java-enum-conversion

Java enum conversion micro-framework demonstration. Focus is on the way how to map enum instances to values and how to do it consistently. Examples are all based on JPA 2.1 converters.

Full length blog story is here: http://virgo47.wordpress.com/2014/08/02/converting-java-enums-to-values-and-back/

Follow-up for Java 8 solution is here: http://virgo47.wordpress.com/2014/08/20/converting-java-enums-to-values-and-back-with-java-8/

Naive approach

Package _0naive. Enum to Integer mapping is in JPA AttributeConverter and it's based on switch/case. Fragile. We need to encapsulate it into enum type.

Encapsulated

Package _1encapsulated solves this. This is the most important step - to get rid of switch. And especially if it's out of the enum, that is recipe for disaster (as it's mostly not just on one place).

Introducing "framework"

Package _2framework tries to bring some unity into conversion. Enums implement ConvertedEnum and there is base class EnumAttributeConverter for AttributeConverters.

Refinement

Package _3refined introduces ConvertedEnumResolver which makes static reverse resolving to enum instances a breeze.

Let's not go overboard

Package _4toomuch tries to push even more to base converter class. There is demonstration main in SomeEntityTypeConverter that shows what work and what doesn't. But considering we didn't save a single line of code and lost compile time safety for static fromDbValue method (no way to enforce the same name on all enums) I'd not go this far.

More flexible Java 8 solution

Package _5java8function utilizes Java 8 method references. Previously our ConvertedEnumResolver was dependent on the existence of ConvertedEnum and used its method. With Java 8, you can provide reference to the method returning value from enum instance directly to the reverse resolver. That is why this time we don't use "converted" in the name, but call it ReverseEnumResolver.

This reverse resolution is completely separated from the JPA conversion framework now! :-) That's why I didn't bother with JPA surroundings in this package. In case of JPA conversion you still have to implement ConvertedEnum if you want to use subclasses of EnumAttributeConverter. Alternatively you may use solution from package _1encapsulated - that is no conversion framework and just benefit from "componentized" reverse resolving.

With this resolving you can have more (in our case two) different other representations, which was not possible before with a single class ConvertedEnumResolver.

Enjoy