functional-programming-2-optional.md

函数式编程2-Optional

本节我们讲一下Optional的使用，Optional主要用来对null的处理，我们将常规需要判断null的对象用Optional包装起来。

1.获取Optional对象的静态方法

• Optional.of(value):value必须不能为空,

示例:Optional<String> firstName = Optional.of(person.getFirstName());

• Optional.ofNullable(value):value可以为空

示例：Optional<String> middleName = Optional.ofNullable(person.getMiddleName());

2.Optional对象的主要方法

• get()

T get()：获得Optional中的对象包装的value,T为value的类型.

示例：String firstNameGreetings = firstNameOptional.get()

• isPresent()

boolean isPresent():判断Optional中的value是否存在，返回值是boolean.

示例：boolean isPresent = middleNameOptional.isPresent()

• ifPresent

ifPresent(Consumer<? super T> consumer)：如果Optional中的value存在，执行什么操作. 对比《函数式编程1-Lambda表达式》的Function和BiFunction，这里的Consumer<T>接口接受一个入参T类型，没有返回值.

示例：

firstNameOptional.ifPresent(name -> {
                            String firstNameGreetings = firstNameOptional.get().toUpperCase();
                            log.info("hello " + firstNameGreetings);
                          })

• map()

Optional<U> map(Function<? super T, ? extends U> mapper): 对Optional中的value(类型为T)进行处理并返回一个新的Optional<U>,U为返回值类型.

示例:

Optional<String> lastNameOptionalNew = lastNameOptional
								.map(name ->  name.toUpperCase());

• orElse()

T orElse(T other)：和get()一样获得value的值，区别是有value的时候返回value，没有value的时候返回other.

示例

String middleNameGreetings = middleNameOptional
					.map(name ->  name.toUpperCase())
					.orElse("no middle name")

3.完整示例

• Person

@Data
public class Person {

    private String firstName;

    private String middleName;

    private String lastName;

}

@Bean
CommandLineRunner testOptional(){
    return p -> {
        Person person = new Person();

        person.setFirstName("Will");
        Optional<String> firstNameOptional = Optional.of(person.getFirstName());
        firstNameOptional.ifPresent(name -> {
                                        String firstNameGreetings = firstNameOptional.get().toUpperCase();
                                        log.info("hello " + firstNameGreetings);
                                      });



        Optional<String> middleNameOptional = Optional.ofNullable(person.getMiddleName());
        String middleNameGreetings = middleNameOptional
                .map(name ->  name.toUpperCase())
                .orElse("no middle name");
        log.info("hello " + middleNameGreetings);

        log.info(new Boolean(middleNameOptional.isPresent()).toString());


        person.setLastName("Smith");
        Optional<String> lastNameOptional = Optional.ofNullable(person.getLastName());
        Optional<String> lastNameOptionalNew = lastNameOptional
                                        .map(name ->  name.toUpperCase());
        log.info("hello " + lastNameOptionalNew.get());



    };
}