Stream使用Collector.tomap方法value值为null时报空指针异常解决方案

柔情只为你懂 2024-03-27 10:38 124阅读 0赞

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<User> 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<String, String> 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<String, String> 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<String, String> map03 = list.stream().collect(HashMap::new, (map, item) -> map.put(item.getUserName(), item.getSex()), HashMap::putAll);
            System.out.println(map03);
        }
    }