JDK1.8新特性之Optional类（一）

• 认识Optional

类注解源码：

/**
 * A container object which may or may not contain a non-null value.
 * If a value is present, {@code isPresent()} will return {@code true} and
 * {@code get()} will return the value.
 *
 * <p>Additional methods that depend on the presence or absence of a contained
 * value are provided, such as {@link #orElse(java.lang.Object) orElse()}
 * (return a default value if value not present) and
 * {@link #ifPresent(java.util.function.Consumer) ifPresent()} (execute a block
 * of code if the 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.
 *
 * @since 1.8
 */
public final class Optional<T> {

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

    /**
     * If non-null, the value; if null, indicates no value is present
     */
    private final T value;
   
}

从类注解中我们可以看出，Optional是一个可以容纳空对象的object容器，可通过isPresent()方法来判断值是否存在，通过get()方法获取值。附加方法的使用取决于容器中的值是否存在。看来，Optional类的主要特性是区分对象是否为空，并分别作出处理的容器类。下面，我们看看这个容器在代码中是怎么使用的。

• API讲解

1.empty() 获取一个空容器。返回一个指向new Optional<>()的私有静态常量；

/**
     * 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;
    }

2.of(T value)  获取一个容纳非空对象的容器。

/**
     * 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);
    }

3.ofNullable(T value) 获取一个可容纳空对象的容器。

/**
     * Returns an {@code Optional} describing the specified value, if non-null,
     * otherwise returns an empty {@code Optional}.
     *
     * @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);
    }

4.get() 获取容器容纳的对象，当对象为空时抛出NoSuchElementException。

 /**
     * 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()
     */
    public T get() {
        if (value == null) {
            throw new NoSuchElementException("No value present");
        }
        return value;
    }

通过代码实践一下：

package optional;

import java.util.Optional;

public class User {

	private String name, id;

	public String getName() {
		return name;
	}

	public void setName(String name) {
		this.name = name;
	}

	public String getId() {
		return id;
	}

	public void setId(String id) {
		this.id = id;
	}

	@Override
	public String toString() {
		return "User{" +
				"name='" + name + '\'' +
				", id='" + id + '\'' +
				'}';
	}

	public static void main(String[] args) {
		User user = new User();
		User nullUser = null;
		Optional<User> userOpt = Optional.ofNullable(user);
		Optional<User> nullUserOpt = Optional.ofNullable(nullUser);
		System.out.println(userOpt.get());
		System.out.println(nullUserOpt.get());
	}
}

 控制台输出内容：

User{name='null', id='null'}
Exception in thread "main" java.util.NoSuchElementException: No value present
	at java.util.Optional.get(Optional.java:135)
	at optional.User.main(User.java:39)

5.isPresent() 判断容纳的对象是否为空。

   /**
     * 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 value != null;
    }

代码实践：

        System.out.println(userOpt.isPresent());
	System.out.println(nullUserOpt.isPresent());

控制台输出：
        true
        false

6.ifPresent(Consumer<? super T> consumer)当容纳的对象存在时，执行参数中的方法。

    /**
     * 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
     */
    public void ifPresent(Consumer<? super T> consumer) {
        if (value != null)
            consumer.accept(value);
    }

代码实践：

        userOpt.ifPresent(item -> System.out.println(item));
	nullUserOpt.ifPresent(item -> System.out.println(item));
控制台：
        User{name='null', id='null'}

7.filter(Predicate<? super T> predicate)通过一定条件过滤容器，返回null，this，或者空容器。

/**
     * 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();
    }

代码实践：

	Optional<User> optional = nullUserOpt.filter((User item) -> item == null);
	System.out.println(optional.isPresent());
	Optional<User> opt1 = userOpt.filter((User item) -> item == null);
	System.out.println(opt1.isPresent());
	Optional<User> opt2 = userOpt.filter((User item) -> item != null);
	System.out.println(opt2.get());
控制台输出：
        false
        false
        User{name='null', id='null'}

8.map(Function<? super T, ? extends U> mapper)获取容纳参数方法的返回值的容器。

  /**
     * 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>}:
     *
     * <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.
     *
     * @param <U> The type of the result of the mapping function
     * @param mapper a mapping function to apply to the value, if present
     * @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}
     * @throws NullPointerException if the mapping function is null
     */
    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));
        }
    }

代码实践：

		user.setId("123");
		Optional<String> idOpt = userOpt.map(User::getId);
		System.out.println(idOpt.get());
控制台：
    123

 9.flatMap(Function<? super T, Optional<U>> mapper)目的同上，不同的是参数方法的返回值必须由Optional包装。

代码实践：

user.setId("123");
Optional<String> idOpt = userOpt.flatMap(item -> Optional.ofNullable(item.getId()));
System.out.println(idOpt.get());

控制台：123

10.orElse(T other) 当容纳的对象为空时，返回参数中的对象。

    /**
     * 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
     * @return the value, if present, otherwise {@code other}
     */
    public T orElse(T other) {
        return value != null ? value : other;
    }

代码实践：

	User user1 = nullUserOpt.orElse(user);
	System.out.println(user1);
控制台：
        User{name='null', id='null'}

11.orElseGet(Supplier<? extends T> other)当容纳的对象为空时，返回参数中方法的返回值，注意泛型一致。

    /**
     * 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
     */
    public T orElseGet(Supplier<? extends T> other) {
        return value != null ? value : other.get();
    }

代码实践（本次直接使用userOpt）：

	User user1 = nullUserOpt.orElseGet(userOpt::get);
	System.out.println(user1);
控制台输出：
        User{name='null', id='null'}

12.orElseThrow(Supplier<? extends X> exceptionSupplier) throws X 当容器中对象为空时，抛出参数中的异常。

  /**
     * 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();
        }
    }

代码实践：

		try {
			nullUserOpt.orElseThrow(Exception::new);
		}catch (Exception e){
			System.err.println(e);
		}
控制台输出：
java.lang.Exception

13.equals(Object obj)对比容器中的对象

    /**
     * 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) {
        if (this == obj) {
            return true;
        }

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

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

代码实践：

		boolean equals = userOpt.equals(Optional.ofNullable(user));
		System.out.println(equals);
控制台输出：
true

14.hashCode() 获取容器中对象的哈希值

/**
     * 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);
    }