Java 8 新特性 - Optional

目录：Java 8 新特性-终极指南

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文章目录

• ​​Java 8 新特性 - Optional​​

• ​​Optional源码-各个方法​​

Java 8 新特性 - Optional

Java应用中最常见的bug就是​​空值异常​​​。在Java 8之前，​​Google Guava​​引入了Optionals类来解决NullPointerException，从而避免源码被各种null检查污染，以便开发者写出更加整洁的代码。

Java 8也将Optional加入了官方库。

Optional仅仅是一个功能：存放T类型的值或者null。它提供了一些有用的接口来避免显式的null检查，可以参考​​Java 8官方文档​​了解更多细节。

接下来看一点使用Optional的例子：可能为空的值或者某个类型的值：

Optional<String> fullName = Optional.ofNullable( null );
System.out.println( "Full Name is set? " + fullName.isPresent() );
System.out.println( "Full Name: " + fullName.orElseGet( () -> "[none]" ) );
System.out.println( fullName.map( s -> "Hey " + s + "!" ).orElse( "Hey Stranger!" ) );

如果Optional 实例持有一个非空值，则 isPresent() 方法返回true，否则返回false；

orElseGet() 方法，Optional 实例持有null，则可以接受一个lambda表达式生成的默认值；

map() 方法可以将现有的 Opetional 实例的值转换成新的值；

orElse() 方法与 orElseGet() 方法类似，但是在持有null的时候返回传入的默认值。

上述代码的输出结果如下：

Full Name is set? false
Full Name: [none]
Hey Stranger!

再看下另一个简单的例子：

Optional<String> firstName = Optional.of( "Tom" );
System.out.println( "First Name is set? " + firstName.isPresent() );
System.out.println( "First Name: " + firstName.orElseGet( () -> "[none]" ) );
System.out.println( firstName.map(s -> "Hey " + s + "!" ).orElse( "Hey Stranger!" ) );
System.out.println();

这个例子的输出是：

First Name is set? true
First Name: Tom
Hey Tom!

如果想了解更多的细节，请参考​​官方文档​​。

Optional源码-各个方法

package java.util;

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

/**
* 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;

/**
* Constructs an empty instance.
*
* @implNote Generally only one empty instance, {@link Optional#EMPTY},
* should exist per VM.
*
* 无参构造
*/
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}
*
* 返回一个空Optional实例。
*/
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
* @throws NullPointerException if value is null
*
* 有参构造
*/
private Optional(T value) {
this.value = Objects.requireNonNull(value);
}

/**
* 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
*
* 返回一个具有指定当前非空值的Optional实例。
*/
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
* @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}
*
* 如果非空，返回一个指定值Optional实例，否则返回一个空Optional。
*/
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()
*
* 如果 this 中存在Optional值，则返回该值，否则抛出NoSuchElementException。
*/
public T get() {
if (value == null) {
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}
*
* 如果存在值（不为null）则返回true，否则返回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
*
* 如果存在值，则使用该值调用指定的使用者，否则不执行任何操作。
*/
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
*
* 如果存在一个值，并且该值与给定的谓词匹配，则返回一个Optional描述该值的值，否则返回一个空值Optional。
*/
public Optional<T> filter(Predicate<? super T> predicate) {
Objects.requireNonNull(predicate);
if (!isPresent())
return this;
else
return predicate.test(value) ? this : empty();
}

/**
* 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
*
* 如果存在值，则对其应用提供的映射函数，如果结果为非 null，则返回Optional描述结果。
*/
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
*
* 如果存在值，则将提供的Optional-bearing 映射函数应用于它，返回该结果，否则返回空 Optional。
*/
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));
}
}

/**
* 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}
*
* 对自身判断，如果存在则返回自身，否则返回other。
* 对自身判断，如果不为null则返回自身，否则返回一个指定的值。
*/
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
*
* 如果存在则返回该值，否则调用other并返回该调用的结果。
*/
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
*
* 如果存在（不为null），则返回该值，否则抛出由提供的供应商创建的异常。
*/
public <X extends Throwable> T orElseThrow(Supplier<? extends X> exceptionSupplier) throws X {
if (value != null) {
return value;
} else {
throw exceptionSupplier.get();
}
}

/**
* 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}
*
* 判断某个其他对象是否“等于”此 Optional。 相等返回true，不想等返回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);
}

/**
* 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
*
* 返回当前值的哈希码值（如果有）；如果不存在值，则返回 0（零）。
*/
@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
*
* 返回此 Optional 适合调试的非空字符串表示形式。
*/
@Override
public String toString() {
return value != null
? String.format("Optional[%s]", value)
: "Optional.empty";
}
}