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Chapter 11
The Collections Framework
When writing object-oriented programs, we often work with groups of objects, either objects of the same type or of different types. In Chapter 5, “Core Classes” you have learned that arrays can be used to group objects of the same type. Unfortunately, arrays lack the flexibility you need to rapidly develop applications. They cannot be resized and they cannot hold objects of different types. Fortunately, Java comes with a set of interfaces and classes that make working with groups of objects easier: the Collections Framework. This chapter deals with the most important types in the Collections Framework that a Java beginner should know. Most of them are very easy to use and there’s no need to provide extensive examples. More attention is paid to the last section of the chapter, “Making Your Objects Comparable and Sortable” where carefully designed examples are given because it is important for every Java programmer to know how to make objects comparable and sortable.
Note on Generics
Discussing the Collections Framework would not be complete without generics. On the other hand, explaining generics without previous knowledge of the Collections Framework would be difficult. Therefore, there needs to be a compromise: The Collections Framework will be explained first in this chapter and will be revisited in Chapter 12, “Generics.” Since up to this point no knowledge of generics is assumed, the discussion of the Collections Framework in this chapter will have to use class and method signatures as they appear in pre-5 JDK’s, instead of signatures used in Java 5 that imply the presence of generics. As long as you read both this chapter and Chapter 12, you will have up-to date knowledge of both the Collections Framework and generics.