Money { public Money subtract(Money other) { return new Money(amount - other.getAmount()); } Money();  Money(long amount);  Money(String amount); @Override boolean equals(Object o); @Override int hashCode(); long getAmount(); void setAmount(long amount); Money subtract(Money other); Money add(Money other); boolean isGreaterOrEqualThan(Money otherAmount); @Override String toString();  }
@Test public void shouldSubtract() { assertEquals(new Money("5"), new Money("6").subtract(new Money("1"))); }