Understanding Java Classes and Objects

Introduction to Java Classes:
A class is nothing but a blueprint or a template for creating different objects which defines its properties and behaviors. Java class objects exhibit the properties and behaviors defined by its class. A class can contain fields and methods to describe the behavior of an object.
Methods are nothing but members of a class that provide a service for an object or perform some business logic. Java fields and member functions names are case sensitive. Current states of a class’s corresponding object are stored in the object’s instance variables. Methods define the operations that can be performed in java programming.

A class has the following general syntax:

<class modifiers>class<class name>
<extends clause> <implements clause>{
// Dealing with Classes (Class body)
<field declarations (Static and Non-Static)>
<method declarations (Static and Non-Static)>
<Inner class declarations>
<nested interface declarations>
<constructor declarations>
<Static initializer blocks>
}

Below is an example showing the Objects and Classes of the Cube class that defines 3 fields namely length, breadth and height. Also the class contains a member function getVolume().

public class Cube
{
    int length;
    int breadth;
    int height;
    public int getVolume()
    {
         return (length * breadth * height);
    }
}

How do you reference a data member/function?
This is accomplished by stating the name of the object reference, followed by a period (dot), followed by the name of the member inside the object.
( objectReference.member ). You call a method for an object by naming the object followed by a period (dot), followed by the name of the method and its argument list, like this: objectName.methodName(arg1, arg2, arg3).
For example:
cubeObject.length = 4;
cubeObject.breadth = 4;
cubeObject.height = 4;
cubeObject.getvolume()

Class Variables – Static Fields
We use class variables also know as Static fields when we want to share characteristics across all objects within a class. When you declare a field to be static, only a single instance of the associated variable is created common to all the objects of that class. Hence when one object changes the value of a class variable, it affects all objects of the class. We can access a class variable by using the name of the class, and not necessarily using a reference to an individual object within the class. Static variables can be accessed even though no objects of that class exist. It is declared using static keyword.

Class Methods – Static Methods
Class methods, similar to Class variables can be invoked without having an instance of the class. Class methods are often used to provide global functions for Java programs. For example, methods in the java.lang.Math package are class methods.
You cannot call non-static methods from inside a static method.

Instance Variables
Instance variables stores the state of the object. Each class would have its own copy of the variable. Every object has a state that is determined by the values stored in the object. An object is said to have changed its state when one or more data values stored in the object have been modified. When an object responds to a message, it will usually perform an action, change its state etc. An object that has the ability to store values is often said to have persistence.

Consider this simple Java program showing the use of static fields and static methods:

    // Class and Object initialization showing the Object Oriented concepts in Java
            class Cube
             {
                 int length = 10;
                 int breadth = 10;
                 int height = 10;
                public static int numOfCubes = 0; // static variable
                public static int getNoOfCubes()
                {
                    //static method
                    return numOfCubes;
                }
                public Cube()
                {
                    numOfCubes++; //
                }
             }

         public class CubeStaticTest
        {
             public static void main(String args[])
             {
                 System.out.println("Number of Cube objects = " + Cube.numOfCubes);
                System.out.println("Number of Cube objects = " + Cube.getNoOfCubes());
             }
        }

Output
Number of Cube objects = 0
Number of Cube objects = 0

Final Variable, Methods and Classes:
In Java we can mark fields, methods and classes as final. Once marked as final, these items cannot be changed.
Variables defined in an interface are implicitly final. You can’t change value of a final variable (is a constant). A final class can’t be extended i.e., final class may not be subclassed. This is done for security reasons with basic classes like String and Integer. It also allows the compiler to make some optimizations, and makes thread safety a little easier to achieve. A final method can’t be overridden when its class is inherited. Any attempt to override or hide a final method will result in a compiler error.

Posted in: J2SE, Java