java8 Optional的使用，Optional实例，使你的代码高大上

Optional是为了解决空指针异常，本质是一个包含可选值的包装类，或者说是一个容器

实例代码

package com.dume.boot.code;


import java.util.Optional;
/**
 * @author dume
 * @create 2021-09-13 10:53
 *
 *  Optional是为了解决空指针异常，本质是一个包含可选值的包装类，或者说是一个容器
 **/


public class OptionalDemo {
    public static void main(String[] args) {
        Student studentnull = null;
        Student student = new Student("john1",11);
        Student student2 = new Student("john2",22);
        Student student3 = new Student("john3",33);
        Student student4 = new Student("john4",44);

        //isPresent判断值不为空，get返回对象
        System.out.println("*******************isPresent判断值不为空，get返回对象**********************");
        Optional<Student> optional = Optional.ofNullable(student);
        if(optional.isPresent()){
            Student back1 = optional.get();
            System.out.println("back1:"+back1.toString());
        }
        System.out.println("*********************ifPresent判断不为空, 并执行Lambda 表达式********************");
        //ifPresent判断不为空，Consumer(消费者) 参数，如果对象不是空的，就执行传入的 Lambda 表达式
        //注意没有返回值
        optional.ifPresent(u->ifFuction(u));

        System.out.println("*********************orElse判断不为空，否则返回默认值********************");
        //orElse判断不为空，否则返回默认值
        Student back2 =  Optional.ofNullable(studentnull).orElse(student2);
        System.out.println("back2:"+back2.toString());

        System.out.println("*******************orElseGet不为空时返回值；为空时作为参数传入函数式接口，并返回执行后的参数**********************");
        //orElseGet不为空时返回值；为空时，他会作为参数传入Supplier(供应者) 函数式接口，并返回执行后的参数
        Student back3 = Optional.ofNullable(studentnull).orElse(createStudent());
        Student back4 = Optional.ofNullable(studentnull).orElseGet(()->createStudent());
        System.out.println("back3:"+back3.toString());
        System.out.println("back4:"+back4.toString());
        /**
         * 可见当都为空时，orElse与orElseGet并无差别
         * 当都不为空时，orElse依然会执行，orElseGet不会执行。所以orElseGet效率更高
         */
        System.out.println("*********************当都不为空时，orElse依然会执行，orElseGet不会执行********************");
        Student back5 = Optional.ofNullable(student).orElse(createStudent());
        Student back6 = Optional.ofNullable(student).orElseGet(()->createStudent());
        System.out.println("back5:"+back5.toString());
        System.out.println("back6:"+back6.toString());

        //orElseThrow当为空时，抛出自定义异常
        System.out.println("*********************orElseThrow当为空时，抛出自定义异常********************");
        try {
            Student back7 = Optional.ofNullable(studentnull).orElseThrow(()-> new IllegalArgumentException());
        }catch (Exception e){
            e.printStackTrace();
        }

        //map转换值,map() 对值调用作为参数的函数，然后将返回的值包装在 Optional
        System.out.println("*********************map转换值********************");
        String back8 = Optional.ofNullable(studentnull).map(u->u.getName()).orElse("DEFUAFT");
        String back9 = Optional.ofNullable(studentnull).map(Student::getName).orElse("DEFUAFT");
        System.out.println("back8:"+back8);
        System.out.println("back9:"+back9);

        //flatMap() 也需要函数作为参数，并对值调用这个函数，然后直接返回结果;其实就是需要传入Optional类型参数，并返回Optional类型
        System.out.println("********************flatMap()转换值 需要传入Optional类型*********************");
        student.setPosition("Position");
        String position = Optional.ofNullable(student)
                .flatMap(u -> u.getPosition()).orElse("defaultPosition");
        System.out.println("back10:"+position);

        //filter() 接受一个 Predicate 参数，返回测试结果为 true 的值。
        // 如果测试结果为 false，会返回一个空的 Optional。
        System.out.println("*******************filter() 接受判断表达式，返回判断结果**********************");
        Optional<Student> optional11 = Optional.ofNullable(student).filter(u->u.getName()!=null&&u.getName().contains("1"));
        System.out.println("back11:"+optional11.isPresent());

        //使用Optional避免空指针来取值
        System.out.println("*******************使用Optional避免空指针来取值**********************");
        String code = Optional.ofNullable(new User())
                .flatMap(User::getAddress)
                .flatMap(Address::getCountry)
                .map(Country::getCode)
                .orElse("defaultcode");
        System.out.println("back12:"+code);
        System.out.println("*****************************************");
    }





    public static Student createStudent(){
        System.out.println("CreateStudent loading......");
        return new Student("Createjohn",0);
    }
    public static  void  ifFuction(Student student){
        System.out.println("ifFuction loading......");

    }

    static  class Student{
        public  String name;
        public  int age;
        public  String position;

        @Override
        public String toString() {
            return "Student{" +
                    "name='" + name + '\'' +
                    ", age=" + age +
                    ", position=" + position +
                    '}';
        }

        public Student(String name, int age) {
            this.name = name;
            this.age = age;
        }

        public void setPosition(String position) {
            this.position = position;
        }

        public Optional<String> getPosition() {
            return Optional.ofNullable(position);
        }

        public String getName() {
            return name;
        }

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

        public int getAge() {
            return age;
        }

        public void setAge(int age) {
            this.age = age;
        }


    }

    static  class User{
        Address address;


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

    }
    static  class Address{
        Country country;
        public Optional<Country> getCountry() {
            return Optional.ofNullable(country);
        }

    }
    static class Country{
        String code;

        public String getCode() {
            return code;
        }

        public void setCode(String code) {
            this.code = code;
        }
    }




}

打印

"C:\Program Files\Java\jdk1.8.0_92\bin\java.exe" -agentlib:jdwp=transport=dt_socket,address=127.0.0.1:60721,suspend=y,server=n -javaagent:C:\Users\du'me\.IntelliJIdea2019.2\system\captureAgent\debugger-agent.jar -Dfile.encoding=UTF-8 -classpath "C:\Program Files\Java\jdk1.8.0_92\jre\lib\charsets.jar;C:\Program Files\Java\jdk1.8.0_92\jre\lib\deploy.jar;C:\Program Files\Java\jdk1.8.0_92\jre\lib\ext\access-bridge-64.jar;C:\Program Files\Java\jdk1.8.0_92\jre\lib\ext\cldrdata.jar;C:\Program Files\Java\jdk1.8.0_92\jre\lib\ext\dnsns.jar;C:\Program Files\Java\jdk1.8.0_92\jre\lib\ext\jaccess.jar;C:\Program Files\Java\jdk1.8.0_92\jre\lib\ext\jfxrt.jar;C:\Program Files\Java\jdk1.8.0_92\jre\lib\ext\localedata.jar;C:\Program Files\Java\jdk1.8.0_92\jre\lib\ext\nashorn.jar;C:\Program Files\Java\jdk1.8.0_92\jre\lib\ext\sunec.jar;C:\Program Files\Java\jdk1.8.0_92\jre\lib\ext\sunjce_provider.jar;C:\Program Files\Java\jdk1.8.0_92\jre\lib\ext\sunmscapi.jar;C:\Program 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Files\JetBrains\IntelliJ IDEA 2019.2.4\lib\idea_rt.jar" com.dume.boot.code.OptionalDemo
Connected to the target VM, address: '127.0.0.1:60721', transport: 'socket'
*******************isPresent判断值不为空，get返回对象**********************
back1:Student{name='john1', age=11, position=null}
*********************ifPresent判断不为空, 并执行Lambda 表达式********************
ifFuction loading......
*********************orElse判断不为空，否则返回默认值********************
back2:Student{name='john2', age=22, position=null}
*******************orElseGet不为空时返回值；为空时作为参数传入函数式接口，并返回执行后的参数**********************
CreateStudent loading......
CreateStudent loading......
back3:Student{name='Createjohn', age=0, position=null}
back4:Student{name='Createjohn', age=0, position=null}
*********************当都不为空时，orElse依然会执行，orElseGet不会执行********************
CreateStudent loading......
back5:Student{name='john1', age=11, position=null}
back6:Student{name='john1', age=11, position=null}
*********************orElseThrow当为空时，抛出自定义异常********************
java.lang.IllegalArgumentException
	at com.dume.boot.code.OptionalDemo.lambda$main$3(OptionalDemo.java:58)
	at java.util.Optional.orElseThrow(Optional.java:290)
	at com.dume.boot.code.OptionalDemo.main(OptionalDemo.java:58)
*********************map转换值********************
back8:DEFUAFT
back9:DEFUAFT
********************flatMap()转换值 需要传入Optional类型*********************
back10:Position
*******************filter() 接受判断表达式，返回判断结果**********************
back11:true
*******************使用Optional避免空指针来取值**********************
back12:defaultcode
*****************************************
Disconnected from the target VM, address: '127.0.0.1:60721', transport: 'socket'

Process finished with exit code 0