XXX.stream().collect(Collectors.groupingby(E:: id));

1. http://nilhcem.com/FakeSMTP/

• nio

1. https://docs.oracle.com/javase/8/docs/technotes/guides/io/
2. https://docs.oracle.com/javase/tutorial/essential/io/fileio.html
3. https://www.cnblogs.com/shawshawwan/p/10027679.html
4. https://blog.csdn.net/qq_37267015/article/details/79015278
5. http://tutorials.jenkov.com/java-nio/index.html

• java.util.function
  • Function
  • BiFunction

• BiConsumer
• BinaryOperator
• Function
• org.apache.commons.lang3.tuple.Pair
• import com.google.common.collect.HashBasedTable;
• import com.google.common.collect.Table;
• map.getOrDefault
• map.computeIfAbsent