Learning Java, 4th Edition

Chapter 8. Generics

It’s been over 15 years since the introduction of the Java programming language (and the first edition of this book). In that time, the Java language has matured and come into its own. But it wasn’t until Java 5.0, the sixth major release of Java, that the core language itself changed in a significant way. Yes, there were subtle changes and drop-ins over the years. Inner classes, added very early on, were important. But no language improvements prior to that point affected all Java code or all Java developers in the way that generic types did in Java 5.0.

Generics are about abstraction. Generics let you create classes and methods that work in the same way on different types of objects. The term generic comes from the idea that we’d like to be able to write general algorithms that can be broadly reused for many types of objects rather than having to adapt our code to fit each circumstance. This concept is not new; it is the impetus behind object-oriented programming itself. Java generics do not so much add new capabilities to the language as they make reusable Java code easier to write and easier to read.

Generics take reuse to the next level by making the type of the objects with which we work into an explicit parameter of the generic code. For this reason, generics are also referred to as parameterized types. In the case of a generic class, the developer specifies a type as a parameter (an argument) whenever she uses the generic type. The class is parameterized ...