java8 Optional源碼

import java.util.NoSuchElementException;
import java.util.Objects;
import java.util.function.Consumer;
import java.util.function.Function;
import java.util.function.Predicate;
import java.util.function.Supplier;

/**
 * 一個容器對象,可能會或可能不會包含一個非空值。
 * 若是一個值是存在的, {@code isPresent()} will return {@code true} and
 * {@code get()} will return the value.
 * @since 1.8
 */
public final class Optional<T> {
    /**
     * 用於方法empty()
     */
    private static final Optional<?> EMPTY = new Optional<>();

    /**
     * 若是非空值,就是該值;若是爲空,表示沒有值
     */
    private final T value;

    /**
     * 私有化空參構造
     *
     * @implNote 通常只有一個空的實例, {@link Optional#EMPTY},
     * 每一個VM應該存在。
     */
    private Optional() {
        this.value = null;
    }

    /**
     * 返回一個空的 {@code Optional} 對象.
     *
     * @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> 無參
     * @return 返回一個Optional對象 {@code Optional}
     */
    public static<T> Optional<T> empty() {
        @SuppressWarnings("unchecked")
        Optional<T> t = (Optional<T>) EMPTY;
        return t;
    }

    /**
     * 私有化有參構造
     *
     * @param value 當前值不能爲null
     * @throws NullPointerException 若是值爲null,拋出NPE異常
     */
    private Optional(T value) {
        //校驗是否對象是否爲空,爲空拋出異常,不爲空(返回傳入的對象)
        this.value = Objects.requireNonNull(value);
     /*
        * public static <T> T requireNonNull(T obj) {
        *    if (obj == null){
        *       throw new NullPointerException();
        *    }
        *    return obj;
        * }
        * */

    }

    /**
     * 參數不能爲null
     * @param <T> the class of the value
     * @param value 必須非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);
    }

    /**
     * 若是傳入的值爲null,就調用empty()方法,返回一個對象,不然調用of(value)方法,返回一個對象
     * @param <T> the class of the value
     * @param value the possibly-null value to describe
     * @return an {@code Optional} with a present value if the specified value
     * is non-null, otherwise an empty {@code Optional}
     */
    public static <T> Optional<T> ofNullable(T value) {
        return value == null ? empty() : of(value);
    }

    /**
     * 若是存在一個值{ @code Optional},返回值,
     * 不然 throws {@code NoSuchElementException}.
     * 此方法不經常使用
     *          User user = new User();
     *         Optional userOptional =   Optional.ofNullable(user);
     *         Object o1 = userOptional.get();
     *         //com.shilvfei.web.User@19f2327
     *         System.out.println(o1);
     * @return the non-null value held by this {@code Optional}
     * @throws NoSuchElementException if there is no value present
     *
     * @see Optional#isPresent()
     */
    public T get() {
        if (value == null) {
            throw new NoSuchElementException("No value present");
        }
        return value;
    }

    /**
     * 判斷值是否爲空,爲空返回false，不爲空返回true
     *          User user = new User();
     *         Optional userOptional =   Optional.ofNullable(user);
     *         //true
     *         System.out.println(userOptional.isPresent());
     * @return {@code true} if there is a value present, otherwise {@code false}
     */
    public boolean isPresent() {
        return value != null;
    }

    /**
     * 若是value != null,調用指定的Consumer實現類中的accept(value)方法,
     * 不然什麼都不作。
     *
     * @param consumer block to be executed if a value is present
     * @throws NullPointerException if value is present and {@code consumer} is
     * null
     */
    public void ifPresent(Consumer<? super T> consumer) {
        if (value != null)
            consumer.accept(value);
    }

    /**
     * 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
     */
    public Optional<T> filter(Predicate<? super T> predicate) {
        Objects.requireNonNull(predicate);
        if (!isPresent())
            return this;
        else
            return predicate.test(value) ? this : empty();
    }

    public<U> Optional<U> map(Function<? super T, ? extends U> mapper) {
        //判斷對象是否爲空
        Objects.requireNonNull(mapper);
        //若是value爲空,返回Optional對象
        if (!isPresent())
            return empty();
        else {
            //不爲空,返回mapper.apply(value)的值
            return Optional.ofNullable(mapper.apply(value));
        }
    }

    /**
     * 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}.
     *
     * @param <U> The type parameter to the {@code Optional} returned by
     * @param mapper a mapping function to apply to the value, if present
     *           the mapping function
     * @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
     */
    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));
        }
    }

    /**
     * 判斷value是否爲空,爲空返回other，不爲空返回value {@code other}.
     *
     * @param other the value to be returned if there is no value present, may
     * be null
     * @return the value, if present, otherwise {@code other}
     */
    public T orElse(T other) {
        return value != null ? value : other;
    }

    /**
     * 判斷value是否爲空，爲空調用指定方法，並返回該調用方法返回的結果，不爲空返回value的值
     *
     * @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
     */
    public T orElseGet(Supplier<? extends T> other) {
        return value != null ? 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}
     *
     * @param <X> Type of the exception to be thrown
     * @param exceptionSupplier The supplier which will return the exception to
     * be thrown
     * @return the present value
     * @throws X if there is no value present
     * @throws NullPointerException if no value is present and
     * {@code exceptionSupplier} is null
     */
    public <X extends Throwable> T orElseThrow(Supplier<? extends X> exceptionSupplier) throws X {
        if (value != null) {
            return value;
        } else {
            throw exceptionSupplier.get();
        }
    }

    @Override
    public boolean equals(Object obj) {
        if (this == obj) {
            return true;
        }

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

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

    @Override
    public int hashCode() {
        return Objects.hashCode(value);
    }

    @Override
    public String toString() {
        return value != null
            ? String.format("Optional[%s]", value)
            : "Optional.empty";
    }
}