jshell> Stream.of("A quick brown fox jumps over the lazy dog.".split(" ")).collect(Collectors.toMap(s->s.length(),s->s)
   ...> )
|  java.lang.IllegalStateException thrown: Duplicate key 5 (attempted merging values quick and brown)
|        at Collectors.duplicateKeyException (Collectors.java:131)
|        at Collectors.lambda$uniqKeysMapAccumulator$1 (Collectors.java:178)
|        at ReduceOps$3ReducingSink.accept (ReduceOps.java:169)
|        at Spliterators$ArraySpliterator.forEachRemaining (Spliterators.java:948)
|        at AbstractPipeline.copyInto (AbstractPipeline.java:484)
|        at AbstractPipeline.wrapAndCopyInto (AbstractPipeline.java:474)
|        at ReduceOps$ReduceOp.evaluateSequential (ReduceOps.java:913)
|        at AbstractPipeline.evaluate (AbstractPipeline.java:234)
|        at ReferencePipeline.collect (ReferencePipeline.java:511)
|        at (#9:1)

jshell> Stream.of("A quick brown fox jumps over the lazy dog.".split(" ")).collect(Collectors.toMap(s->s.length(),s->s,(s1,s2)->s2))
$11 ==> {1=A, 3=the, 4=dog., 5=jumps}

jshell> Stream.of("A quick brown fox jumps over the lazy dog.".split(" ")).collect(Collectors.groupingBy(s->s.length()))
$12 ==> {1=[A], 3=[fox, the], 4=[over, lazy, dog.], 5=[quick, brown, jumps]}