closed-adt.java

abstract class IntOrString {
  private IntOrString() {}

  public static class IsInt extends IntOrString {
    private final int i;

    public IsInt(int i) {
      this.i = i;
    }

    int get() {
      return i;
    }
  }

  public static class IsString extends IntOrString {
    private final String s;

    public IsString(String s) {
      this.s = s;
    }

    String get() {
      return s;
    }
  }

  // todo, library support, knowing that this invariant holds:
  // given IntOrString x, then (x instanceof IsInt) || (x instanceof IsString)
}