this);
    }
}