A DESCRIPTION OF THE PROBLEM :
There is a typo at the end of this section of the package overview (right parenthesis instead of comma).

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
     int sumOfWeights = widgets.stream()
                               .reduce(0,
                                       (sum, b) -> sum + b.getWeight(),
                                       Integer::sum);
 
ACTUAL -
     int sumOfWeights = widgets.stream()
                               .reduce(0,
                                       (sum, b) -> sum + b.getWeight())
                                       Integer::sum);
 

URL OF FAULTY DOCUMENTATION :
http://docs.oracle.com/javase/8/docs/api/java/util/stream/package-summary.html#Reduction