My most noteworthy answers:

• java-stream

  1. Stream.map with two different mapping functions (Collectors.teeing)
  2. Get a list of unique objects from an ArrayList
  3. Putting new elements to an existing Stream
  4. Create a Stream from an object's method
  5. List of Maps to a reduced List of Maps

• java:

  1. generics: Nested generic and wildcards
  2. enums: Finding enum value with Java 8 Stream API
  3. volatile: Thread safety and the volatile keyword in Java
  4. comparator: Sort a list with known values before unknown values
  5. optional: Calling different methods based on values of two Optionals

• java libraries:

  1. guava: How to do map inversion with Guava with non-unique values?
  2. log4j2: Logging with abstract classes in Log4j2
  3. lombok: Singular Builder and a default value
  4. kryo: Difference between readClassAndObject/readObject and writeClassAndObject/writeObject
• regex: Match a specific line that is not followed by another specific line before [...]