@defaultMessage spawns threads with vague names; use a custom thread factory and name threads so that you can tell (by its name) which executor it is associated with

java.util.concurrent.Executors#newFixedThreadPool(int)
java.util.concurrent.Executors#newSingleThreadExecutor()
java.util.concurrent.Executors#newCachedThreadPool()
java.util.concurrent.Executors#newSingleThreadScheduledExecutor()
java.util.concurrent.Executors#newScheduledThreadPool(int)
java.util.concurrent.Executors#defaultThreadFactory()
java.util.concurrent.Executors#privilegedThreadFactory()

java.lang.Character#codePointBefore(char[],int) @ Implicit start offset is error-prone when the char[] is a buffer and the first chars are random chars
java.lang.Character#codePointAt(char[],int) @ Implicit end offset is error-prone when the char[] is a buffer and the last chars are random chars

@defaultMessage Collections.sort dumps data into an array, sorts the array and reinserts data into the list, one should rather use Lucene's CollectionUtil sort methods which sort in place

java.util.Collections#sort(java.util.List)
java.util.Collections#sort(java.util.List,java.util.Comparator)

java.io.StringReader#<init>(java.lang.String) @ Use FastStringReader instead

org.apache.lucene.util.RamUsageEstimator#sizeOf(java.lang.Object) @ This can be a perfromance trap