There is an easy, but boilerplate heavy way to seal classes in Java. You put a private constructor in the base class then make subclasses inner classes of it.
public abstract class List<A> { // private constructor is uncallable by any sublclasses except inner classes private List() { } public static final class Nil<A> extends List<A> { } public static final class Cons<A> extends List<A> { public final A head; public final List<A> tail; public Cons(A head, List<A> tail) { this.head = head; this.tail = tail; } } } Tack on a visitor pattern for dispatch.
My project jADT: Java Algebraic DataTypes generates all that boilerplate for you.
