import junit.framework.TestCase;

import java.util.ArrayList;

import java.util.HashMap;

import java.util.Optional;

import java.util.stream.Collectors;

public class TestCollect extends TestCase {

class User {

String userName;

String sex;

String idCard;

//省略Getter and Setter and Constructor方法

}

/**

* @param

* @description: 取出集合中User对象的userName和idCard属性取出放在map中

* @return: void

* @author: Sue

* @date: 2020/8/14

*/

public void testCollect() {

User user01 = new User("user01", "male", "000000001");

User user02 = new User("user02", "male", "000000002");

User user03 = new User("user03", "male", "000000003");

User user04 = new User(null, "male", "000000004");

User user05 = new User("user05", "male", null);

ArrayList list = new ArrayList();

list.add(user01);

list.add(user02);

list.add(user03);

list.add(user04);

list.add(user05);

/**

* 当存在value值为空时，使用Collectors.toMap()会报NPE

* 原因：底层调用了Map的merge方法，而map方法规定了此处的vlue不能为null，从而抛出空指针异常

* Objects.requireNonNull(value);

*/

// HashMap map01 = list.stream().collect(Collectors.toMap(User::getUserName, User::getIdCard, (a, b) -> b, HashMap::new));

// System.out.println(map01);

//解决方案一，使用Optional类处理null

HashMap map02 = list.stream().collect(Collectors

.toMap(s -> Optional.ofNullable(s.getUserName()).orElse("空的"), s -> Optional.ofNullable(s.getSex()).orElse("空的"), (a, b) -> b, HashMap::new));

System.out.println(map02);

//解决方案二，直接使用collect()方法进行规约操作，关于这里collect方法的使用可以看这篇文章 https://www.cnblogs.com/sueyyyy/p/13502116.html

HashMap map03 = list.stream().collect(HashMap::new, (map, item) -> map.put(item.getUserName(), item.getSex()), HashMap::putAll);

System.out.println(map03);

}

}

　　

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