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.
* 一个可能包含也可能不包含非null值的容器对象
* If a value is present, {@code isPresent()} will return {@code true} and
* {@code get()} will return the value.
* 如果存在值,isPresent()将返回 true,get()将返回该值。
*
* <p>Additional methods that depend on the presence or absence of a contained
* value are provided, such as {@link #orElse(Object) orElse()}
* (return a default value if value not present) and
* {@link #ifPresent(Consumer) ifPresent()} (execute a block
* of code if the value is present).
* 还提供了依赖于包含值是否存在的其他方法,例如 orElse(Object) orElse()(如果值不存在,则返回默认值)
* 和 ifPresent(Consumer) ifPresent()(如果值存在,则执行代码块)。
*
* <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.
*
* 这是一个 value-based 的类;
* 在Optional实例上的身份敏感操作(包括引用相等性==、标识哈希代码或同步)可能会产生不可预知的结果,应避免使用。
*
* @since 1.8
*/
public final class Optional<T> {
/**
* Common instance for {@code empty()}.
* empty()的常见实例
*/
private static final Optional<?> EMPTY = new Optional<>();
/**
* If non-null, the value; if null, indicates no value is present
* 如果非空,则为值;如果为 null,则表示不存在任何值
*/
private final T value;
/**
* Constructs an empty instance.
* 构造一个空实例
*
* @implNote Generally only one empty instance, {@link Optional#EMPTY},
* should exist per VM.
* 通常,每个 VM 只能存在一个空实例
*/
private Optional() {
this.value = null;
}
/**
* Returns an empty {@code Optional} instance. No value is present for this
* Optional.
* 返回一个空的Optional实例。此Optional不存在任何值。
*
* @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()}.
* todo 此处解释不是太明白,比较对象为空? 确保单例?
* 尽管这样做可能很诱人,但请避免通过 == 与 Option.empty() 返回的实例来比较测试对象是否为空。
* 不能保证它是单例。相反,请使用 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;
}
/**
* Constructs an instance with the value present.
* 构造一个有值的实例
*
* @param value the non-null value to be present
* @throws NullPointerException if value is null
*/
private Optional(T value) {
this.value = Objects.requireNonNull(value);
}
/**
* Returns an {@code Optional} with the specified present non-null value.
*
* 返回具有指定的非空值的 Optional实例
*
* @param <T> the class of the value
* @param value the value to be present, which must be non-null 必须非空
* @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);
}
/**
* Returns an {@code Optional} describing the specified value, if non-null,
* otherwise returns an empty {@code Optional}.
* 返回描述指定值的 Optional实例,如果非 null,则返回空的Optional实例
*
* @param <T> the class of the value
* @param value the possibly-null value to describe 要描述的可能为空值
* @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);
}
/**
* If a value is present in this {@code Optional}, returns the value,
* otherwise throws {@code NoSuchElementException}.
* 如果Optional里面有值,则返回该值,否则抛出异常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;
}
/**
* Return {@code true} if there is a value present, otherwise {@code false}.
* 如果存在值,则返回 true,否则返回 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.
* 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
*
* @param consumer block to be executed if a value is present
* @throws NullPointerException if value is present and {@code consumer} is
* null 如果值存在且consumer为空 就会抛出异常
*/
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}.
* 如果存在值,并且该值与Predicate.test()匹配,则返回描述该值的Optional,否则返回空值的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();
}
/**
* 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}.
* 如果存在值,则对其应用提供的映射函数,如果结果为非 null,则返回描述结果的 {@code Optional}。
* 否则不存在值,返回空值的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>}:
* 此方法支持对可选值进行后处理,而无需显式检查返回状态。
* 例如,以下代码遍历文件名流,选择尚未处理的文件名流,然后打开该文件,返回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.
* 在这里,findFirst返回一个Optional<String>,然后map返回所需文件的Optional<FileInputStream>(如果存在)。
*
* @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}.
*
* 如果存在值,请对其应用提供的 {@code 可选} 轴承映射函数,返回该结果,否则返回空的 {@code 可选}。
*
* 此方法类似于map,但提供的映射器的结果已经是 Optional,如果调用flatMap会用额外的Optional包装它。
*
* @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));
}
}
/**
* Return the value if present, otherwise return {@code other}.
* 如果存在,则返回值,否则返回入参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.
* 如果存在,则返回该值,否则返回 Supplier.get()的返回值
*
* @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.
* 如果存在,返回该值,否则抛出异常(此异常是Supplier.get()返回的)
*
* @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();
}
}
/**
* Indicates whether some other object is "equal to" this Optional. The
* other object is considered equal if:
* 指示某个其他对象是否“等于”此可选对象。如果出现以下情况,则另一个对象被视为相等:
* <ul>
* <li>it is also an {@code Optional} and;
* 它也是一个Optional
* <li>both instances have no value present or;
* 两个实例都不存在值或
* <li>the present values are "equal to" each other via {@code equals()}.
* 当前值通过 equals() 彼此“相等”
* </ul>
*
* @param obj an object to be tested for equality
* 一个要测试相等的对象
* @return {code true} if the other object is "equal to" this object
* otherwise {@code false}
*
* <ul> @param obj @return {code true} 如果另一个对象“等于”这个对象,否则 {@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);
}
/**
* Returns the hash code value of the present value, if any, or 0 (zero) if
* no value is present.
* 返回当前值的哈希代码值(如果有)或 0(零)(如果不存在任何值)
*
* @return hash code value of the present value or 0 if no value is present
*/
@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.
* 返回此 Optional 适合调试的非空字符串表示形式。
* 确切的演示格式未指定,可能因实现和版本而异。
*
* @implSpec If a value is present the result must include its string
* representation in the result. Empty and present Optionals must be
* unambiguously differentiable.
* 如果存在值,则结果必须在结果中包含其字符串表示形式。空和存在值的Optionals 必须明确可区分。
*
* @return the string representation of this instance
*/
@Override
public String toString() {
return value != null
? String.format("Optional[%s]", value)
: "Optional.empty";
}
}