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計算，也可以不判斷非null

     * 2.三個方法：Optional.empty(), Optional.of,   Optional.ofNullable
 

     * 3.orElse()用法

     * 4.ifPresent()區別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("劉一", 59));
 

         stuList.add(new Student("陳二", 58));

         stuList.add(new Student("張三", 98));

         stuList.add(new Student("吳九", 100));

        //計算分數在60分一下的分數總和    注意：以前沒有60分以下的，不加判斷就會出現空指標異常: score.orElse(0)= 有值取值，無值為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,"大黃");

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

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

    }

    public static void emptyOptional() {

//            以下會導致空指標：No value present

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

//        emptyOpt.get();

        //當用.of的時候必須不為null，否則也會報空指標

        User user = new User();

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

        opt.get();

        //如果不確定傳入的值是否為null，用ofNullable

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

        System.out.println(opf);

        //ofNullable方法也要isPresent判斷是否有值，或者用.orElse賦值

        String name = "John";

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

        if (opu.isPresent()) {

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

        } else {

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

        }

        //ifPresent區別isPresent，是用labam表示式

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

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

    }

    public static void orelse() {

        //orElse:如果有值則返回該值user，否則返回傳遞給它的引數值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);

        //結論：兩個 Optional物件都包含非空值，兩個方法都會返回對應的非空值。

        //不過，orElse() 方法仍然建立了 User 物件。 與之相反，orElseGet() 方法不建立 User 物件。

        //在執行較密集的呼叫時，比如呼叫 Web 服務或資料查詢，這個差異會對效能產生重大影響。

    }

    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("過濾值：" + result.get().getName());

        });

        //第二種：

        if (result.isPresent()) {

            System.out.println("過濾值2：" + result.get().getName());

        }

    }

}