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;/*** 一个容器对象,可能会或可能不会包含一个非空值。* 如果一个值是存在的, {@code isPresent()} will return {@code true} and* {@code get()} will return the value.* @since 1.8*/
public final class Optional<T> {/*** 用于方法empty()*/private static final Optional<?> EMPTY = new Optional<>();/*** 如果非空值,就是该值;如果为空,表示没有值*/private final T value;/*** 私有化空参构造** @implNote 一般只有一个空的实例, {@link Optional#EMPTY},* 每个VM应该存在。*/private Optional() {this.value = null;}/*** 返回一个空的 {@code Optional} 对象.** @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> 无参* @return 返回一个Optional对象 {@code Optional}*/public static<T> Optional<T> empty() {@SuppressWarnings("unchecked")Optional<T> t = (Optional<T>) EMPTY;return t;}/*** 私有化有参构造** @param value 当前值不能为null* @throws NullPointerException 如果值为null,抛出NPE异常*/private Optional(T value) {//校验是否对象是否为空,为空抛出异常,不为空(返回传入的对象)this.value = Objects.requireNonNull(value);/** public static <T> T requireNonNull(T obj) {*    if (obj == null){*       throw new NullPointerException();*    }*    return obj;* }* */}/*** 参数不能为null* @param <T> the class of the value* @param value 必须非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);}/*** 如果传入的值为null,就调用empty()方法,返回一个对象,否则调用of(value)方法,返回一个对象* @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);}/*** 如果存在一个值{ @code Optional},返回值,* 否则 throws {@code NoSuchElementException}.* 此方法不常用*          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;}/*** 判断值是否为空,为空返回false，不为空返回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;}/*** 如果value != null,调用指定的Consumer实现类中的accept(value)方法,* 否则什么都不做。** @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;elsereturn predicate.test(value) ? this : empty();}public<U> Optional<U> map(Function<? super T, ? extends U> mapper) {//判断对象是否为空Objects.requireNonNull(mapper);//如果value为空,返回Optional对象if (!isPresent())return empty();else {//不为空,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));}}/*** 判断value是否为空,为空返回other，不为空返回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;}/*** 判断value是否为空，为空调用指定方法，并返回该调用方法返回的结果，不为空返回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();}}@Overridepublic 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);}@Overridepublic int hashCode() {return Objects.hashCode(value);}@Overridepublic String toString() {return value != null? String.format("Optional[%s]", value): "Optional.empty";}
}