Optional类如何解决NullPointerException异常

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。

1.empty

static <T> Optional<T> empty()
//返回空的 Optional 实例。

Optional<Insurance> insuranceOptional = Optional.empty();
insuranceOptional.get();

//使用get方法，会抛出java.util.NoSuchElementException: No value present

2.of

static <T> Optional<T> of(T value)
//返回一个指定非null值的Optional。

Optional<Insurance> insuranceOptional = Optional.of(new Insurance());
insuranceOptional.get();

//使用get方法，不会抛出异常

3.ofNullable

static <T> Optional<T> ofNullable(T value)
//如果为非空，返回 Optional 描述的指定值，否则返回空的 Optional。
//相当于empty+of

Optional<Insurance> insuranceOptional = Optional.ofNullable(null); --Optional.empty()
Optional<Insurance> insuranceOptional = Optional.ofNullable(new Insurance());

4.toString()

String toString()
返回一个Optional的非空字符串，用来调试

Optional<Insurance> insuranceOptional = Optional.ofNullable(new Insurance());
System.out.println(insuranceOptional.toString());
//输出：Optional[Insurance{name='picc'}]

5.equals

boolean equals(Object obj)
判断其他对象是否等于 Optional。

Insurance insurance =  new Insurance();
Optional<Insurance> insuranceOptional1 = Optional.ofNullable(insurance);
Optional<Insurance> insuranceOptional2 = Optional.ofNullable(insurance);

System.out.println(insuranceOptional1.equals(insuranceOptional2));
//输出：true

6.filter

Optional<T> filter(Predicate<? super <T> predicate)
//如果值存在，并且这个值匹配给定的 predicate，返回一个Optional用以描述这个值，
//否则返回一个空的Optional。

Optional<Insurance> insuranceOptional = Optional.of(new Insurance());
Insurance insurance = insuranceOptional.filter(i -> i.getName() != null).get();
System.out.println(insurance);
//输出：Insurance{name='picc'}

Optional<Insurance> insuranceOptional = Optional.of(new Insurance());
Insurance insurance = insuranceOptional.filter(i -> i.getName() == null).get(); 
//输出：java.util.NoSuchElementException: No value present       

7.orElse, orElseGet, orElseThrow

T orElse(T other)
如果存在该值，返回值， 否则返回 other。

T orElseGet(Supplier<? extends T> other)
如果存在该值，返回值， 否则触发 other，并返回 other 调用的结果。

<X extends Throwable> T orElseThrow(Supplier<? extends X> exceptionSupplier)
如果存在该值，返回包含的值，否则抛出由 Supplier 继承的异常

Optional<Insurance> insuranceOptional = Optional.of(new Insurance("PICC"));
Insurance insurance = insuranceOptional.orElse(new Insurance("AIX"));
System.out.println(insurance);
//输出：Insurance{name='PICC'}

Optional<Insurance> insuranceOptional = Optional.ofNullable(null);
Insurance insurance = insuranceOptional.orElse(new Insurance("AIX"));
System.out.println(insurance);
//输出：Insurance{name='AIX'}
 
Optional<Insurance> insuranceOptional = Optional.ofNullable(null);
Insurance insurance = insuranceOptional.orElseGet(()->new Insurance("PICC"));
System.out.println(insurance);
//输出：Insurance{name='PICC'} 

Optional<Insurance> insuranceOptional = Optional.ofNullable(null);
Insurance insurance = insuranceOptional.orElseThrow(() -> new RuntimeException("Not have reference"));
System.out.println(insurance);
//输出：java.lang.RuntimeException: Not have reference 

8.isPresent

boolean isPresent()
//如果值存在则方法会返回true，否则返回 false。

Optional<Insurance> insuranceOptional = Optional.ofNullable(null);
System.out.println(insuranceOptional.isPresent());
//输出: false

Insurance insurance =  new Insurance("PICC");
Optional<Insurance> insuranceOptional = Optional.ofNullable(insurance);
System.out.println(insuranceOptional.isPresent());
//输出: true

9.ifPresent

void ifPresent(Consumer<? super T> consumer)
//如果值存在则使用该值调用 consumer , 否则不做任何事情。

Insurance insurance =  new Insurance("PICC");
Optional<Insurance> insuranceOptional = Optional.ofNullable(insurance);
insuranceOptional.ifPresent(System.out::println);
//输出: Insurance{name='PICC'}

10.map

<U>Optional<U> map(Function<? super T,? extends U> mapper)
//如果有值，则对其执行调用映射函数得到返回值。如果返回值不为 null，
//则创建包含映射返回值的Optional作为map方法返回值，否则返回空Optional。

private static String getInsuranceName(Insurance insurance) {
        if (null == insurance)
            return "unknown";
        return insurance.getName();
}

private static String getInsuranceNameByOptional(Insurance insurance) {
    return Optional.ofNullable(insurance).map(Insurance::getName).orElse("unknown");
}

11.flatmap

<U> Optional<U> flatMap(Function<? super T,Optional<U>> mapper)
//如果值存在，返回基于Optional包含的映射方法的值，否则返回一个空的Optional

• map中获取的返回值自动被Optional包装,即返回值 -> Optional<返回值>
• flatMap中返回值保持不变,但必须是Optional类型,即Optional<返回值> -> Optional<返回值>

class FlightTicketInfo {

    private String orderNumber;

    public String getOrderNumber() {
        return orderNumber;
    }

}


public class OptionalTest {

    @Test
    public void testMap() {
        FlightTicketInfo flightTicketInfo = null;

        Optional<Optional<String>> s1 = Optional.ofNullable(flightTicketInfo).map(OptionalTest::getOrderNumber);

        Optional<String> s2 = Optional.ofNullable(flightTicketInfo).map(FlightTicketInfo::getOrderNumber);

        Optional<String> s3 = Optional.ofNullable(flightTicketInfo).flatMap(OptionalTest::getOrderNumber);
    }

    private static Optional<String> getOrderNumber(FlightTicketInfo flightTicketInfo) {
        return Optional.ofNullable(flightTicketInfo).map(f -> f.getOrderNumber());
    }

}

本文地址：https://blog.csdn.net/weixin_42868638/article/details/110230138