Many programmers will never need to implement their own Collections classes. Someday you might want to write your own implementation. It is fairly easy to do this with the aid of the abstract implementations provided by the Java platform. Before we discuss how to write an implementation, let's discuss why you might want to write one.
The following list illustrates the sort of custom Collections you might want to implement. It is not intended to be exhaustive:
Persistent All of the built-in Collection implementations reside in main memory and vanish when the program exits. If you want a collection that will still be present the next time the program starts, you can implement it by building a veneer over an external database. Such a collection might be concurrently accessible by multiple programs.
Application-specific This is a very broad category. One example is an unmodifiable Map containing real-time telemetry data. The keys could represent locations, and the values could be read from sensors at these locations in response to the get operation.
High-performance, special-purpose Many data structures take advantage of restricted usage to offer better performance than is possible with general-purpose implementations. For instance, consider a List containing long runs of identical element values. Such lists, which occur frequently in text processing, can be runlength encodedruns can be represented as a single object containing the repeated element and the number of consecutive repetitions. This example is interesting because it trades off two aspects of performance: It requires less space but more time than an ArrayList.
High-performance, general-purpose The Java Collections Framework's designers tried to provide the best general-purpose implementations for each interface, but many, many data structures could have been used, and new ones are invented every day. Maybe you can come up with something faster!
Enhanced functionality Suppose you need an efficient bag implementation (also known as a multiset): a Collection that offers constant-time containment checks while allowing duplicate elements. It's reasonably straightforward to implement such a collection atop a HashMap.
Convenience You may want additional implementations that offer conveniences beyond those offered by the Java platform. For instance, you may frequently need List instances representing a contiguous range of Integers.
Adapter Suppose you are using a legacy API that has its own ad hoc collections' API. You can write an adapter implementation that permits these collections to operate in the Java Collections Framework. An adapter implementation is a thin veneer that wraps objects of one type and makes them behave like objects of another type by translating operations on the latter type into operations on the former.
