java8 Optional 类

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());
        }
    }

}