Provide annotation on Enum constant that is used on JSON mapping

[nbartels]

Our JSON mapper has support for enums as types in our objects, but we only support the constant itself or an upper case version. This means if Facebook uses the enum value test the enum constant may be test or TEST.

This is ok so far, but we are not that flexible in naming our enum constants. To improve this, I suggest using an annotation on the constants and use the provided value in the annotation for the mapping in both directions.

GSON uses some similar approach:

enum Flavor {
    @SerializedName("van")
    VANILLA,
    @SerializedName("choc")
    CHOCOLATE
}

To access this value, there is this method:

String getSerializedName(Enum<?> e) {
    try {
        Field f = e.getClass().getField(e.name());
        SerializedName a = f.getAnnotation(SerializedName.class);
        return a == null ? null : a.value();
    } catch (NoSuchFieldException ignored) {
        return null;
    }
}

[nbartels]

The enum constants can be annotated with our Facebook annotation and can have a different value.

For example @Facebook("test") FOO will convert the incoming String test to the FOO enum, if the enum is used in a type and the other way around. The FOO constant will be converted to the String "test" on converting the object to JSON.

Important: It is not necessary to set the annotation to allow the usage of the constant. The former default behavior is still active.