java 8 stream java.lang.NullPointerException

Java8 Stream 操作 Collectors.toMap(）会出现NullPointerException异常_miskss的博客-CSDN博客_java.lang.nullpointerexception: the mono returned
https://blog.csdn.net/wanping321/article/details/97389347

java - Streams: Avoid NullPointerException - Stack Overflow
https://stackoverflow.com/questions/39617625/streams-avoid-nullpointerexception

Avoid NullPointerException in Java 9 with Stream.ofNullable - TURRETA.COM
https://turreta.com/2017/07/11/avoid-nullpointerexception-in-java-9-with-stream-ofnullable/

避免在Java中检查Null语句 Baeldung
https://www.baeldung.com/java-avoid-null-check#IDE contracts

Java 8 NullPointerException in Collectors.toMap - Intellipaat Community
https://intellipaat.com/community/24895/java-8-nullpointerexception-in-collectors-tomap

Java 8 NullPointerException in Collectors.toMap - ExceptionsHub
https://exceptionshub.com/java-8-nullpointerexception-in-collectors-tomap.html