一、例子
package com.qiang.test.service;
import com.qiang.test.service.entity.UserInfo;
import com.qiang.test.service.exception.BusinessException;
import org.junit.jupiter.api.Test;
import java.util.Optional;
import static jdk.nashorn.internal.runtime.regexp.joni.Config.log;
/**
* @author: 小强崽
* @create: 2021-06-12 22:24
* @description: 对象判空
*/
public class OptionalTest {
UserInfo userInfo = null;
@Test
public void test1() {
// 一般写法
if (userInfo == null) {
log.println("用户信息为空");
} else {
log.println(userInfo);
}
// ofNullable 不会抛出空指针异常
Optional.ofNullable(userInfo);
// of 会抛出空指针异常
Optional.of(userInfo);
}
@Test
public void test2() {
// orElse 当userInfo为空和不为空时都会去执行后面的方法
UserInfo orElseUserInfo = Optional.ofNullable(userInfo).orElse(createUserInfo());
log.println(orElseUserInfo);
// orElseGet 当userInfo只为空时才会去执行后面的方法
userInfo = null;
UserInfo orElseGetUserInfo = Optional.ofNullable(userInfo).orElseGet(() -> createUserInfo());
log.println(orElseGetUserInfo);
// orElseThrow 当 userInfo 为空时抛出异常
Optional.ofNullable(userInfo).orElseThrow(() -> new BusinessException("用户信息不存在"));
}
@Test
public void test3() {
// map 判空后获取实体的某个值
userInfo = createUserInfo();
String nickName = Optional.ofNullable(userInfo).map(userInfo -> userInfo.getNickname()).get();
log.println(nickName);
}
@Test
public void test4() {
// flatMap 判空后获取实体的某个值，但是要求实体的get方法为
// public Optional<String> getNickname() {
// return Optional.ofNullable(nickname);
// }
userInfo = createUserInfo();
// String nickName = Optional.ofNullable(userInfo).flatMap(userInfo -> userInfo.getNickname()).get();
// log.println(nickName);
}
@Test
public void test5() {
userInfo = createUserInfo();
// isPresent 判断 userInfo 是否不为空
boolean present = Optional.ofNullable(userInfo).isPresent();
log.println(present);
// isPresent 判断 userInfo 是否不为空 后做一些操作
Optional.ofNullable(userInfo).ifPresent(userInfo -> {
log.println(userInfo.getNickname());
});
}
@Test
public void test6() {
// userInfo = createUserInfo();
// filter 如果包含的值满足条件，那么还是返回这个 Optional；否则返回 Optional.empty。
Optional<UserInfo> optional = Optional.ofNullable(userInfo).filter(userInfo -> userInfo.getNickname().equals("小强崽"));
// 结果1 Optional[UserInfo(id=1, nickname=小强崽, username=xiaoqiang, password=123456, salt=null, gender=null, portrait=null, phone=null, mail=null, card=null, role=null, address=null, status=null, createTime=null, updateTime=null, deleteTime=null)]
// 结果2 Optional.empty
log.println(optional);
}
@Test
public void test7() throws Exception {
userInfo = createUserInfo();
// 获取用户密码后，再获取密码的hashCode，适用获取对象里面的对象
int hashCode = Optional.ofNullable(userInfo)
.map(u -> u.getPassword())
.map(p -> p.hashCode())
.orElseThrow(() -> new Exception("取指错误"));
log.println(hashCode);
}
@Test
public void test8() {
// 不注释返回id为1的用户信息; 注释则返回id为2的用户信息
userInfo = createUserInfo();
// 如果不为空，则返回符合条件的对象; 如果为空则返回创建的对象
UserInfo result = Optional.ofNullable(userInfo).filter(userInfo -> userInfo.getNickname().equals("小强崽")).orElseGet(() -> {
return new UserInfo()
.setId(2)
.setUsername("xiaoqiang")
.setNickname("小强崽")
.setPassword("123456");
});
log.println(result);
}
public UserInfo createUserInfo() {
return new UserInfo()
.setId(1)
.setUsername("xiaoqiang")
.setNickname("小强崽")
.setPassword("123456");
}
}
二、源码
/*
* 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.
*
* <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.
*/
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";
}
}
