/*
* Copyright (c) 2012, 2013, Oracle and/or its affiliates. All rights reserved.
* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*/
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.
*
* 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).
*
*
This is a value-based
* 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.
*
* 一个判断实体是否为空的容器,如果有值,返回true, 无值的话返回false
* 并扩展了一些其他的获取当前值得方法
*
* 容器的创建方式:
* @method Optional() 创建一个空实例
* @method Optional(T) 创建一个实例 如果参数为null的话,会抛出空指针异常
* @methid empty() 创建一个空实例,等同于Optional()
* @methid of() 创建带参实例,等同于Optional(T)
* @method ofNullable(T) 如果参数不为空,返回带参实例,为空的话返回空实例
*
* 判断当前容器的状态
* @method isPresent() 判断当前实例是否有值
* @method isPresent(Consumer super T>) 判断当前实例是否有值,有值的话执行消费信息
*
* 获取当前容器的值
* @method get() 获取当前实例的值,如果实例为空的话会抛出异常
* @method orElse(T) 获取当前实例的值,如果值不存在返回当前参数
* @method orElseGet(Supplier extends T>) 获取当前值,值不存在返回指定实现的返回值
*
* 对当前容器的值进行数据处理
* @method filter(Predicate super T>) 过滤实例,如果当前值符合规则,则返回, 否则返回空实例
* @method map(Function super T, ? extends U>)
* @method flatMap(Function super T, Optional>)
*
* @since 1.8
*/
public final class Optional {
/**
* Common instance for {@code empty()}.
* 通用的空实例
*/
private static final Optional> EMPTY = new Optional<>();
/**
* If non-null, the value; if null, indicates no value is present
* 保存当前容器的值
*/
private final T value;
/**
* Constructs an empty instance.
* 构造方法, 返回空的容器
* @implNote Generally only one empty instance, {@link Optional#EMPTY},
* should exist per VM.
*/
private Optional() {
this.value = null;
}
/**
* Returns an empty {@code Optional} instance. No value is present for this
* Optional.
* 返回一个空的实例, 该实例没有value
*
* @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 Type of the non-existent value
* @return an empty {@code Optional}
*/
public static Optional empty() {
@SuppressWarnings("unchecked")
Optional t = (Optional) EMPTY;
return t;
}
/**
* Constructs an instance with the value present.
* 创建一个实例, 实例的值为传递参数
* 如果参数为null的话,会抛出空指针异常
* @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.
* 返回一个带参数的实例
* 如果参数为null的话,会抛出空指针异常
* @param 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 Optional of(T value) {
return new Optional<>(value);
}
/**
* Returns an {@code Optional} describing the specified value, if non-null,
* otherwise returns an empty {@code Optional}.
* 如果参数不为空的话,返回一个带参的实例
* 否则的话,返回一个空的实例
*
* @param 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 Optional 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}.
* 获取当前实例的值
* 如果存在,返回当前值
* 如果不存在,抛出元素不存在异常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}.
* 判断当前实例是否有值
* true 有值
* false 无值
* @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.
* 如果当前实例有值, 执行实现了Consumer接口的回调函数实例
*
* @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}.
* 判断当前实例是否符合匹配规则
* 如果符合匹配规则, 返回当前实例
* 如果不符合匹配规则,返回空实例
* @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 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}:
*
* {@code
* Optional fis =
* names.stream().filter(name -> !isProcessedYet(name))
* .findFirst()
* .map(name -> new FileInputStream(name));
* }
*
* Here, {@code findFirst} returns an {@code Optional}, and then
* {@code map} returns an {@code Optional} for the desired
* file if one exists.
*
* @param 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 Optional 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 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 Optional flatMap(Function super T, Optional> mapper) {
Objects.requireNonNull(mapper);
if (!isPresent())
return empty();
else {
return Objects.requireNonNull(mapper.apply(value));
}
}
/**
* Return the value if present, otherwise return {@code other}.
* 获取当前值
* 如果值存在的话, 返回当前值,
* 如果值不存在的话,返回指定的参数值
* @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.
* 获取当前实例的值
* 如果值存在的话,返回当前值, 如果值不存在的话,返回回调函数里的返回值
* @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.
*
* 获取当前实例的值,
* 如果当前实例无值的话,抛出回调异常
*
* @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 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 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:
*
* - it is also an {@code Optional} and;
*
- both instances have no value present or;
*
- the present values are "equal to" each other via {@code equals()}.
*
* 比较两个实例的地址 及两个实例的值
*
* @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);
}
/**
* 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);
}
/**
* 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
*/
@Override
public String toString() {
return value != null
? String.format("Optional[%s]", value)
: "Optional.empty";
}
}