The main changes are to add ConcurrentHashMap specific implementations of the new Java 8 default Map methods with better concurrent implementations that rely on the internal details. These changes required lots of new inner classes which bloat the .java file
For example, some of these methods include:
compute(K key, BiFunction remappingFunction)
forEach(BiConsumer action)
merge(K key, V value, BiFunction remappingFunction)
Just to name a few.
I think this also shows why you usually shouldn't care about implementation details on how a class you don't have to maintain works. As long as the class follows the contract laid out in its javadoc, you should be agnostic of how it works since the implementation details can change in the future.
