Chapter 3. Object-Oriented Programming in Java

Now that we've covered fundamental Java syntax, we are ready to begin object-oriented programming in Java. All Java programs use objects, and the type of an object is defined by its class or interface. Every Java program is defined as a class, and nontrivial programs usually include a number of classes and interface definitions. This chapter explains how to define new classes and interfaces and how to do object-oriented programming with them.^[1]

^[1] If you do not have object-oriented (OO) programming background, don't worry; this chapter does not assume any prior experience. If you do have experience with OO programming, however, be careful. The term "object-oriented" has different meanings in different languages. Don't assume that Java works the same way as your favorite OO language. This is particularly true for C++ programmers. Although Java and C++ borrow much syntax from C, the similarities between the two languages do not go far beyond the level of syntax. Don't let your experience with C++ lull you into a false familiarity with Java.

This is a relatively long and detailed chapter, so we begin with an overview and some definitions. A class is a collection of fields that hold values and methods that operate on those values. Classes are the most fundamental structural element of all Java programs. You cannot write Java code without defining a class. All Java statements appear within methods, and all methods are implemented within classes.

A class defines a new reference type, such as the Point type defined in Chapter 2. An object is an instance of a class. The Point class defines a type that is the set of all possible two-dimensional points. A Point object is a value of that type: it represents a single two-dimensional point.

Objects are usually created by instantiating a class with the new keyword and a constructor invocation, as shown here:

Point p = new Point(1.0, 2.0);

Constructors are covered in Section 3.3 later in this chapter.

A class definition consists of a signature and a body. The class signature defines the name of the class and may also specify other important information. The body of a class is a set of members enclosed in curly braces. The members of a class may include fields and methods, constructors and initializers, and nested types.

Members can be static or nonstatic. A static member belongs to the class itself while a nonstatic member is associated with the instances of a class (see Section 3.2 later in this chapter).

The signature of a class may declare that the class extends another class. The extended class is known as the superclass and the extension is known as the subclass. A subclass inherits the members of its superclass and may declare new members or override inherited methods with new implementations.

The signature of a class may also declare that the class implements one or more interfaces. An interface is a reference type that defines method signatures but does not include method bodies to implement the methods. A class that implements an interface is required to provide bodies for the interface's methods. Instances of such a class are also instances of the interface type that it implements.

The members of a class may have access modifiers public , protected, or private, which specify their visibility and accessibility to clients and to subclasses. This allows classes to hide members that are not part of their public API. When applied to fields, this ability to hide members enables an object-oriented design technique known as data encapsulation .

Classes and interfaces are the most important of the five fundamental reference types defined by Java. Arrays, enumerated types (or "enums") and annotation types are the other three. Arrays are covered in Chapter 2. Enumerated types and annotation types were introduced in Java 5.0 (see Chapter 4). Enums are a specialized kind of class and annotation types are a specialized kind of interface.