Objects and Classes in Java

The Class Declaration

The left side of the following diagram shows the possible components of a class declaration in the order they should or must appear in your class declaration. The right side describes their purposes. The required components are the class keyword and the class name and are shown in bold. All the other components are optional, and each appears on a line by itself (thus "extends Super" is a single component). Italics indicates an identifier such as the name of a class or interface. If you do not explicitly declare the optional items, the Java compiler assumes certain defaults: a nonpublic, nonabstract, nonfinal subclass of Object that implements no interfaces.

The following list provides a few more details about each class declaration component. It also provides references to sections later in this trail that talk about what each component means, how to use each, and how it affects your class, other classes, and your Java program.

public

By default, a class can be used only by other classes in the same package. The public modifier declares that the class can be used by any class regardless of its package. Look in Creating and Using Packages for information about how to use public to limit access to your classes and how it affects your access to other classes.

abstract

Declares that the class cannot be instantiated. For a discussion about when abstract classes are appropriate and how to write them, see Writing Abstract Classes and Methods.

final

Declares that the class cannot be subclassed. Writing Final Classes and Methods shows you how to use final and discusses the reasons for using it.

class NameOfClass

The class keyword indicates to the compiler that this is a class declaration and that the name of the class is NameOfClass.

extends Super

The extends clause identifies Super as the superclass of the class, thereby inserting the class within the class hierarchy. Reality Break! The Spot Applet in this lesson shows you a subclass of Applet and talks briefly about the responsibilities and benefits of subclasses. Managing Inheritance in the next lesson goes into further detail on this subject.

implements Interfaces

To declare that your class implements one or more interfaces, use the keyword implements followed by a comma-delimited list of the names of the interfaces implemented by the class. Implementing an Interface explains how the Spot applet implements an interface. Details about writing your own interfaces can be found in Creating Interfaces.

Objects and Classes in Java