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if(user !=null){
    Address address = user.getAddress();
    if(address != null){
        PhoneNumber phoneNumber = address.getPhoneNumber();
        if(phoneNumber != null){
            log.info("用户的电话号码={}", phoneNumber.getNumber());
        }
    }
}

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import java.util.Optional;

public class PhoneNumberExtractor {

    public final static Integer extract(User user) {
        return Optional.ofNullable(user)
                // address 为 null 时，可以返回默认值 0
                .map(User::getAddress).orElse(Optional.empty())
                // phoneNumber 为 null 时，抛出空指针异常
                .flatMap(Address::getPhoneNumber)
                .map(PhoneNumber::getNumber)
                .orElse(0);
    }
}

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import java.util.Optional;

import static org.assertj.core.api.AssertionsForClassTypes.assertThat;

class PhoneNumberExtractorTest {

    @Test
    void return_0_when_user_is_null() {
        Integer phoneNumber = PhoneNumberExtractor.extract(null);

        assertThat(phoneNumber).isEqualTo(0);
    }

    @Test
    void return_0_when_address_is_null() {
        Integer phoneNumber = PhoneNumberExtractor.extract(User.builder().address(null).build());

        assertThat(phoneNumber).isEqualTo(0);
    }

    @Test
    void throws_NullPointerException_when_phone_number_is_null() {
        Optional<Address> address = Optional.of(Address.builder().phoneNumber(null).build());
        User user = User.builder().address(address).build();

        Assertions.assertThrows(NullPointerException.class, () -> PhoneNumberExtractor.extract(user));
    }
}

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import lombok.Builder;
import lombok.Getter;

import java.util.Optional;

@Builder
@Getter
public class User {
    private Optional<Address> address;
}

@Builder
@Getter
public class Address {
    private Optional<PhoneNumber> phoneNumber;
}

@Builder
@Getter
public class PhoneNumber {
    private Integer number;
}