Optional的简单使用

public class OptionalDemo {
    public static void main(String[] args) {
        Insurance pacificOcean = new Insurance("太平洋", 3456d);
        Insurance life = new Insurance("人寿", 2456d);
        Car bmw = new Car("BMW", 320000d, null);
        Car audi = new Car("AUDI", 350000d, life);
        User lucy = new User("Lucy", bmw);
        User lily = new User("Lily", null);

        // 1、Optional.of()只能接受非空值，
        Optional<Object> o = Optional.of(null);

        // 2、Lily如果没有车子，默认给她一个audi
        Car car = Optional.ofNullable(lily.getCar()).orElse(audi);
        System.out.println(car);
        // Lucy有一辆BMW车子
        car = Optional.ofNullable(lucy.getCar()).orElse(bmw);
        System.out.println(car);

        // 3、如果 Optional 中有值，则对该值调用 consumer.accept，否则什么也不做。
        Optional<Car> car1 = Optional.ofNullable(lily.getCar());
        if (car1.isPresent()) {
            car1.ifPresent(item -> System.out.println(item.getInsurance()));
        } else {
            System.out.println("Lily没有车子");
        }
        car1 = Optional.ofNullable(lucy.getCar());
        if (car1.isPresent()) {
            car1.ifPresent(item -> System.out.println(item.getInsurance()));
        } else {
            System.out.println("Lucy没有车子");
        }

        // 4、Lily如果没有车子，现在给她指定一辆别克
        car = Optional.ofNullable(lily.getCar()).orElseGet(() -> new Car("BUICK", 230000d, pacificOcean));
        System.out.println(car);

        // 5、Lily没有车子，抱怨一下
        car = Optional.ofNullable(lily.getCar()).orElseThrow(() -> new RuntimeException("别人都有车子，我没有车子"));
        System.out.println(car);

        // 6、如果当前 Optional 为 Optional.empty，则依旧返回 Optional.empty；否则返回一个新的 Optional
        String temp = Optional.ofNullable(lucy).map(User::getCar).map(Car::getInsurance).map(Insurance::getCompany).orElse("没有保险");
        System.out.println(temp);

        // 7、flatMap 方法与 map 方法的区别在于，map 方法参数中的函数 mapper 输出的是值，然后 map 方法会使用 Optional.ofNullable 将其包装为 Optional
        temp = Optional.ofNullable(lucy)
                .flatMap(item -> Optional.ofNullable(item.getCar()))
                .flatMap(item -> Optional.ofNullable(item.getInsurance()))
                .flatMap(item -> Optional.ofNullable(item.getCompany()))
                .orElse("我车都没得，哪里有保险");
        System.out.println(temp);
    }
}

@Data
@AllArgsConstructor
class User {
    private String name;

    private Car car;
}

@Data
@AllArgsConstructor
class Car {
    private String brand;

    private double price;

    private Insurance insurance;
}

@Data
@AllArgsConstructor
class Insurance {
    private String company;

    private double cost;
}