Chapter 14
Generics
Since the original 1.0
release in 1995, many new features have been added to Java. One that has had a
profound impact is generics.
Introduced by JDK 5, generics changed Java in two important ways. First, it
added a new syntactical element to the language. Second, it caused changes to
many of the classes and methods in the core API. Today, generics are an
integral part of Java programming, and a solid understanding of this important
feature is required. It is examined here in detail.
Through the use of generics,
it is possible to create classes, interfaces, and methods that will work in a
type-safe manner with various kinds of data. Many algorithms are logically the
same no matter what type of data they are being applied to. For example, the
mechanism that supports a stack is the same whether that stack is storing items
of type Integer, String, Object, or Thread. With
generics, you can define an algorithm once, independently of any specific type of data, and then apply
that algorithm to a wide variety of data types without any additional effort.
The expressive power generics added to the language fundamentally changed the
way that Java code is written.
Perhaps the one feature of
Java that has been most significantly affected by generics is the Collections Framework. The Collections
Framework is part of the Java API and is described in detail in Chapter 18, but
a brief mention is useful now. A collection
is a group of objects. The Collections Framework defines several classes, such
as lists and maps, that manage collections. The collection classes have always
been able to work with any type of object. The benefit that generics added is
that the collection classes can now be used with complete type safety. Thus, in
addition to being a powerful language element on its own, generics also enabled
an existing feature to be substantially improved. This is another reason why
generics were such an important addition to Java.
This chapter describes the
syntax, theory, and use of generics. It also shows how generics provide type
safety for some previously difficult cases. Once you have completed this
chapter, you will want to examine Chapter 18, which covers the Collections
Framework. There you will find many examples of generics at work.
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