Optional.java

Optional.java

public class Optional<T> {
  private final boolean isSet;
  private final T value;

  public static <T> Optional<T> nothing() {
    return new Optional<T>(false, null);
  }

  public static <T> Optional<T> value(T value) {
    return new Optional<T>(true, value);
  }

  private Optional(boolean isSet, T value) {
    this.isSet = isSet;
    this.value = value;
  }

  public boolean isSet() { return isSet; }
  public T getValue() { return value; }

  public String toString() {
      if (!isSet) {
	  return "#<nothing>";
      } else {
	  return "#<value: " + String.valueOf(value) + ">";
      }
  }

  public static void main(String[] args) {
      System.out.println("Nothing:       " + Optional.nothing());
      System.out.println("Explicit int:  " + Optional.value(42));
      System.out.println("Explicit null: " + Optional.value(null));
  }
}