Functional Programming in Java, Second Edition: p.51 - Recall what the "office-space pattern" is

On page 51, “Sorting with a Comparator”, we find:

Rather than writing this code, we can use the office-space pattern—i.e., ask the Java compiler to do the routing again, using a method reference.

One should refer the reader to page 26, where that pattern was introduced.

Also, at this point, we have the following examples of :: shortenings to describe calls: