文章目录
关于Optional理解
概述
- jdk1.7 - 资源的自动释放
- jdk1.8 - Stream
- jdk1.8 - Lambda
- jdk1.8 - 函数接口
jdk1.8 - Optional- jdk1.8 -日期处理
一、Optional的应用和使用
官方参考网址：https://docs.oracle.com/javase/8/docs/api/java/util/Optional.html
1.1 简介
以前一直不懂Optional有啥用，感觉太无语了，Java8还把它当做一个噱头来宣传，最近终于发现它的用处了，当然不用函数式编程的话，是没感觉的；
举例：一个非洲的Zoo，提供add一个animal进来的功能，可是有可能是用Future来捕捉动物的，也就是说可能没有catch到animal，但是为了防止以后使用的时候，有NPE错误，只有做判断；
个人觉得Optional实现的功能，有很多替代方案，if-else、三目等都可以；但Optional是用于函数式的一个整体中的一环，让函数式更流畅
NullPointerException相信每个JAVA程序员都不陌生，是JAVA应用程序中最常见的异常。之前，
Google Guava项目曾提出用Optional类来包装对象从而解决NullPointerException。受此影响，JDK8的类中也引入了Optional类，在新版的SpringData Jpa和Spring Redis Data中都已实现了对该方法的支持。
- 出现：Guava
1.2 认识使用
- 简化程序的逻辑
- 可以修复程序代码中的逻辑判断的问题
1.3 Optional类
package java.util;
import java.util.function.Consumer;
import java.util.function.Function;
import java.util.function.Predicate;
import java.util.function.Supplier;
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.
*/
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}
*/
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
*/
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
*/
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}
*/
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()
*/
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}
*/
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);
}
/**
* 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();
}
/**
* 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));
}
}
/**
* 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();
}
}
/**
* 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);
}
/**
* 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";
}
}
通过源码分析 ，Optional是一个final类，
- 说明不能被继承
- 它的构造函数是私有的，是单列的对象
- 提供一系列的方法，对其java程序逻辑的判断和优化处理
| 序号 | 方法 | 方法说明 |
|---|---|---|
| 1 | private Optional() |
无参构造，构造一个空Optional |
| 2 | private Optional(T value) |
根据传入的非空value构建Optional |
| 3 | public static<T> Optional<T> empty() |
返回一个空的Optional，该实例的value为空 |
| 4 | public static <T> Optional<T> of(T value) |
根据传入的非空value构建Optional，与Optional(T value)方法作用相同 |
| 5 | public static <T> Optional<T> ofNullable(T value) |
与of(T value)方法不同的是，ofNullable(T value)允许你传入一个空的value，当传入的是空值时其创建一个空Optional，当传入的value非空时，与of()作用相同 |
| 6 | public T get() |
返回Optional的值，如果容器为空，则抛出NoSuchElementException异常 |
| 7 | public boolean isPresent() |
判断当家Optional是否已设置了值 |
| 8 | public void ifPresent(Consumer<? super T> consumer) |
判断当家Optional是否已设置了值，如果有值，则调用Consumer函数式接口进行处理 |
| 9 | public Optional<T> filter(Predicate<? super T> predicate) |
如果设置了值，且满足Predicate的判断条件，则返回该Optional，否则返回一个空的Optional |
| 10 | public<U> Optional<U> map(Function<? super T, ? extends U> mapper) |
如果Optional设置了value，则调用Function对值进行处理，并返回包含处理后值的Optional，否则返回空Optional |
| 11 | public<U> Optional<U> flatMap(Function<? super T, Optional<U>> mapper) |
与map()方法类型，不同的是它的mapper结果已经是一个Optional，不需要再对结果进行包装 |
| 12 | public T orElse(T other) |
如果Optional值不为空，则返回该值，否则返回other |
| 13 | public T orElseGet(Supplier<? extends T> other) |
如果Optional值不为空，则返回该值，否则根据other另外生成一个 |
| 14 | public <X extends Throwable> T orElseThrow(Supplier<? extends X> exceptionSupplier)throws X |
如果Optional值不为空，则返回该值，否则通过supplier抛出一个异常 |
1.4 Optional的作用
- 简化程序的逻辑判断
二、 isPresent
判断当下Optional是否已设置了值
- 也就如果被Optional使用的对象，如果不是null,就返回:true
- 也就如果被Optional使用的对象，如果是null,就返回:false
User user = null;
Optional<User> optional = Optional.ofNullable(user);
if( ! optional.isPresent()){
throw new RuntimeException("用户找不到!!!");
}
- 上面的结果其实就出现异常，因为user对象是null。所以
optional.isPresent()结果就是false。所以出现异常
三、orElseThrow() （ if + throws）
public static User getUser(Integer userId){
User user = null;
user = Optional.ofNullable(user)
.orElseThrow(()->new RuntimeException("用户找不到!!!"));//简化 简化 if + throws / if/else
return user;
}
- 判断一个对象是否为null,如果为null，就会抛出异常 它可以简化：if+throws场景 + springmvc统一异常处理
四、 orElse() + if + new (复默认值)
public static User getUser(Integer userId) {
User user = null;
user = Optional.ofNullable(user).orElse(new User());
return user;
}
- 判断一个对象是否为null,如果为null，给你默认对象：if+throws场景 + springmvc统一异常处理
五、oElseGet() + if + 加逻辑处理
在开发中，有些时候我们数据查询是有值的，有些是没有值，但是我统计出，那些错误访问数据。
user = Optional.ofNullable(user).orElseGet(()->{
User user1 = new User();
// 增加各种处理和逻辑
return user1;
});
六、 ofNullable妙用（赋予默认值）
User user = new User();
user.setNickname(Optional.ofNullable(user.getNickname()).orElse("学生"));
user.setPassword(Optional.ofNullable(user.getPassword()).orElse("123456"));
七、 伪代码举例
// 保存用户
@GetMapping("/user/{userid}")
public User getUser(@PathVariable("userid") Integer userId){
User user = userService.getById(userId);
user = Optional.ofNullable(user).orElseThrow(()->new RuntimeException("用户找不到!!") );
return user;
}
// 保存用户 mybatis
@PostMapping("/user/saveupdate")
public void saveupdateUser(@RequestBody User user){
User user1 = userService.getById(user.getId());
user1 = Optional.ofNullable(user1).orElse(new User());
if(!Optional.ofNullable(user1.getId()).isPresent()){
user1.setNickname(Optional.ofNullable(user.getNickname()).orElse("学生"));
user1.setPassword(Optional.ofNullable(user.getPassword()).orElse("123456"));
userservice.save(user1);
}else{
userservice.update(user);
}
}
八、 of 和offNullAble区别
Optional<String> fullName = Optional.of(null);
Optional<String> fullName2 = Optional.ofNullable(null);
-
of()和ofNullable() 它们的目标都是一致
- 两者都是去创建Optional对象
- 两者都是给value成员变量赋值
-
为什么 of 如果传递null的时候，会报空指针异常呢？是因为底层在创建Optional对象的时候，如果value是null，就出现空指针异常。同时告诉你一个道理，of只能去处理那些非空对象。
private Optional(T value) { this.value = Objects.requireNonNull(value); }public static <T> T requireNonNull(T obj) { if (obj == null) throw new NullPointerException(); // 如果value是null，就出现空指针异常 return obj; }
九、 Optional中filter ,map,flatmap的认识
- filter ,map,flatmap它们是属于java.util.Stream的中间方法。但是这里的filter方法和前面的stream流中间无关。
- 个人的理解不应该出现，因为会给很多的初学者造成错了现象。
- 它在Optional中的存在，其实还是去解决一个程序开发过程中集合为空问题。
- 可以达到，集合中的元素如果不为空，我就直接filter/map/flatmap处理，如果为空给要么抛出异常。要么给默认初始化
package com.kuangstudy.demo09;
import com.kuangstudy.demo08.User;
import java.util.ArrayList;
import java.util.List;
import java.util.Optional;
import java.util.function.Predicate;
import java.util.stream.Collectors;
import java.util.stream.Stream;
public class OptionnalDemo3 {
public static void main(String[] args) {
List<User> users = null;
users = Optional.ofNullable(users) // 1： 创建一个Optional，同时给Optional的value赋值null
.filter((u) -> u.get(0).getId().equals(1))// 2: 如果你调用的Optional中filter，
// 如果内部出现异常的话，直接放一个空的Optional对象
.orElse(new ArrayList<>()); // 3: 如果出现
System.out.println(users);
//List<User> collect = users.stream().filter(u -> u.getId() > 0).collect(Collectors.toList());
}
}