written based on Java 8 Optional to support Android API Level lower than 24

Optional.java

public class Optional<M> {

  /**
   * Common instance for {@code empty()}.
   */
  private static final Optional<?> EMPTY = new Optional<>(null);

  private final M value;

  public Optional(@Nullable M value) {
    this.value = value;
  }

  /**
   * Returns an {@code Optional} with the specified present non-null value.
   *
   * @param <T> the class of the value
   * @param value the value to be present, which must be non-null
   * @return an {@code Optional} with the value present
   * @throws NullPointerException if value is null
   */
  public static <T> Optional<T> of(T value) {
    return new Optional<>(value);
  }

  /**
   * Returns an empty {@code Optional} instance.  No value is present for this
   * Optional.
   *
   * @apiNote Though it may be tempting to do so, avoid testing if an object
   * is empty by comparing with {@code ==} against instances returned by
   * {@code Option.empty()}. There is no guarantee that it is a singleton.
   * Instead, use {@link #isPresent()}.
   *
   * @param <T> Type of the non-existent value
   * @return an empty {@code Optional}
   */
  public static<T> Optional<T> empty() {
    @SuppressWarnings("unchecked") Optional<T> t = (Optional<T>) EMPTY;
    return t;
  }

  /**
   * Return {@code true} if there is a value present, otherwise {@code false}.
   *
   * @return {@code true} if there is a value present, otherwise {@code false}
   */
  public boolean isPresent() {
    return this.value != null;
  }


  /**
   * Return {@code true} if the value is empty, the value may not be null but empty
   *
   * @return {@code true} if the value is empty, the value may not be null but empty
   */
  public boolean isEmpty() {
    return this.value == null ||
        (value instanceof Collection && ((Collection) value).isEmpty()) ||
        (value instanceof Map && ((Map) value).isEmpty()) ||
        (value instanceof Set && ((Set) value).isEmpty());
  }

  public @NonNull M get() {
    if (value == null) {
      throw new NoSuchElementException("No value present");
    }
    return value;
  }

  public @Nullable M getNullable() {
    return value;
  }

}