Optional接口简记

Posted 2020-11-23 lothlorien

@Data
public class Employee {
    private String name;
}

@Data
public class Company {

    private String name;

    private List<Employee> employees;
}

/**
 * optional接口, optional的使用都是基于函数式的用法, 不写if-else判断
 * optional 无法序列化, 不要将她作为方法参数 或 类的成员变量, 只用在方法的返回值上,
 * 用来规避空指针异常
 */
public class OptionalTest {

    @Test
    public void test() {
        Optional<String> optional = Optional.of("hello");
        // 无用的使用方式
        if (optional.isPresent()) {
            System.out.println(optional.get());
        }

        // 推荐的使用方式
        optional.ifPresent(item -> System.out.println(item));
    }

    @Test
    public void test2() {
        Optional<String> optional = Optional.empty();
        // 推荐的使用方式, 没有值将不执行传入的方法
        optional.ifPresent(item -> System.out.println(item));
        System.out.println("---------------------------------");

        System.out.println(optional.orElse("world"));
        System.out.println("---------------------------------");

        System.out.println(optional.orElseGet(() ->  "nihao"));
        System.out.println("---------------------------------");

    }

    @Test
    public void Test3() {
        Employee employee = new Employee();
        employee.setName("zhangsan");

        Employee employee2 = new Employee();
        employee2.setName("lisi");

        Company company = new Company();
        company.setName("company");

        List<Employee> employees = Arrays.asList(employee, employee2);
        company.setEmployees(employees);

        List<Employee> list = company.getEmployees();

        // 传统手工艺做法:
        /*
        if (null != list) {
            return list;
        } else {
            return new ArrayList<Employee>();
        }
        */
        // 崭新的函数式风格
        Optional<Company> optional = Optional.ofNullable(company);
        System.out.println(
                optional.map(c -> c.getEmployees()).orElse(Collections.emptyList())
        );
    }
}