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

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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);     } }