Java in a Nutshell, 5th Edition

Reference Types

Now that we’ve covered arrays and introduced classes and objects, we can turn to a more general description of reference types. Classes and arrays are two of Java’s five kinds of reference types. Classes were introduced earlier and are covered in complete detail, along with interfaces, in Chapter 3. Enumerated types and annotation types are reference types introduced in Java 5.0 (see Chapter 4).

This section does not cover specific syntax for any particular reference type, but instead explains the general behavior of reference types and illustrates how they differ from Java’s primitive types. In this section, the term object refers to a value or instance of any reference type, including arrays.

Reference vs. Primitive Types

Reference types and objects differ substantially from primitive types and their primitive values:

• Eight primitive types are defined by the Java language. Reference types are user-defined, so there is an unlimited number of them. For example, a program might define a class named Point and use objects of this newly defined type to store and manipulate X,Y points in a Cartesian coordinate system. The same program might use an array of characters—of type char[ ]—to store text and might use an array of Point objects—of type Point[ ]—to store a sequence of points.

• Primitive types represent single values. Reference types are aggregate types that hold zero or more primitive values or objects. Our hypothetical Point class, for example, might hold two