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sealed-enum

Java 20 library to use sealed classes as if they were enums! Enums are great but they can't handle generic arguments. This is where sealed classes come in.

Example usage

public sealed MySealedEnum extends SealedEnum<T>{
   // this can only be called once!
   public static MySealedEnum INSTANCE = new MySealedEnum();
   private static final class EnumOptionA extends MySealedEnum{}
   private static final class EnumOptionB extends MySealedEnum{}
   private static final class EnumOptionC extends MySealedEnum{}
   
   {
      System.out.println(INSTANCE.values()); // EnumOptionA, EnumOptionB, EnumOptionC. 
      MySealedEnum enumOption = SealedEnum.getSealedEnum(EnumOptionA.class);
      switch(enumOption) {
         case EnumOptionA -> ...
         case EnumOptionB -> ...
         default -> ...
      }
   }
}

Please see SealedEnumExample for a decent example how it can be used with generics!

todo / other thoughts

possible naughty usage

public sealed MySealedEnum extends SealedEnum<T>{
    protected MySealedEnum() {
        super(MySealedEnum.class)
        SealedEnum badInstance = new SealedEnum(MySealedEnum.class);
        SealedEnum anotherBadInstance = new MySealedEnum();
            // attempting to circumvent security checks :(
    }

    public static final Enum1 extends MySealedEnum{
        public Enum1(otherArgs) {
            super(otherArgs);
        }
        public Enum1() {
            this(someStaticFunction());
            Enum1 badInstance = new Enum1();
        }
    }
}

What we do: have a set of the classes we have "seen" by calling getClass() in SealedEnum's constructor. If there is already a class we have seen, this implies we are attempting to create the same object for the class twice. We throw an exception as a result.

todo

  • todo: could we just have SealedEnum myEnum = SealedEnum.of(MySealedEnum.class);
    • I like this better. We can also return the same object using this method.
    • This would also not have an instantiation of the base class like we currently do.
    • What would this API look like?
    • interface EnumDescriptor<T extends SealedEnum> -- interface common to enum values and base type
      • + values() : List<T>
      • + ordinal() : int
      • + getDelaringClass(): Class<? super T>
    • SealedEnumType<T extends SealedEnum> implements EnumDescriptor<T> -- handle used to describe base class
    • MyEnumVal extends MyEnum -- user implementation of an enum's val
    • MyEnum extends SealedEnum -- user implementation of an enum
    • SealedEnum implements EnumDescriptor
    • SealedEnumType::values() : MyEnum[] -- returns values possible
    • SealedEnumType::ordinal() : int -- returns -1
    • SealedEnumType::getDeclaringClass() -- returns (this) base class
    • SealedEnum::ordinal() : int
    • SealedEnum::getDeclaringClass() : MyEnum.class
    • SealedEnum.getInstance(MyEnumVal.class) : MyEnumVal (extends SealedEnum)
    • SealedEnum.getSealedEnum(MyEnum.class) : SealedEnumType<MyEnum>
    • SealedEnum.values(MyEnum.class | MyEnumVal.class) : MyEnum[]
    • SealedEnum.ordinal(MyEnum.class | MyEnumVal.class) : int
    • SealedEnum.declaringClass(MyEnumVal.class | MyEnum.class) : MyEnum.class