A Guide to Java Enums

1. Overview

In this tutorial, we’ll learn what Java enums are, what problems they solve, and how some of their design patterns can be used in practice.

Java 5 first introduced the enum keyword. It denotes a special type of class that always extends the java.lang.Enum class. For the official documentation on usage, we can head over to the documentation.

Constants defined this way make the code more readable, allow for compile-time checking, document the list of accepted values upfront, and avoid unexpected behavior due to invalid values being passed in.

Here’s a quick and simple example of an enum that defines the status of a pizza order; the order status can be ORDERED, READY or DELIVERED:

public enum PizzaStatus {
    ORDERED,
    READY, 
    DELIVERED; 
}

Additionally, enums come with many useful methods that we would otherwise need to write if we were using traditional public static final constants.

2. Custom Enum Methods

Now that we have a basic understanding of what enums are and how we can use them, we’ll take our previous example to the next level by defining some extra API methods on the enum:

public class Pizza {
    private PizzaStatus status;
    public enum PizzaStatus {
        ORDERED,
        READY,
        DELIVERED;
    }

    public boolean isDeliverable() {
        if (getStatus() == PizzaStatus.READY) {
            return true;
        }
        return false;
    }
    
    // Methods that set and get the status variable.
}

3. Comparing Enum Types Using “==” Operator

Since enum types ensure that only one instance of the constants exist in the JVM, we can safely use the “==” operator to compare two variables, like we did in the above example. Furthermore, the “==” operator provides compile-time and run-time safety.

First, we’ll look at run-time safety in the following snippet, where we’ll use the “==” operator to compare statuses. Either value can be null and we won’t get a NullPointerException. Conversely, if we use the equals method, we will get a NullPointerException:

if(testPz.getStatus().equals(Pizza.PizzaStatus.DELIVERED)); 
if(testPz.getStatus() == Pizza.PizzaStatus.DELIVERED); 

As for compile-time safety, let’s look at an example where we’ll determine that an enum of a different type is equal by comparing it using the equals method. This is because the values of the enum and the getStatus method coincidentally are the same; however, logically the comparison should be false. We avoid this issue by using the “==” operator.

The compiler will flag the comparison as an incompatibility error:

if(testPz.getStatus().equals(TestColor.GREEN));
if(testPz.getStatus() == TestColor.GREEN);

4. Using Enum Types in Switch Statements

We can use enum types in switch statements also:

public int getDeliveryTimeInDays() {
    switch (status) {
        case ORDERED: return 5;
        case READY: return 2;
        case DELIVERED: return 0;
    }
    return 0;
}

5. Fields, Methods and Constructors in Enums

We can define constructors, methods, and fields inside enum types, which makes them very powerful.

Next, let’s extend the example above by implementing the transition from one stage of a pizza order to another. We’ll see how we can get rid of the if and switch statements used before:

public class Pizza {

    private PizzaStatus status;
    public enum PizzaStatus {
        ORDERED (5){
            @Override
            public boolean isOrdered() {
                return true;
            }
        },
        READY (2){
            @Override
            public boolean isReady() {
                return true;
            }
        },
        DELIVERED (0){
            @Override
            public boolean isDelivered() {
                return true;
            }
        };

        private int timeToDelivery;

        public boolean isOrdered() {return false;}

        public boolean isReady() {return false;}

        public boolean isDelivered(){return false;}

        public int getTimeToDelivery() {
            return timeToDelivery;
        }

        PizzaStatus (int timeToDelivery) {
            this.timeToDelivery = timeToDelivery;
        }
    }

    public boolean isDeliverable() {
        return this.status.isReady();
    }

    public void printTimeToDeliver() {
        System.out.println("Time to delivery is " + 
          this.getStatus().getTimeToDelivery());
    }
    
    // Methods that set and get the status variable.
}

The test snippet below demonstrates how this works:

@Test
public void givenPizaOrder_whenReady_thenDeliverable() {
    Pizza testPz = new Pizza();
    testPz.setStatus(Pizza.PizzaStatus.READY);
    assertTrue(testPz.isDeliverable());
}

6. EnumSet and EnumMap

6.1. EnumSet

The EnumSet is a specialized Set implementation that’s meant to be used with Enum types.

Compared to a HashSet, it’s a very efficient and compact representation of a particular Set of Enum constants, owing to the internal Bit Vector Representation that’s used. It also provides a type-safe alternative to traditional int-based “bit flags,” allowing us to write concise code that’s more readable and maintainable.

The EnumSet is an abstract class that has two implementations, RegularEnumSet and JumboEnumSet, one of which is chosen depending on the number of constants in the enum at the time of instantiation.

Therefore, it’s a good idea to use this set whenever we want to work with a collection of enum constants in most scenarios (like subsetting, adding, removing, and bulk operations like containsAll and removeAll), and use Enum.values() if we just want to iterate over all possible constants.

In the code snippet below, we can see how to use EnumSet to create a subset of constants:

public class Pizza {

    private static EnumSet<PizzaStatus> undeliveredPizzaStatuses =
      EnumSet.of(PizzaStatus.ORDERED, PizzaStatus.READY);

    private PizzaStatus status;

    public enum PizzaStatus {
        ...
    }

    public boolean isDeliverable() {
        return this.status.isReady();
    }

    public void printTimeToDeliver() {
        System.out.println("Time to delivery is " + 
          this.getStatus().getTimeToDelivery() + " days");
    }

    public static List<Pizza> getAllUndeliveredPizzas(List<Pizza> input) {
        return input.stream().filter(
          (s) -> undeliveredPizzaStatuses.contains(s.getStatus()))
            .collect(Collectors.toList());
    }

    public void deliver() { 
        if (isDeliverable()) { 
            PizzaDeliverySystemConfiguration.getInstance().getDeliveryStrategy()
              .deliver(this); 
            this.setStatus(PizzaStatus.DELIVERED); 
        } 
    }
    
    // Methods that set and get the status variable.
}

Executing the following test demonstrates the power of the EnumSet implementation of the Set interface:

@Test
public void givenPizaOrders_whenRetrievingUnDeliveredPzs_thenCorrectlyRetrieved() {
    List<Pizza> pzList = new ArrayList<>();
    Pizza pz1 = new Pizza();
    pz1.setStatus(Pizza.PizzaStatus.DELIVERED);

    Pizza pz2 = new Pizza();
    pz2.setStatus(Pizza.PizzaStatus.ORDERED);

    Pizza pz3 = new Pizza();
    pz3.setStatus(Pizza.PizzaStatus.ORDERED);

    Pizza pz4 = new Pizza();
    pz4.setStatus(Pizza.PizzaStatus.READY);

    pzList.add(pz1);
    pzList.add(pz2);
    pzList.add(pz3);
    pzList.add(pz4);

    List<Pizza> undeliveredPzs = Pizza.getAllUndeliveredPizzas(pzList); 
    assertTrue(undeliveredPzs.size() == 3); 
}

6.2. EnumMap

EnumMap is a specialized Map implementation meant to be used with enum constants as keys. Compared to its counterpart HashMap, it’s an efficient and compact implementation that’s internally represented as an array:

EnumMap<Pizza.PizzaStatus, Pizza> map;

Let’s look at an example of how we can use it in practice:

public static EnumMap<PizzaStatus, List<Pizza>> 
  groupPizzaByStatus(List<Pizza> pizzaList) {
    EnumMap<PizzaStatus, List<Pizza>> pzByStatus = 
      new EnumMap<PizzaStatus, List<Pizza>>(PizzaStatus.class);
    
    for (Pizza pz : pizzaList) {
        PizzaStatus status = pz.getStatus();
        if (pzByStatus.containsKey(status)) {
            pzByStatus.get(status).add(pz);
        } else {
            List<Pizza> newPzList = new ArrayList<Pizza>();
            newPzList.add(pz);
            pzByStatus.put(status, newPzList);
        }
    }
    return pzByStatus;
}

Executing the following test demonstrates the power of the EnumMap implementation of the Map interface:

@Test
public void givenPizaOrders_whenGroupByStatusCalled_thenCorrectlyGrouped() {
    List<Pizza> pzList = new ArrayList<>();
    Pizza pz1 = new Pizza();
    pz1.setStatus(Pizza.PizzaStatus.DELIVERED);

    Pizza pz2 = new Pizza();
    pz2.setStatus(Pizza.PizzaStatus.ORDERED);

    Pizza pz3 = new Pizza();
    pz3.setStatus(Pizza.PizzaStatus.ORDERED);

    Pizza pz4 = new Pizza();
    pz4.setStatus(Pizza.PizzaStatus.READY);

    pzList.add(pz1);
    pzList.add(pz2);
    pzList.add(pz3);
    pzList.add(pz4);

    EnumMap<Pizza.PizzaStatus,List<Pizza>> map = Pizza.groupPizzaByStatus(pzList);
    assertTrue(map.get(Pizza.PizzaStatus.DELIVERED).size() == 1);
    assertTrue(map.get(Pizza.PizzaStatus.ORDERED).size() == 2);
    assertTrue(map.get(Pizza.PizzaStatus.READY).size() == 1);
}

7. Implement Design Patterns Using Enums

7.1. Singleton Pattern

Normally, implementing a class using the Singleton pattern is quite non-trivial. Enums provide a quick and easy way of implementing singletons.

In addition, since the enum class implements the Serializable interface under the hood, the class is guaranteed to be a singleton by the JVM. This is unlike the conventional implementation, where we have to ensure that no new instances are created during deserialization.

In the code snippet below, we see how we can implement a singleton pattern:

public enum PizzaDeliverySystemConfiguration {
    INSTANCE;
    PizzaDeliverySystemConfiguration() {
        // Initialization configuration which involves
        // overriding defaults like delivery strategy
    }

    private PizzaDeliveryStrategy deliveryStrategy = PizzaDeliveryStrategy.NORMAL;

    public static PizzaDeliverySystemConfiguration getInstance() {
        return INSTANCE;
    }

    public PizzaDeliveryStrategy getDeliveryStrategy() {
        return deliveryStrategy;
    }
}

7.2. Strategy Pattern

Conventionally, the Strategy pattern is written by having an interface that is implemented by different classes.

Adding a new strategy means adding a new implementation class. With enums, we can achieve this with less effort, and adding a new implementation means simply defining another instance with some implementation.

The code snippet below shows how to implement the Strategy pattern:

public enum PizzaDeliveryStrategy {
    EXPRESS {
        @Override
        public void deliver(Pizza pz) {
            System.out.println("Pizza will be delivered in express mode");
        }
    },
    NORMAL {
        @Override
        public void deliver(Pizza pz) {
            System.out.println("Pizza will be delivered in normal mode");
        }
    };

    public abstract void deliver(Pizza pz);
}

Then we add the following method to the Pizza class:

public void deliver() {
    if (isDeliverable()) {
        PizzaDeliverySystemConfiguration.getInstance().getDeliveryStrategy()
          .deliver(this);
        this.setStatus(PizzaStatus.DELIVERED);
    }
}

@Test
public void givenPizaOrder_whenDelivered_thenPizzaGetsDeliveredAndStatusChanges() {
    Pizza pz = new Pizza();
    pz.setStatus(Pizza.PizzaStatus.READY);
    pz.deliver();
    assertTrue(pz.getStatus() == Pizza.PizzaStatus.DELIVERED);
}

8. Java 8 and Enums

We can rewrite the Pizza class in Java 8, and see how the methods getAllUndeliveredPizzas() and groupPizzaByStatus() become so concise with the use of lambdas and the Stream APIs:

public static List<Pizza> getAllUndeliveredPizzas(List<Pizza> input) {
    return input.stream().filter(
      (s) -> !deliveredPizzaStatuses.contains(s.getStatus()))
        .collect(Collectors.toList());
}

public static EnumMap<PizzaStatus, List<Pizza>> 
  groupPizzaByStatus(List<Pizza> pzList) {
    EnumMap<PizzaStatus, List<Pizza>> map = pzList.stream().collect(
      Collectors.groupingBy(Pizza::getStatus,
      () -> new EnumMap<>(PizzaStatus.class), Collectors.toList()));
    return map;
}

9. JSON Representation of Enum

Using Jackson libraries, it’s possible to have a JSON representation of enum types as if they’re POJOs. In the code snippet below, we’ll see how we can use the Jackson annotations for the same:

@JsonFormat(shape = JsonFormat.Shape.OBJECT)
public enum PizzaStatus {
    ORDERED (5){
        @Override
        public boolean isOrdered() {
            return true;
        }
    },
    READY (2){
        @Override
        public boolean isReady() {
            return true;
        }
    },
    DELIVERED (0){
        @Override
        public boolean isDelivered() {
            return true;
        }
    };

    private int timeToDelivery;

    public boolean isOrdered() {return false;}

    public boolean isReady() {return false;}

    public boolean isDelivered(){return false;}

    @JsonProperty("timeToDelivery")
    public int getTimeToDelivery() {
        return timeToDelivery;
    }

    private PizzaStatus (int timeToDelivery) {
        this.timeToDelivery = timeToDelivery;
    }
}

We can use the Pizza and PizzaStatus as follows:

Pizza pz = new Pizza();
pz.setStatus(Pizza.PizzaStatus.READY);
System.out.println(Pizza.getJsonString(pz));

This will generate the following JSON representation of the Pizzas status:

{
  "status" : {
    "timeToDelivery" : 2,
    "ready" : true,
    "ordered" : false,
    "delivered" : false
  },
  "deliverable" : true
}

For more information on JSON serializing/deserializing (including customization) of enum types, we can refer to the Jackson – Serialize Enums as JSON Objects.

10. Conclusion

In this article, we explored the Java enum, from the language basics to more advanced and interesting real-world use cases.

Code snippets from this article can be found in the Github repository.