如何在基于可选值调用空方法时使用optional的map和orElseGet方法

import java.util.Optional;

class Person {
    private Optional<Address> address;

    public Person(Optional<Address> address) {
        this.address = address;
    }

    public Optional<Address> getAddress() {
        return address;
    }
}

class Address {
    private Optional<String> city;

    public Address(Optional<String> city) {
        this.city = city;
    }

    public Optional<String> getCity() {
        return city;
    }
}

public class OptionalExample {
    public static void main(String[] args) {
        // 创建一个Person实例，其地址和城市都存在
        Person personWithCity = new Person(Optional.of(new Address(Optional.of("New York"))));
        
        // 创建一个Person实例，其地址存在但城市不存在
        Person personWithoutCity = new Person(Optional.of(new Address(Optional.empty())));
        
        // 创建一个Person实例，其地址不存在
        Person personWithoutAddress = new Person(Optional.empty());

        // 使用map和orElseGet方法安全地获取城市名称
        String cityName1 = personWithCity.getAddress()
                                       .map(Address::getCity)
                                       .map(city -> city.orElseGet(() -> "Unknown City"))
                                       .orElseGet(() -> "Unknown City");
        System.out.println(cityName1); // 输出: New York

        String cityName2 = personWithoutCity.getAddress()
                                            .map(Address::getCity)
                                            .map(city -> city.orElseGet(() -> "Unknown City"))
                                            .orElseGet(() -> "Unknown City");
        System.out.println(cityName2); // 输出: Unknown City

        String cityName3 = personWithoutAddress.getAddress()
                                                .map(Address::getCity)
                                                .map(city -> city.orElseGet(() -> "Unknown City"))
                                                .orElseGet(() -> "Unknown City");
        System.out.println(cityName3); // 输出: Unknown City
    }
}