jdk1.8_Optional

概述

主要用于解决空指针异常（NullPointerException）
类似包含有可选值的包装类，Optional 类既可以含有对象也可以为空

/**
 * 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> {...}

of()&ofNullable()

构造带有值的Optional类
of()方法中的参数不允许为null，传入null会导致NullPointerException异常
若调用ofNullable()方法并传入空值则实际上相当于直接调用empty()方法实例化一个空的Optional

Optional<String> optional = Optional.of("value");
Optional<String> optionalNull = Optional.ofNullable(null);
Optional<String> emptyOpt = Optional.empty();
System.out.println(optional.get());//value
System.out.println(emptyOpt.get());//NoSuchElementException
System.out.println(optionalNull.get());//NoSuchElementException

/**
 * 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);
}

isPresent()&ifPresent()

isPresent() 判断是否为空
ifPresent() 判断是否为空,不为空则执行方法内的lambda表达式(实现消费型接口)

/**
 * 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;
}

/**
 * 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);
}

Optional<String> optional = Optional.of("value");
Optional<String> optionalNull = Optional.ofNullable(null);
optional.ifPresent((s) -> System.out.println(s));//value
optionalNull.ifPresent(System.out::println);//不执行

orElse()&orElseGet()&orElseThrow()

orElse() 若Optional对象为空值则返回方法传入的值
orElseGet() 若Optional对象为空值则执行方法内的lambda表达式(供给型功能函数)
orElseThrow() 若Optional对象为空值则抛出指定的异常(供给型功能函数)

/**
 * 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;
}
	
/**
 * 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();
}
	
/**
 * 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();
    }
}

Optional<String> optionalNull = Optional.ofNullable(null);
System.out.println(optionalNull.orElse("值为null"));//值为null
System.out.println(optionalNull.orElseGet(()->"值为null"));
try {
	optionalNull.orElseThrow(Exception::new);
} catch (Exception e) {
	e.printStackTrace();
}

map()&flatMap()

map()方法，若传入的方法存在值，则返回方法对应类型的Optional对象，否则返回一个空的Optional
同样，但入参需要是Optional对象

/**
 * 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));
    }
}

/**
 * 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));
    }
}

class FlightTicketInfo {

    private String orderNumber;

    public String getOrderNumber() {
        return orderNumber;
    }

}

public class OptionalTest {

    @Test
    public void testMap() {
        FlightTicketInfo flightTicketInfo = null;

        Optional<Optional<String>> s1 = Optional.ofNullable(flightTicketInfo).
			map(OptionalTest::getOrderNumber);

        Optional<String> s2 = Optional.ofNullable(flightTicketInfo).
			map(FlightTicketInfo::getOrderNumber);

        Optional<String> s3 = Optional.ofNullable(flightTicketInfo).
			flatMap(OptionalTest::getOrderNumber);
    }

    private static Optional<String> getOrderNumber(FlightTicketInfo flightTicketInfo) {
        return Optional.ofNullable(flightTicketInfo).map(f -> f.getOrderNumber());
    }

}

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