Many programmers will never need to implement their own Collections classes. You can go pretty far using the implementations described in the preceding sections of this chapter. However, 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.

Reasons to Write an Implementation

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 run-length encoded — runs 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.

How to Write a Custom Implementation

Writing a custom implementation is surprisingly easy. The Java Collections Framework provides abstract implementations designed expressly to facilitate custom implementations. We'll start with the following example of an implementation of Arrays.asList.

public static <T> List<T> asList(T[] a) {
    return new MyArrayList<T>(a);
}

private static class MyArrayList<T> extends AbstractList<T> {

    private final T[] a;

    MyArrayList(T[] array) {
        a = array;
    }

    public T get(int index) {
        return a[index];
    }

    public T set(int index, T element) {
        T oldValue = a[index];
        a[index] = element;
        return oldValue;
    }

    public int size() {
        return a.length;
    }
}

Believe it or not, this is very close to the implementation that is contained in java.util.Arrays. It's that simple! You provide a constructor and the get, set, and size methods, and AbstractList does all the rest. You get the ListIterator, bulk operations, search operations, hash code computation, comparison, and string representation for free.

Suppose you want to make the implementation a bit faster. The API documentation for abstract implementations describes precisely how each method is implemented, so you'll know which methods to override to get the performance you want. The preceding implementation's performance is fine, but it can be improved a bit. In particular, the toArray method iterates over the List, copying one element at a time. Given the internal representation, it's a lot faster and more sensible just to clone the array.

public Object[] toArray() {
    return (Object[]) a.clone();
}

With the addition of this override and a few more like it, this implementation is exactly the one found in java.util.Arrays. In the interest of full disclosure, it's a bit tougher to use the other abstract implementations because you will have to write your own iterator, but it's still not that difficult.

The following list summarizes the abstract implementations:

• AbstractCollection — a Collection that is neither a Set nor a List. At a minimum, you must provide the iterator and the size methods.
• AbstractSet — a Set; use is identical to AbstractCollection.
• AbstractList — a List backed up by a random-access data store, such as an array. At a minimum, you must provide the positional access methods (get and, optionally, set, remove, and add) and the size method. The abstract class takes care of listIterator (and iterator).
• AbstractSequentialList — a List backed up by a sequential-access data store, such as a linked list. At a minimum, you must provide the listIterator and size methods. The abstract class takes care of the positional access methods. (This is the opposite of AbstractList.)
• AbstractQueue — at a minimum, you must provide the offer, peek, poll, and size methods and an iterator supporting remove.
• AbstractMap — a Map. At a minimum you must provide the entrySet view. This is typically implemented with the AbstractSet class. If the Map is modifiable, you must also provide the put method.

The process of writing a custom implementation follows:

1. Choose the appropriate abstract implementation class from the preceding list.

2. Provide implementations for all the class's abstract methods. If your custom collection is to be modifiable, you'll have to override one or more of the concrete methods as well. The API documentation for the abstract implementation class will tell you which methods to override.

3. Test and, if necessary, debug the implementation. You now have a working custom collection implementation.

4. If you're concerned about performance, read the abstract implementation class's API documentation for all the methods whose implementations you're inheriting. If any seem too slow, override them. If you override any methods, be sure to measure the performance of the method before and after the override. How much effort you put into tweaking performance should be a function of how much use the implementation will get and how critical to performance its use is. (Often this step is best omitted.)