ConcurrentHashMap Internals

Four threads count visits to one web page. Each thread records 200,000 visits, so the correct total is 800,000. Here is one real result from a program in this lesson:

Expected total:                  800000
HashMap, get then put:           221627
ConcurrentHashMap, get then put: 306233
ConcurrentHashMap, merge:        800000

Two totals are wrong, and one of them comes from ConcurrentHashMap. This class makes each single operation thread-safe, but it does not make several separate calls act as one operation.

This lesson explains how `ConcurrentHashMap