Introduction

It would be nice if we could write a single sort method that could sort the elements in an Integer array, a String array or an array of any type that supports ordering (i.e., its elements can be compared). It would also be nice if we could write a single Stack class that could be used as a Stack of integers, a Stack of floating-point numbers, a Stack of Strings or a Stack of any other type. It would be even nicer if we could detect type mismatches at compile timeknown as compile-time type safety. For example, if a Stack stores only integers, attempting to push a String on to that Stack should issue a compile-time error.

This chapter discusses one of J2SE 5.0's new featuresgenericswhich provides the means to create the general models mentioned above. Generic methods and generic classes enable programmers to specify, with a single method declaration, a set of related methods or, with a single class declaration, a set of related types, respectively. Generics also provide compile-time type safety that allows programmers to catch invalid types at compile time.

We might write a generic method for sorting an array of objects, then invoke the generic method with Integer arrays, Double arrays, String arrays and so on, to sort the array elements. The compiler could perform type checking to ensure that the array passed to the sorting method contains same type elements. We might write a single generic Stack class that manipulates a stack of objects, then instantiate Stack objects for a stack of Integers, a stack of Doubles, a stack of Strings and so on. The compiler could perform type checking to ensure that the Stack stores elements of the same type.

Software Engineering Observation 18.1

Generic methods and classes are among Java's most powerful capabilities for software reuse with compile-time type safety.

This chapter presents generic method and generic class examples. It also considers the relationships between generics and other Java features, such as overloading and inheritance. Chapter 19, Collections, presents an in-depth treatment of the Java Collections Framework's generic methods and classes. A collection is a data structure that maintains references to many objects. The Java Collections Framework uses generics to allow programmers to specify the exact types of objects that a particular collection will store in a program.