2.7. Classes and Objects Introduced

Now that we have introduced operators, expressions, statements, and methods, we can finally talk about classes. A class is a named collection of fields that hold data values and methods that operate on those values. Classes are just one of five reference types supported by Java, but they are the most important type. Classes are thoroughly documented in a chapter of their own, Chapter 3. We introduce them here, however, because they are the next higher level of syntax after methods, and because the rest of this chapter requires a basic familiarity with the concept of class and the basic syntax for defining a class, instantiating it, and using the resulting object.

The most important thing about classes is that they define new data types. For example, you might define a class named Point to represent a data point in the two-dimensional Cartesian coordinate system. This class would define fields (each of type double) to hold the X and Y coordinates of a point and methods to manipulate and operate on the point. The Point class is a new data type.

When discussing data types, it is important to distinguish between the data type itself and the values the data type represents. char is a data type: it represents Unicode characters. But a char value represents a single specific character. A class is a data type; a class value is called an object. We use the name class because each class defines a type (or kind, or species, or class) of objects. The Point class is a data type that represents X,Y points, while a Point object represents a single specific X,Y point. As you might imagine, classes and their objects are closely linked. In the sections that follow, we will discuss both.

2.7.1. Defining a Class

Here is a possible definition of the Point class we have been discussing:

/** Represents a Cartesian (x,y) point */
public class Point {
     public double x, y;                    // The coordinates of the point
     public Point(double x, double y) {     // A constructor that
         this.x = x; this.y = y;            // initializes the fields
     }

     public double distanceFromOrigin(  ) {   // A method that operates on
         return Math.sqrt(x*x + y*y);       // the x and y fields
     }
}

This class definition is stored in a file named Point.java and compiled to a file named Point.class, where it is available for use by Java programs and other classes. This class definition is provided here for completeness and to provide context, but don't expect to understand all the details just yet; most of Chapter 3 is devoted to the topic of defining classes.

Keep in mind that you don't have to define every class you want to use in a Java program. The Java platform includes thousands of predefined classes that are guaranteed to be available on every computer that runs Java.

2.7.2. Creating an Object

Now that we have defined the Point class as a new data type, we can use the following line to declare a variable that holds a Point object:

Point p;

Declaring a variable to hold a Point object does not create the object itself, however. To actually create an object, you must use the new operator. This keyword is followed by the object's class (i.e., its type) and an optional argument list in parentheses. These arguments are passed to the constructor method for the class, which initializes internal fields in the new object:

// Create a Point object representing (2,-3.5).
// Declare a variable p and store a reference to the new Point object in it.
Point p = new Point(2.0, -3.5);

// Create some other objects as well
Date d = new Date(  );        // A Date object that represents the current time
Set words = new HashSet(  );  // A HashSet object to hold a set of objects

The new keyword is by far the most common way to create objects in Java. A few other ways are also worth mentioning. First, a couple of classes are so important that Java defines special literal syntax for creating objects of those types (as we discuss later in this section). Second, Java supports a dynamic loading mechanism that allows programs to load classes and create instances of those classes dynamically. This dynamic instantiation is done with the newInstance( ) methods of java.lang.Class and java.lang.reflect.Constructor. Finally, objects can also be created by deserializing them. In other words, an object that has had its state saved, or serialized, usually to a file, can be recreated using the java.io.ObjectInputStream class.

2.7.3. Using an Object

Now that we've seen how to define classes and instantiate them by creating objects, we need to look at the Java syntax that allows us to use those objects. Recall that a class defines a collection of fields and methods. Each object has its own copies of those fields and has access to those methods. We use the dot character (.) to access the named fields and methods of an object. For example:

Point p = new Point(2, 3);         // Create an object
double x = p.x;                    // Read a field of the object
p.y = p.x * p.x;                   // Set the value of a field
double d = p.distanceFromOrigin(  ); // Access a method of the object

This syntax is central to object-oriented programming in Java, so you'll see it a lot. Note, in particular, the expression p.distanceFromOrigin( ). This tells the Java compiler to look up a method named distanceFromOrigin( ) defined by the class Point and use that method to perform a computation on the fields of the object p. We'll cover the details of this operation in Chapter 3.

2.7.4. Object Literals

In our discussion of primitive types, we saw that each primitive type has a literal syntax for including values of the type literally into the text of a program. Java also defines a literal syntax for a few special reference types, as described next.

2.7.4.1 String literals

The String class represents text as a string of characters. Since programs usually communicate with their users through the written word, the ability to manipulate strings of text is quite important in any programming language. In some languages, strings are a primitive type, on a par with integers and characters. In Java, however, strings are objects; the data type used to represent text is the String class.

Because strings are such a fundamental data type, Java allows you to include text literally in programs by placing it between double-quote (") characters. For example:

String name = "David";
System.out.println("Hello, " + name);

Don't confuse the double-quote characters that surround string literals with the single-quote (or apostrophe) characters that surround char literals. String literals can contain any of the escape sequences char literals can (see Table 2-2). Escape sequences are particularly useful for embedding double-quote characters within double-quoted string literals. For example:

String story = "\t\"How can you stand it?\" he asked sarcastically.\n";

String literals cannot contain comments and may consist of only a single line. Java does not support any kind of continuation-character syntax that allows two separate lines to be treated as a single line. If you need to represent a long string of text that does not fit on a single line, break it into independent string literals and use the + operator to concatenate the literals. For example:

String s = "This is a test of the         // This is illegal; string  literals
            emergency broadcast system";  // cannot be broken across lines.

String s = "This is a test of the " +     // Do this instead
           "emergency broadcast system";

This concatenation of literals is done when your program is compiled, not when it is run, so you do not need to worry about any kind of performance penalty.

2.7.4.2 Type literals

The second type that supports its own special object literal syntax is the class named Class. Instances of the Class class represent a Java data type. To include a Class object literally in a Java program, follow the name of any data type with .class. For example:

Class typeInt = int.class;
Class typeIntArray = int[].class;
Class typePoint = Point.class;

2.7.4.3 The null reference

The null keyword is a special literal value that is a reference to nothing, or an absence of a reference. The null value is unique because it is a member of every reference type. You can assign null to variables of any reference type. For example:

String s = null;
Point p = null;