Using Enums For Descriptive Behavior Grouping

Recently i stumbled upon Javin Pauls interview questions article about enums and although i knew almost everything about it i stopped when i saw the question "Can Enum implement interface in Java". I already had used that feature in several projects but more as a marker interface or to provide a more generic way for a module. But this i time recognized the full effect when it comes to grouping behavior via inheritance.

Consider the following Calculator:

1 public interface Calculator {
2     default double add(int a, int b){
3         return a + b;
4     }
5 }

Now lets assume that there are several other functional calculators where some of them have the same business logic and therefore can be grouped together. Usually (the cleancode way ;) an interface or abstract class is used which can be implemented. In practice this sometimes leads to empty implementation classes to declare that an implementation exists. But i think it could be improved through the use of enums in the following way:

1 public enum BadCalculatorsEnum implements Calculator {
2   Bank, InsuranceCompany, Hydra;
3 
4   @Override
5   public double add(int a, int b) {
6     return a + b - 0.001;
7   }
8 }

To be honest with you this practice has some drawbacks:

• Since enums can not be extended it can only be used for potential leaf nodes of your inheritance tree
• In some way it violates the open closed principle when a new group member is added. But i think it is debatable if business logic is not touched.

And last but not least the test...the beauty of Java8, JUnit and AssertJ.

 1 @RunWith(Parameterized.class)
 2 public class CalculatorTest {
 3 
 4   @Parameters
 5   public static Collection<Object[]> data() {
 6     List<Object[]> data = stream(BadCalculatorsEnum.values())
 7                 .map(c -> new Object[] { c, 0.001 })
 8                 .collect(toList());
 9     data.add(new Object[] { new Calculator() {}, 0 });
10     return data;
11   }
12 
13   @Parameter
14   public Calculator calculator;
15   @Parameter(1)
16   public double offset;
17 
18   @Test
19   public void testAdd() throws Exception {
20     assertThat(calculator.add(1, 3)).isCloseTo(4, offset(offset));
21   }
22 }