Chapter 11. Collections

The term "collections" refers to the notion of objects that contain or manage other objects. Most developers are familiar with the basic collections, including the classic array (such as String[]) or the built-in Java classes (such as java.util.Hashtable). The introduction of a rich Java Collections Framework in Java 2 (JDK 1.2) brought a standardized implementation of classes for expressing more complex relationships. The Apache Jakarta Commons Collections project adds even more capability, as described in this chapter.

Table 11-1 compares some of the key differences between basic collection types. When possible, you should try to rely on the interfaces shown, not on the underlying implementation. For example, even if you have marked a HashMap as typed, sorted, and unmodifiable, you should still interface with it as a java.util.Map whenever possible:

Table 11-1. Collection Features

	Interface	Duplicates	Keys	Indexed	Ordered
Array	Object[]	Yes	No	Yes	Yes
Bag	org.apache.commons .collections.Bag	Yes	No	No	No
List	java.util.List	Yes	No	Yes	Yes
Map	java.util.Map	No	Yes	No	Optional
Primitive Array	primitive[]	Yes	No	Yes	Yes
Set	java.util.Set	No	No	No	Optional

 java.util.Map myMap = new java.util.HashMap(); 

For more information on the built-in JDK 1.4 collections, see http://java.sun.com/j2se/1.4.2/docs/guide/collections/reference.html. Note that this chapter does not duplicate information included in the default JDK documentationfor example, the Stack and SortedMap interfaces are concepts covered as part of the standard Java collection interface.

The meaning of each column heading in Table 11-1 is as follows:

Interface: The expected Java interface for this collection type.

Duplicates: Are duplicate elements allowed?

Keys: Are elements accessible by a key value (e.g., java.util.Hashtable)?

Indexed: Is the order of the elements maintained by a column in the database?

Ordered: Are the results returned as a sorted collection?