Java Pocket Guide, 4th Edition

Chapter 4. Reference Types

Reference types hold references to objects and provide a means to access those objects stored somewhere in memory. The memory locations are irrelevant to programmers. All reference types are a subclass of type java.lang.Object.

Table 4-1 lists the five Java reference types.

Table 4-1. Reference types

Reference type	Brief description
Annotation	Provides a way to associate metadata (data about data) with program elements.
Array	Provides a fixed-size data structure that stores data elements of the same type.
Class	Designed to provide inheritance, polymorphism, and encapsulation. Usually models something in the real world and consists of a set of values that holds data and a set of methods that operates on the data.
Enumeration	A reference for a set of objects that represents a related set of choices.
Interface	Provides a public API and is “implemented” by Java classes.

Comparing Reference Types to Primitive Types

There are two type categories in Java: reference types and primitive types. Table 4-2 shows some of the key differences between them. See Chapter 3 for more details.

Table 4-2. Reference types compared with primitive types

Reference types	Primitive types
Unlimited number of reference types, as they are defined by the user.	Consists of boolean and numeric types: char, byte, short, int, long, float, and double.
Memory location stores a reference to the data.	Memory location stores actual data held by the primitive type. ...