Introduction

The class is at the core of Java. It is the logical construct upon which the entire Java language is built because it defines the shape and nature of an object. As such, the class forms the basis for object-oriented programming in Java. Any concept you wish to implement in a Java program must be encapsulated within a class. Because the class is so fundamental to Java. Here, you will be introduced to the basic elements of a class and learn how a class can be used to create objects. You will also learn about methods, constructors.

Class
A class is a template for an object, and an object is an instance of a class.

The General Form of a Class

When you define a class, you declare its exact form and nature. You do this by specifying the data that it contains and the code that operates on that data. While very simple classes may contain only code or only data, most real-world classes contain both. As you will see, a class’ code defines the interface to its data. A class is declared by use of the class keyword. The classes that have been used up to this point are actually very limited examples of its complete form. Classes can (and usually do) get much more complex. The general form of a class definition is shown here:

class classname {
type instance-variable1;
type instance-variable2; // ...
type instance-variableN;
type methodname1(parameter-list) { // body of method }
type methodname2(parameter-list) { // body of method} // ...
type methodnameN(parameter-list) { // body of method } }
The data, or variables, defined within a class are called instance variables. The code is contained within methods. Collectively, the methods and variables defined within a class are called members of the class. In most classes, the instance variables are acted upon and accessed by the methods defined for that class. Thus, it is the methods that determine how a class’ data can be used. Variables defined within a class are called instance variables because each instance of the class (that is, each object of the class) contains its own copy of these variables. Thus, the data for one object is separate and unique from the data for another. All methods have the same general form as main( ), which we have been using thus far.
A Simple Class

Let’s begin our study of the class with a simple example. Here is a class called Box that defines three instance variables: width, height, and depth. Currently, Box does not contain any methods (but some will be added soon).
class Box
{
double width;
double height;
double depth;
}
As stated, a class defines a new type of data. In this case, the new data type is called
Box. You will use this name to declare objects of type Box. It is important to remember
that a class declaration only creates a template; it does not create an actual object. Thus, the preceding code does not cause any objects of type Box to come into existence. To actually create a Box object, you will use a statement like the following:

Box mybox = new Box(); // create a Box object called mybox

After this statement executes, mybox will be an instance of Box. Thus, it will have “physical” reality. For the moment, don’t worry about the details of this statement. Again, each time you create an instance of a class, you are creating an object that contains its own copy of each instance variable defined by the class. Thus, every Box
object will contain its own copies of the instance variables width, height, and depth. To access these variables, you will use the dot (.) operator. The dot operator links the name of the object with the name of an instance variable. For example, to assign the width
variable of mybox the value 100, you would use the following statement:
mybox.width = 100;

This statement tells the compiler to assign the copy of width that is contained within the mybox object the value of 100. In general, you use the dot operator to access both the instance variables and the methods within an object. Here is a complete program that uses the Box class:


/* A program that uses the Box class. Call this file BoxDemo.java
*/ class Box { double width; double height; double depth; }
// This class declares an object of type Box.
class BoxDemo
{
public static void main(String args[])
{
Box mybox = new Box();
double vol;// assign values to mybox's instance variables
mybox.width = 10;
mybox.height = 20;
mybox.depth = 15;
// compute volume of box
vol = mybox.width * mybox.height * mybox.depth;
System.out.println("Volume is " + vol);
}
}
You should call the file that contains this program BoxDemo.java, because the main( )
method is in the class called BoxDemo, not the class called Box. When you compile this program, you will find that two .class files have been created, one for Box and one for
BoxDemo. The Java compiler automatically puts each class into its own .class file. It is not necessary for both the Box and the BoxDemo class to actually be in the same source file. You could put each class in its own file, called Box.java and BoxDemo.java, respectively. To run this program, you must execute BoxDemo.class. When you do, you will see the following output:
Volume is 3000.0
As stated earlier, each object has its own copies of the instance variables. This means that if you have two Box objects, each has its own copy of depth, width, and
height. It is important to understand that changes to the instance variables of one object have no effect on the instance variables of another. For example, the following program declares two Box objects:

// This program declares two Box objects.
class Box
{
double width;
double height;
double depth;
}
class BoxDemo2
{
public static void main(String args[])
{
Box mybox1 = new Box();
Box mybox2 = new Box();
double vol; // assign values to mybox1's instance variables
mybox1.width = 10;
mybox1.height = 20;
mybox1.depth = 15;
/* assign different values to mybox2's instance variables */
mybox2.width = 3;
mybox2.height = 6;
mybox2.depth = 9;
// compute volume of first box
vol = mybox1.width * mybox1.height * mybox1.depth;
System.out.println("Volume is " + vol);
// compute volume of second box
vol = mybox2.width * mybox2.height * mybox2.depth;
System.out.println("Volume is " + vol); }
}
The output produced by this program is shown here:
Volume is 3000.0 Volume is 162.0
As you can see, mybox1’s data is completely separate from the data contained in mybox2.
Declaring Objects
As just explained, when you create a class, you are creating a new data type. You can use this type to declare objects of that type. However, obtaining objects of a class is a two-step process. First, you must declare a variable of the class type. This variable does not define an object. Instead, it is simply a variable that can refer to an object. Second, you must acquire an actual, physical copy of the object and assign it to that variable. You can do this using the new operator. The new operator dynamically allocates (that is, allocates at run time) memory for an object and returns a reference to it. This reference is, more or less, the address in memory of the object allocated by new.
This reference is then stored in the variable. Thus, in Java, all class objects must be dynamically allocated. Let’s look at the details of this procedure. In the preceding sample programs, a line similar to the following is used to declare an object of type Box:
Box mybox = new Box();

This statement combines the two steps just described. It can be rewritten like this to show each step more clearly:
Box mybox; // declare reference to object mybox = new Box(); // allocate a Box object
The first line declares mybox as a reference to an object of type Box. After this line executes, mybox contains the value null, which indicates that it does not yet point to an actual object. Any attempt to use mybox at this point will result in a compile-time error. The next line allocates an actual object and assigns a reference to it to mybox. After the second line executes, you can use mybox as if it were a Box object. But in reality, mybox simply holds the memory address of the actual Box object. The effect of these two lines of code is depicted in Figure 6-1.