Discriminated union sort of thing in Java

Measurement.java

import static java.lang.System.out;
import java.util.function.IntUnaryOperator;

/**
 * See https://softwareengineering.stackexchange.com/questions/376845/how-should-i-store-unknown-and-missing-values-in-a-variable-while-still-ret
 */
public abstract class Measurement {

  /**
   * Prints:
   *
   * <pre>
   *   Value(4)
   *   Empty()
   *   Unknown()
   * </pre>
   */
  public static void main(String[] args) {
    var value = Measurement.of(2);
    out.println(value.map(x -> x * 2));
    
    var empty = Measurement.empty();
    out.println(empty.map(x -> x * 2));

    var unknown = Measurement.unknown();
    out.println(unknown.map(x -> x * 2));
  }

  public abstract Measurement map(IntUnaryOperator f);

  public boolean hasValue() {
    return false;
  }

  public boolean isEmpty() {
    return false;
  }

  public boolean isUnknown() {
    return false;
  }

  /**
   * A measured non-empty value.
   */
  public static Value of(int value) {
    return new Value(value);
  }

  /**
   * A measured empty value.
   */
  public static Empty empty() {
    return Empty.INSTANCE;
  }

  /**
   * A measurement that has not been taken.
   */
  public static Unknown unknown() {
    return Unknown.INSTANCE;
  }

  /**
   * A measured non-empty value.
   */
  public static class Value extends Measurement {
    private final int value;

    private Value(int value) {
      this.value = value;
    }

    @Override
    public Measurement map(IntUnaryOperator f) {
      return new Value(f.applyAsInt(value));
    }

    @Override
    public boolean hasValue() {
      return true;
    }

    @Override
    public String toString() {
      return "Value(" + value + ")";
    }
  }

  /**
   * A measured empty value.
   */
  public static class Empty extends Measurement {
    private static final Empty INSTANCE = new Empty();

    private Empty() {
      // not to be instantiated directly
    }

    @Override
    public Empty map(IntUnaryOperator f) {
      return this;
    }

    @Override
    public boolean isEmpty() {
      return true;
    }

    @Override
    public String toString() {
      return "Empty()";
    }
  }

  /**
   * A measurement that has not been taken.
   */
  public static class Unknown extends Measurement {
    private static final Unknown INSTANCE = new Unknown();

    private Unknown() {
      // not to be instantiated directly
    }

    @Override
    public Unknown map(IntUnaryOperator f) {
      return this;
    }

    @Override
    public boolean isUnknown() {
      return true;
    }

    @Override
    public String toString() {
      return "Unknown()";
    }
  }
}