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 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());  
    }  
}

}