This class represents collections which can quickly be iterated over (forward or backward) in a thread-safe manner without creating new objects and without using iterators .

This class represents a comparator to be used for equality as well as for ordering; instances of this class provide a hashcode function consistent with equal (if two objects are equal , they have the same hashcode ), equality with null values is supported.

This class represents a hash map with real-time behavior; smooth capacity increase and no rehashing ever performed.