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Enumerables

Enumerables are similar to standard Java Enum types with the added ability to parse 'yet unknown' values.
They are invaluable if you attempt to build a stable API.

Getting the library

Add the following dependency to your project or download it from maven central.

<dependency>
    <groupId>nl.talsmasoftware.enumerables</groupId>
    <artifactId>enumerables</artifactId>
    <version>[see maven-central badge]</version>
</dependency>

Example

A working example of an Enumerable type:

public final class CarBrand extends Enumerable {
    public static final CarBrand ASTON_MARTIN = new CarBrand("Aston martin");
    public static final CarBrand JAGUAR = new CarBrand("Jaguar");
    public static final CarBrand TESLA = new CarBrand("Tesla");
    // We all know there are more CarBrands than the ones we identified here... 
    // Not a good fit for a java.lang.Enum, but suitable for Enumerable.

    private CarBrand(String value) { super(value); }
}

We can interact with CarBrand similar to Enum:

  • CarBrand[] knownCarBrands = Enumerable.values(CarBrand.class);
  • assert CarBrand.ASTON_MARTIN == Enumerable.valueOf(CarBrand.class, "ASTON_MARTIN");
  • assert CarBrand.JAGUAR.ordinal() == 1;
  • assert CarBrand.TESLA.name().equals("TESLA");

But you can also:

  • assert CarBrand.ASTON_MARTIN.getValue().equals("Aston martin");
  • Parse another 'unknown' car brand: CarBrand porsche = Enumerable.parse(CarBrand.class, "Porsche");
  • Compare all CarBrand instances with each-other.
    Constants sort in declaration order before 'unknown' values, which in turn sort alphabetically, case-insensitive.
  • Consistent with the sorting order, ordinal() of non-constants are by definition:
    assert porsche.ordinal() == Integer.MAX_VALUE;
  • As they aren't defined by a constant, name() of non-constants always returns null:
    assert porsche.name() == null;

Parsing and printing

public static <E extends Enumerable> E parse(Class<E> type, CharSequence value);

The parse method first compares the given value with the values of all known constants for the specified Enumerable type. This results in a constant reference in most cases. A new object instance is only created using the String constructor for non-constant values.

The counterpart of parsing, printing is also covered which simply returns the value:

public static String print(Enumerable enumerable);

Enum-like behaviour

Enumerable.valueOf()

public static <E extends Enumerable> E valueOf(Class<E> type, CharSequence name)
        throws ConstantNotFoundException;

This method is comparable with the Enum.valueOf(Class, String) method.

This method returns the constant with the specified name. If there is no constant within the requested type found by that name, the method will throw a ConstantNotFoundException.

Please note that this method looks at the constant name and not the String value of this enumerable object. To obtain an enumerable object from a specific value, please use the parse(Class, CharSequence) method instead. That method will not throw any exceptions for yet-unknown values, but returns a new enumerable instance containing the value instead.

Enumerable.values()

public static <E extends Enumerable> E[] values(Class<E> enumerableType);

This method is comparable with the Enum.values() method.

It finds all public constants of the requested enumerableType that have been declared in the class of the enumerable itself.

These constants must be public static final fields of the own Enumerable type.

Enumerable.ordinal()

public int ordinal();

Returns the 'enum ordinal' for an enumerable value.

For a constant value, this method returns the index of the constant within the Enumerable.values() array.

For non-constant values, this method will always return Integer.MAX_VALUE. There are various reasons for this choice, but the most obvious one is that the #compareTo(Enumerable) implementation is greatly influenced by this ordinal value, automatically sorting all constants before any non-constant parsed values.

Enumerable.name()

public String name();

Returns the 'enum constant name' for an enumerable value.

For a constant value, this method returns the name of the defined constant as defined in the code (not its value).

For non-constant values, this method will always return null.

Serialization / deserialization

Serialization: The Enumerable implements Serializable. This means that a concrete subclass is serializable if it does not contain any non-serializable and non-transient fields. Note that additional class fields should be either specified in the constant declaration or be deducable from the String constructor if they carry meaning after deserialization.

Deserialization: Similar to parsing, a deserialized Enumerable object resolves back to a listed constant reference if its value matches the constant.
Only unanticipated values will result in new objects.

Add-on modules

Validation

The enumerables-validation module provides several annotations as javax.validation constraints.

JAX-RS

The enumerables-jaxrs module provides an enumerable parameter converter provider for JAX-RS.

JSON serialization

The enumerables-jackson2 and enumerables-gson modules provide serialization and deserialization functionality to and from json.
Jackson also supports other common formats such as yaml.

Swagger documentation

The enumerables-swagger module provides Swagger API model documentation for Enumerable types, including examples.

Background

The Enumerable superclass is very similar to a standard Java Enum type but allows representing currently unknown values as well.

Why represent 'currently unknown' values?

Have you ever had to support an API that returns an actual Enum value?
Can you remember what happened when the product owner decided that an additional value was needed for something?
*Bang!* there goes your API compatibility: You will either have to tell all your existing customers "sorry, the api is now broken" or create a new version beside the existing API and declare the old one deprecated.
Even with a new API version, you still have to think about handling the new value in the old version or create a special exception for this case.
This is nasty in any conceivable scenario!

Change the Enum to a String, problem solved?

You still want to document all those constant values that you do know, right?
So now there's documentation or best-case a list of constants informing users which values are special. You also lose all other Enum advantages.

Conclusion

That is exactly the reason we've created the Enumerable type. It is similar to Enum in that it represents a number of known constants that can be validated, iterated over and used as constants from code. By providing a way to parse yet-unknown additional values it allows API designers to make a slightly lighter promise to users:
I currently know of these possible values which have meaning in the API, but please be prepared to receive any 'other' value as well (handle as you feel fit).
Introducing a new value in a new API release does not break the existing contract since the consumer should have already anticipated unknown values.

License

Apache 2.0 license