Is there a way to implement algebraic types in Java?

Question

Is it possible, in Java, to enforce that a class have a specific set of subclasses and no others? For example:

public abstract class A {}
public final class B extends A {}
public final class C extends A {}
public final class D extends A {}

Can I somehow enforce that no other subclasses of A can ever be created?

Answer 1

Church encoding to the rescue:

public abstract class A {
  public abstract <R> R fold(R b, R c, R d);
}

There are only three implementations possible:

public final class B extends A {
  public <R> R fold(R b, R c, R d) {
    return b;
  }
}

public final class C extends A {
  public <R> R fold(R b, R c, R d) {
    return c;
  }
}

public final class D extends A {
  public <R> R fold(R b, R c, R d) {
    return d;
  }
}

Answer 2

Give class A a constructor with package-level accessibility (and no other constructors).

^{Thanks, Dave L., for the bit about no other constructors.}

Answer 3

You probably want an enum (Java >= 1.5). An enum type can have a set of fixed values. And it has all the goodies of a class: they can have fields and properties, and can make them implement an interface. An enum cannot be extended.

Example:

enum A {

  B,
  C,
  D;

  public int someField;

  public void someMethod() {
  }


}

Answer 4

You could put class A,B,C,D in a seperate package and make class A not public.