package jdk180reduce;

import java.util.ArrayList;

import java.util.HashMap;

import java.util.List;

import java.util.Map;

import java.util.Optional;

import test.Teacher;

import test.User;

public class optional {

    /**

     * 1.Optional的用法，可以，用了reduce计算，也可以不判断非null

     * 2.三个方法：Optional.empty(), Optional.of,   Optional.ofNullable

     * 3.orElse()用法

     * 4.ifPresent()区别isPresent()

     * 5.orElse和orElseGet()不同*/

    public static void main(String[] args) {

        emptyOptional();

        orelse();

        orelseDifferent();

        whenMap_thenOk();

         List<Student> stuList = new ArrayList<>(4);

         stuList.add(new Student("刘一", 59));

         stuList.add(new Student("陈二", 58));

         stuList.add(new Student("张三", 98));

         stuList.add(new Student("吴九", 100));

        //计算分数在60分一下的分数总和    注意：以前没有60分以下的，不加判断就会出现空指针异常: score.orElse(0)= 有值取值，无值为0

        Optional<Integer> score = stuList.stream().map(Student :: getScore).filter(s -> s<60).reduce((a,b) -> a+b);

        System.out.println("score="+score.orElse(0));   //

        Map<Integer,String> map = new HashMap<>();

        map.put(20180001,"章子");

        map.put(20180002,"小米");

        map.put(20180003,"大黄");

        String name = Optional.ofNullable(map.get(20180001)).orElse("无");

        System.out.println(name);  //无

    }

    public static void emptyOptional() {

//            以下会导致空指针：No value present

//        Optional<User> emptyOpt = Optional.empty();

//        emptyOpt.get();

        //当用.of的时候必须不为null，否则也会报空指针

        User user = new User();

        Optional<User> opt = Optional.of(user);

        opt.get();

        //如果不确定传入的值是否为null，用ofNullable

        Object opf = Optional.ofNullable(null).orElse("null");

        System.out.println(opf);

        //ofNullable方法也要isPresent判断是否有值，或者用.orElse赋值

        String name = "John";

        Optional<String> opu = Optional.ofNullable(null);

        if (opu.isPresent()) {

            System.out.println(opu.get());

        } else {

            System.out.println("opu无值");

        }

        //ifPresent区别isPresent，是用labam表达式

        Optional<String> opp = Optional.ofNullable(name);

        opp.ifPresent(it -> System.out.println(opp.get()));

    }

    public static void orelse() {

        //orElse:如果有值则返回该值user，否则返回传递给它的参数值user2：

        User user = null;

        User user2 = new User();

        user2.setUserId(2);

        User result = Optional.ofNullable(user).orElse(user2);

        System.out.println("result="+result);

    }

    /**

     * orElse和orElseGet()不同

     */

    public static void orelseDifferent() {

        User user = null;

        System.out.println("1----Using orElse");

        User result = Optional.ofNullable(user).orElse(createNewUser());

        System.out.println("1----Using orElseGet");

        User result2 = Optional.ofNullable(user).orElseGet(() -> createNewUser());

        User user4 = new User();

        user4.setAge(2);

        System.out.println("2----Using orElse");

        User result4 = Optional.ofNullable(user4).orElse(createNewUser());

        System.out.println("result4====="+result4);

        System.out.println("2----Using orElseGet");

        User result3 = Optional.ofNullable(user4).orElseGet(() -> createNewUser());

        System.out.println("result3===="+result3);

        //结论：两个 Optional对象都包含非空值，两个方法都会返回对应的非空值。

        //不过，orElse() 方法仍然创建了 User 对象。 与之相反，orElseGet() 方法不创建 User 对象。

        //在执行较密集的调用时，比如调用 Web 服务或数据查询，这个差异会对性能产生重大影响。

    }

    private static User createNewUser() {

        System.out.println("Creating New User");

        User user = new User();

        user.setName("java");

        return user;

    }

    public static void whenMap_thenOk() {

        User user = createNewUser();

        User userNull = new User();

        String name = Optional.ofNullable(user).map(u -> u.getName()).orElse("vivi");

        Optional<User> result = Optional.ofNullable(user).filter(u -> u.getName() != null && u.getName().contains("li"));

        result.ifPresent(u -> {

            String myName = result.get().getName();

            //-------------

            System.out.println("过滤值：" + result.get().getName());

        });

        //第二种：

        if (result.isPresent()) {

            System.out.println("过滤值2：" + result.get().getName());

        }

    }

}