Question

我的课堂定义如下。

Insurance.java

package Chapter10;

public class Insurance {

    private String name;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
}

Car.java

package Chapter10;

import java.util.Optional;

public class Car {

    private Optional<Insurance> insurance;

    public Optional<Insurance> getInsurance() {
        return insurance;
    }

    public void setInsurance(Optional<Insurance> insurance) {
        this.insurance = insurance;
    }
}

Person.java

package Chapter10;

import java.util.Optional;

public class Person {

    private Optional<Car> car;

    public Optional<Car> getCar() {
        return car;
    }

    public void setCar(Optional<Car> car) {
        this.car = car;
    }
}

现在，我有下面的主类，但是我收到了NullPoinerException 为人p。

package Chapter10;

import java.util.Optional;

public class OptionalTester {

    public static String getCarInsuranceName(Optional<Person> person) {
        return person.flatMap(Person::getCar)
                .flatMap(Car::getInsurance)
                .map(Insurance::getName)
                .orElse("Unknown");
    }

    public static void main(String[] args) {

        Insurance iffco = new Insurance();
        iffco.setName("IFFCO TOKIO");

        Insurance argo = new Insurance();
        argo.setName("HDFC ARGO");

        Car i10 = new Car();
        i10.setInsurance(Optional.of(iffco));

        Car santro = new Car();

        Person p = new Person();
        p.setCar(Optional.of(santro));

        Person p1 = new Person();
        p1.setCar(Optional.of(i10));

        System.out.println("---------------FROM PERSON GET INSURANCE----------------------");

        Optional<Insurance> optInsurance = Optional.ofNullable(argo);
        Optional<String> name = optInsurance.map(Insurance::getName);


        System.out.println(getCarInsuranceName(Optional.of(p1)));
        System.out.println(getCarInsuranceName(Optional.of(p)));

    }
}

我在Optional.of（p）行中获得了NPE，我知道p1有一辆汽车，那辆汽车有保险，因此可以正常工作。

在p的情况下，没有保险，所以我得到了NPE。但是，如果我使用Optional.empty（）执行此操作，则可以正确获取 Unknown 作为输出。

如何更概括地说，如果发生任何问题，我会得到未知？

Answer 1

基本上，简短的回答是“你不能”。原因是Optional应该是monad（一个高阶对象，可以使您将某些操作包装在一个特定的容器中，简称），但是在Java中则不是。因此，flatMap无法正确链接。对于更长的讨论，您可能希望查看https://www.sitepoint.com/how-optional-breaks-the-monad-laws-and-why-it-matters/

Answer 2

在Santro中，汽车保险无效。

import java.util.Optional;

公共类OptionalTester {

public static String getCarInsuranceName(Optional<Person> person) {
    return person.flatMap(Person::getCar)
            .flatMap(Car::getInsurance)
            .map(Insurance::getName)
            .orElse("Unknown");
}

public static void main(String[] args) {

    Insurance iffco = new Insurance();
    iffco.setName("IFFCO TOKIO");

    Insurance argo = new Insurance();
    argo.setName("HDFC ARGO");

    Car i10 = new Car();
    i10.setInsurance(Optional.of(iffco));

    Car santro = new Car();
    santro.setInsurance(Optional.of(argo)); \\ Insurence was null.

    Person p = new Person();
    p.setCar(Optional.of(santro));

    Person p1 = new Person();
    p1.setCar(Optional.of(i10));

    System.out.println("---------------FROM PERSON GET INSURANCE----------------------");

    Optional<Insurance> optInsurance = Optional.ofNullable(argo);
    Optional<String> name = optInsurance.map(Insurance::getName);


    System.out.println(getCarInsuranceName(Optional.of(p1)));
    System.out.println(getCarInsuranceName(Optional.of(p)));

}

}

Answer 3

换车应该可以。

公共课车{

    private Insurance insurance;

    public Optional<Insurance> getInsurance() {
       return Optional.ofNullable(insurance);
    }

    public void setInsurance(Optional<Insurance> insurance) {
        this.insurance = insurance.orElse(null);
    }
}

如何为对象链制作flatMap链？

3 个答案: