/**
 * A container object which may or may not contain a non-{@code null} value.
 * If a value is present, {@code isPresent()} returns {@code true}. If no
 * value is present, the object is considered <i>empty</i> and
 * {@code isPresent()} returns {@code false}.
 *
 * <p>Additional methods that depend on the presence or absence of a contained
 * value are provided, such as {@link #orElse(Object) orElse()}
 * (returns a default value if no value is present) and
 * {@link #ifPresent(Consumer) ifPresent()} (performs an
 * action if a value is present).
 *
 * <p>This is a <a href="../lang/doc-files/ValueBased.html">value-based</a>
 * class; use of identity-sensitive operations (including reference equality
 * ({@code ==}), identity hash code, or synchronization) on instances of
 * {@code Optional} may have unpredictable results and should be avoided.
 *
 * @apiNote
 * {@code Optional} is primarily intended for use as a method return type where
 * there is a clear need to represent "no result," and where using {@code null}
 * is likely to cause errors. A variable whose type is {@code Optional} should
 * never itself be {@code null}; it should always point to an {@code Optional}
 * instance.
 *
 * @param <T> the type of value
 * @since 1.8
 */