為什么說(shuō)JDK 8 Optional<T>的實(shí)現(xiàn)挺不合理的？因?yàn)?code>Optional<T>值容器對(duì)“包含的值是否存在”是基于value != null或value == null判斷的末盔，而非基于isPresent()方法筑舅。這樣實(shí)現(xiàn)的后果是很難做自定義擴(kuò)展，如果要做擴(kuò)展需要重新實(shí)現(xiàn)大部分接口陨舱。其基本數(shù)據(jù)類型的實(shí)現(xiàn)類OptionalInt翠拣，也是基于isPresent字段做判斷條件，藍(lán)瘦香菇游盲。maybe他們就不想讓我們做擴(kuò)展误墓，不然真想不出什么理由。public final class Optional<T>益缎，final類??????谜慌。

擴(kuò)展性設(shè)計(jì)貫穿在集合框架的抽象類實(shí)現(xiàn)中，核心集合接口的基礎(chǔ)實(shí)現(xiàn)是全部基于抽象方法實(shí)現(xiàn)的莺奔。例如欣范，AbstractCollection<E>、AbstractList<E>令哟。

為什么會(huì)聊這個(gè)話題恼琼？因?yàn)槲覀冊(cè)趯?shí)際編程中都會(huì)通過(guò)HTTP請(qǐng)求調(diào)用他人的API，其成功/失敗的返回一般都是基于返回碼(code)來(lái)判斷的励饵，這時(shí)想基于Optional<T>做擴(kuò)展驳癌，發(fā)現(xiàn)幾乎要重新實(shí)現(xiàn)所有方法，很杯具??役听。

下面是在JDK原有實(shí)現(xiàn)上颓鲜，基于擴(kuò)展性考慮修改的代碼。如有哪里不合理的地方甜滨，感謝指出來(lái)。

// [函數(shù)式接口] 可能包含null值的容器對(duì)象
public final class Optional<T> {
    /**
     * Common instance for {@code empty()}.
     * 空實(shí)例({@linkplain #empty()})
     */
    private static final Optional<?> EMPTY = new Optional<>();

    /**
     * If non-null, the value; if null, indicates no value is present
     * null表示值不存在
     */
    private final T value;

    /**
     * Constructs an empty instance.
     *
     * @implNote Generally only one empty instance, {@link Optional#EMPTY},
     * should exist per VM.
     */
    private Optional() {
        this.value = null;
    }

    /**
     * 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}
     */
    // 核心方法 返回空實(shí)例
    public static<T> Optional<T> empty() {
        @SuppressWarnings("unchecked")
        Optional<T> t = (Optional<T>) EMPTY;
        return t;
    }

    /**
     * Constructs an instance with the value present.
     *
     * @param value the non-null value to be present 存在的非null值
     * @throws NullPointerException if value is null
     */
    private Optional(T value) {
        this.value = Objects.requireNonNull(value); // 必須是非null值
    }

    /**
     * Returns an {@code Optional} with the specified present non-null value.
     *
     * @param <T> the class of the value 值的類型
     * @param value the value to be present, which must be non-null 存在的值瘤袖，必須不是非null
     * @return an {@code Optional} with the value present
     * @throws NullPointerException if value is null 如果值是null艾扮，則拋出NPE
     */
    // 核心方法 返回一個(gè)指定存在的非空值的可選實(shí)例
    public static <T> Optional<T> of(T value) {
        return new Optional<>(value);
    }

    /**
     * Returns an {@code Optional} describing the specified value, if non-null,
     * otherwise returns an empty {@code Optional}.
     *
     * @param <T> the class of the value 值的類型
     * @param value the possibly-null value to describe 描述可為null的值
     * @return an {@code Optional} with a present value if the specified value
     * is non-null, otherwise an empty {@code Optional}
     */
    // 核心方法 如果值非空占婉，返回一個(gè)描述指定值的可選實(shí)例泡嘴；否則，返回一個(gè)空實(shí)例
    public static <T> Optional<T> ofNullable(T value) {
        return value == null ? empty() : of(value);
    }

    /**
     * If a value is present in this {@code Optional}, returns the value,
     * otherwise throws {@code NoSuchElementException}.
     *
     * @return the non-null value held by this {@code Optional}
     * @throws NoSuchElementException if there is no value present
     *
     * @see Optional#isPresent()
     */
    // 核心方法 如果值存在逆济，則返回該值酌予；否則磺箕，拋出沒(méi)有這個(gè)元素異常
    public T get() {
//        if (value == null) {
        if (!isPresent()) {
            throw new NoSuchElementException("No value present");
        }
        return value;
    }

    /**
     * Return {@code true} if there is a value present, otherwise {@code false}.
     *
     * @return {@code true} if there is a value present, otherwise {@code false}
     */
    // 核心方法 如果是存在的值，則返回true抛虫；否則松靡，返回false
    public boolean isPresent() {
        return value != null;
    }

    // 函數(shù)式接口
    /**
     * If a value is present, invoke the specified consumer with the value,
     * otherwise do nothing.
     *
     * @param consumer block to be executed if a value is present
     * @throws NullPointerException if value is present and {@code consumer} is
     * null
     */
    // 核心方法 如果值是存在的，則調(diào)用指定的操作消費(fèi)該值建椰；否則雕欺，什么都不做
    public void ifPresent(Consumer<? super T> consumer) {
//        if (value != null)
        if (isPresent()) {
            consumer.accept(value);
        }
    }

    // 第一層：過(guò)濾
    /**
     * If a value is present, and the value matches the given predicate,
     * return an {@code Optional} describing the value, otherwise return an
     * empty {@code Optional}.
     *
     * @param predicate a predicate to apply to the value, if present
     * @return an {@code Optional} describing the value of this {@code Optional}
     * if a value is present and the value matches the given predicate,
     * otherwise an empty {@code Optional}
     * @throws NullPointerException if the predicate is null
     */
    // 核心方法 如果值是存在的，并且值匹配給定的謂詞棉姐，則返回描述該值的可選實(shí)例阅茶；否則，返回空實(shí)例
    public Optional<T> filter(Predicate<? super T> predicate) {
        Objects.requireNonNull(predicate);
        if (!isPresent()) {
            return this;
        } else {
            return predicate.test(value) ? this : empty();
        }
    }

    // 使用規(guī)則：flatMap(mapper) { map(mapper) }
    /**
     * If a value is present, apply the provided mapping function to it,
     * and if the result is non-null, return an {@code Optional} describing the
     * result.  Otherwise return an empty {@code Optional}.
     *
     * @apiNote This method supports post-processing on optional values, without
     * the need to explicitly check for a return status.  For example, the
     * following code traverses a stream of file names, selects one that has
     * not yet been processed, and then opens that file, returning an
     * {@code Optional<FileInputStream>}:
     * <p>
     * 本方法支持可選值的后置處理谅海，不需要顯示地檢查返回狀態(tài)脸哀。
     * 例如，下面的代碼遍歷文件名稱列表的流扭吁，選擇一個(gè)尚未被處理的撞蜂，然后打開(kāi)該文件，并返回文件輸出流的可選實(shí)例：
     *
     * <pre>{@code
     *     Optional<FileInputStream> fis =
     *         names.stream().filter(name -> !isProcessedYet(name))
     *                       .findFirst()
     *                       .map(name -> new FileInputStream(name));
     * }</pre>
     *
     * Here, {@code findFirst} returns an {@code Optional<String>}, and then
     * {@code map} returns an {@code Optional<FileInputStream>} for the desired
     * file if one exists.
     * <p>
     * {@code findFirst}返回一個(gè)字符串的可選實(shí)例侥袜，然后通過(guò)映射函數(shù)為存在的所需文件返回一個(gè)文件輸入流的可選實(shí)例蝌诡。
     *
     * @param <U> The type of the result of the mapping function 映射函數(shù)的結(jié)果類型
     * @param mapper a mapping function to apply to the value, if present 如果值存在，應(yīng)用到值的映射函數(shù)
     * @return an {@code Optional} describing the result of applying a mapping
     * function to the value of this {@code Optional}, if a value is present,
     * otherwise an empty {@code Optional} 返回一個(gè)描述應(yīng)用映射函數(shù)到該可選實(shí)例值的結(jié)果的可選實(shí)例
     * @throws NullPointerException if the mapping function is null
     */
    // 核心方法 如果值是存在的枫吧，則應(yīng)用給定的映射函數(shù)浦旱，并且結(jié)果是非空的，則返回描述該結(jié)果的可選實(shí)例九杂；否則颁湖，返回空實(shí)例
    public <U> Optional<U> map(Function<? super T, ? extends U> mapper) {
        Objects.requireNonNull(mapper);
        if (!isPresent()) {
            return empty();
        } else {
            return Optional.ofNullable(mapper.apply(value)); // 映射結(jié)果可能為null，包裝成Optional
        }
    }

    /**
     * If a value is present, apply the provided {@code Optional}-bearing
     * mapping function to it, return that result, otherwise return an empty
     * {@code Optional}.  This method is similar to {@link #map(Function)},
     * but the provided mapper is one whose result is already an {@code Optional},
     * and if invoked, {@code flatMap} does not wrap it with an additional
     * {@code Optional}.
     * <p>
     * 本方法和{@link #map(Function)}相似例隆，但提供的映射函數(shù)的結(jié)果已是一個(gè)可選實(shí)例甥捺，
     * 如果被調(diào)用，本方法并不會(huì)使用一個(gè)額外的可選實(shí)例包裝它镀层。
     *
     * @param <U> The type parameter to the {@code Optional} returned by 返回的可選實(shí)例的類型參數(shù)
     * @param mapper a mapping function to apply to the value, if present
     *           the mapping function 應(yīng)用到值的映射函數(shù)
     * @return the result of applying an {@code Optional}-bearing mapping
     * function to the value of this {@code Optional}, if a value is present,
     * otherwise an empty {@code Optional}
     * @throws NullPointerException if the mapping function is null or returns
     * a null result
     */
    // 核心方法 如果值是可選的镰禾，則應(yīng)用提供的可選關(guān)系映射函數(shù)，然后返回該結(jié)果唱逢；否則吴侦，返回空實(shí)例
    public <U> Optional<U> flatMap(Function<? super T, Optional<U>> mapper) {
        Objects.requireNonNull(mapper);
        if (!isPresent()) {
            return empty();
        } else {
            return Objects.requireNonNull(mapper.apply(value)); // Optional<U>不能為null，原生返回
        }
    }

    /**
     * Return the value if present, otherwise return {@code other}.
     *
     * @param other the value to be returned if there is no value present, may
     * be null 可能為null
     * @return the value, if present, otherwise {@code other}
     */
    // 核心方法 如果值存在坞古，則返回該值备韧；否則，返回默認(rèn)值
    public T orElse(T other) {
//        return value != null ? value : other;
        return isPresent() ? value : other;
    }

    /**
     * Return the value if present, otherwise invoke {@code other} and return
     * the result of that invocation.
     *
     * @param other a {@code Supplier} whose result is returned if no value
     * is present
     * @return the value if present otherwise the result of {@code other.get()}
     * @throws NullPointerException if value is not present and {@code other} is
     * null
     */
    // 核心方法 如果值存在绸贡，則返回該值盯蝴；否則，調(diào)用結(jié)果提供者并返回調(diào)用的結(jié)果
    // 使用場(chǎng)景：數(shù)據(jù)多層保護(hù)設(shè)計(jì)听怕，如Cache + DB
    public T orElseGet(Supplier<? extends T> other) {
//        return value != null ? value : other.get();
        return isPresent() ? value : other.get();
    }

    /**
     * Return the contained value, if present, otherwise throw an exception
     * to be created by the provided supplier.
     *
     * @apiNote A method reference to the exception constructor with an empty
     * argument list can be used as the supplier. For example,
     * {@code IllegalStateException::new}.
     * <p>
     * 一個(gè)引用使用空參數(shù)列表異常構(gòu)造器的方法捧挺，可用于異常供應(yīng)商。
     * 例如尿瞭，{@code IllegalStateException::new}闽烙。
     *
     * @param <X> type of the exception to be thrown 待拋出的異常類型
     * @param exceptionSupplier The supplier which will return the exception to
     * be thrown 返回異常的供應(yīng)商
     * @return the present value
     * @throws X if there is no value present
     * @throws NullPointerException if no value is present and
     * {@code exceptionSupplier} is null
     */
    // 核心方法 如果值存在，則返回包含的值声搁；否則黑竞，拋出一個(gè)由提供的供應(yīng)商創(chuàng)建的異常
    // 使用場(chǎng)景：基于異常的快速失敗設(shè)計(jì)(fail-fast)
    public <X extends Throwable> T orElseThrow(Supplier<? extends X> exceptionSupplier) throws X {
//        if (value != null) {
        if (isPresent()) {
            return value;
        } else {
            throw exceptionSupplier.get(); // 值為null，則拋出異常
        }
    }


    // ----------------- Object -----------------
    /**
     * Indicates whether some other object is "equal to" this Optional. The
     * other object is considered equal if:
     * <ul>
     * <li>it is also an {@code Optional} and;
     * <li>both instances have no value present or;
     * <li>the present values are "equal to" each other via {@code equals()}.
     * </ul>
     *
     * @param obj an object to be tested for equality
     * @return {code true} if the other object is "equal to" this object
     * otherwise {@code false}
     */
    @Override
    public boolean equals(Object obj) {
        // 《Effective Java》總結(jié)的優(yōu)化實(shí)現(xiàn)
        if (this == obj) {
            return true;
        }

        if (!(obj instanceof Optional)) {
            return false;
        }

        Optional<?> other = (Optional<?>) obj;
        return Objects.equals(value, other.value);
    }

    /**
     * Returns the hash code value of the present value, if any, or 0 (zero) if
     * no value is present.
     *
     * @return hash code value of the present value or 0 if no value is present
     */
    @Override
    public int hashCode() {
        return Objects.hashCode(value);
    }

    /**
     * Returns a non-empty string representation of this Optional suitable for
     * debugging. The exact presentation format is unspecified and may vary
     * between implementations and versions.
     *
     * @implSpec If a value is present the result must include its string
     * representation in the result. Empty and present Optionals must be
     * unambiguously differentiable.
     *
     * @return the string representation of this instance
     */
    @Override
    public String toString() {
//        return value != null
        return isPresent()
            ? String.format("Optional[%s]", value)
            : "Optional.empty"; // 空實(shí)例
    }
}

祝玩得開(kāi)心疏旨！ˇ?ˇ
云舒很魂，2017.7.23，杭州