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Trail: Collections

Lesson: Custom Implementations

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. Someday, however, 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.

Reasons to Write an Implementation

The following list illustrates the sort of custom collections that you might want to implement. It is not intended to be exhaustive:

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 an example, an implementation of Arrays.asList (in the API reference documentation):
    public static <T> List<T> asList(T[] a) {
    return new MyArrayList<T>(a);
}

private static class MyArrayList<T> extends AbstractList<T>
    implements java.io.Serializable {

    private final T[] a;

    ArrayWrapList(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 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 that you want to make the implementation a bit faster. The API documentation for the abstract implementations describes precisely how each method is implemented, so you'll know which methods to override in order to get the performance you want. The performance of the preceding implementation 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 interests of full disclosure, it's a bit tougher to use the other abstract implementations, because they require you to write your own iterator, but it's still not that difficult.

The abstract implementations are summarized below:

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 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 of them 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 the performance should be a function of how much use the implementation will get and how performance-critical the use. (Often this step is best omitted.)

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