Java Generics and Collections

Preface

Java now supports generics, the most significant change to the language since the addition of inner classes in Java 1.2—some would say the most significant change to the language ever.

Say you wish to process lists. Some may be lists of integers, others lists of strings, and yet others lists of lists of strings. In Java before generics this is simple. You can represent all three by the same class, called List, which has elements of class Object:

list of integers	List
list of strings	List
list of lists of strings	List

In order to keep the language simple, you are forced to do some of the work yourself: you must keep track of the fact that you have a list of integers (or strings or lists of strings), and when you extract an element from the list you must cast it from Object back to Integer (or String or List). For instance, the Collections Framework before generics made extensive use of this idiom.

Einstein is reputed to have said, “Everything should be as simple as possible but no simpler”. And some might say the approach above is too simple. In Java with generics you may distinguish different types of lists:

list of integers	List<Integer>
list of strings	List<String>
list of lists of strings	List<List<String>>

Now the compiler keeps track of whether you have a list of integers (or strings or lists of strings), and no explicit cast back to Integer (or String or List<String>) is required. In some ways, this is similar to generics in Ada or templates in C++, but the actual inspiration is ...