Java™ How To Program (Early Objects), Tenth Edition

Displaying the Summary Grouped by Starting Letter

Next, lines 27–37 group the key–value pairs in the Map wordCounts by the keys’ first letter. This produces a new Map in which each key is a Character and the corresponding value is a List of the key–value pairs in wordCounts in which the key starts with the Character. The statement performs the following tasks:

• First we need to get a Stream for processing the key–value pairs in wordCounts. Interface Map does not contain any methods that return Streams. So, line 27 calls Map method entrySet on wordCounts to get a Set of Map.Entry objects that each contain one key–value pair from wordCounts. This produces an object of type Set<Map.Entry<String, Long>>.

• Line 28 calls Set method stream to get ...