@Test
  public void test_executeNegative() throws Exception {
    Operation operation = new SubtractOperation(BigDecimal.valueOf(1));
    BigDecimal result = operation.execute(BigDecimal.valueOf(-3)); // -3 - 1

    assertTrue(result.compareTo(BigDecimal.valueOf(-4)) == 0);
  }
 @Test(expected = NullPointerException.class)
 public void test_executeNull_NullPointerException() throws Exception {
   Operation operation = new SubtractOperation(BigDecimal.valueOf(2));
   operation.execute(null);
 }