Optional 教程

1、isPresent

使用isPresent方法来判断非空，isPresent相当于!=null
isPresent 返回一个boolean

Optional<Student> optional = Optional.ofNullable(new Student("王五", 80));
        if (optional.isPresent()){
            // 将输出：student1不为空的操作
            System.out.println("student不为空");
        } else {
            System.out.println("student is 空");
        }

2、ifPresent

ifPresent 相当于相判断 !=null,不为空才执行里面的代码:

 Optional<Student> optional = Optional.ofNullable(new Student("王五", 80));
 optional.ifPresent(student -> System.out.println("学生姓名:"+ student.getName()));

3、获取数据

1. get 如果为空会抛出NoSuchElementException异常，使用前 isPresent
2. orElse 接受T类型的参数
3. orElseGet 接收Supplier类型的参数

// optional不为空则返回student1给 student3，否则返回student2给student3
Student student1 = new Student("王五", 80);
Student student2 = new Student("张三", 90);
Optional<Student> optional = Optional.ofNullable(student1);
Student student3 = optional.orElseGet(()->student2)

4、map

map将谓词作为参数并返回一个Optional对象

List<String> companyNames = Arrays.asList("paypal", "oracle", "", "microsoft", "", "apple");
Optional<List<String>> listOptional = Optional.of(companyNames);
//获取companyNames的长度
int size = listOptional.map(List::size).orElse(0);
System.out.println(size);//6

获取学生名称大写

 Student student1 = new Student("aaa", 80);
 String stName = Optional.ofNullable(student1).map(s -> s.getName().toUpperCase()).orElse(null);

5、filter

filter 将谓词作为参数并返回一个Optional对象

Integer year = 2022;
boolean is2022 = Optional.ofNullable(year).filter(y -> y == 2022).isPresent();
System.out.println(is2022);//true

双重判断
判断价格是否在在10到15

Optional.ofNullable(modem)
                .map(Modem::getPrice)
                .filter(p -> p >= 10)
                .filter(p -> p <= 15)
                .isPresent();