Introduction of Class in java Programming Language

A simple introduction of Class

A class in java  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. Generally the fields are data member and methods are function member. So in a single formula.

Class = data + methods. Everything (data and methods) in Java is contained in classes, Even the main method in java is defined within some class. Classes can also be used to store data. Historically, programming languages had something similar to classes for grouping data, usually called structs, Union or records in c and C++ family languages. These were used for storing only data, not methods.(This is the main disadvantage of structure and union).We can derive one class from another class.

Diagram of simple class and subclass.

Example of a simple class

class Bicycle {

    int cadence = 0;

    int speed = 0;

    int gear = 1;

    void changeCadence(int newValue) {

         cadence = newValue;

    }

    void changeGear(int newValue) {

         gear = newValue;

    }

    void speedUp(int increment) {

         speed = speed + increment;  

    }

    void applyBrakes(int decrement) {

         speed = speed - decrement;

    }

    void printStates() {

         System.out.println("cadence:" +

             cadence + " speed:" +

             speed + " gear:" + gear);

    }

}

Here bicycle is a class, Having three data member and five function member. In java by default the data member are public .Here public is nothing but access specifier , and control the access mechanism and invocation operation. In this above example as the data and methods are declared both within same class then method can access these data easily.

Access Specifiers of class

Generally there are three types of access specifier in java class

1)Public

2)Protected

3)Private

Public mode

public classes, methods, and fields can be accessed from everywhere. The only constraint is that a file with Java source code can only contain one public class whose name must also match with the filename.  If it exists, this public class represents the application or the applet, in which case the public keyword is necessary to enable your Web browser or appletviewer to show the applet. You use public classes, methods, or fields only if you explicitly want to offer access to these entities and if this access cannot do any harm. An example of a square determined by the position of its upper-left corner and its size:

public class Square {  // public class

  public x, y, size;   // public instance variables

}

Generally the methods are declared as publicly, So that from any class it’s possible to invoke.

Protected mode

protected methods and fields can only be accessed within the same class to which the methods and fields belong, within its subclasses, and within classes of the same package, but not from anywhere else. You use the protected access level when it is appropriate for a class's subclasses to have access to the method or field, but not for unrelated classes. Example of a protected data member is.

Class hello

{

  Protected void fun()

                    {

                    System.Out.Println(“Hello function”);

                    }

}

This protected method only accessible from it’s subclass.

 

Private mode

private methods and fields can only be accessed within the same class to which the methods and fields belong. private methods and fields are not visible within subclasses and are not inherited by subclasses. So, the private access specifier is opposite to the public access specifier. It is mostly used for encapsulation: data are hidden within the class and accessor methods are provided. An example, in which the position of the upper-left corner of a square can be set or obtained by accessor methods, but individual coordinates are not accessible to the user.

public class Square {   // public class

  private double x, y   // private (encapsulated) instance variables

  public setCorner(int x, int y) {  // setting values of private fields

    this.x = x;

    this.y = y;

  }

  public getCorner() {  // setting values of

Access Specifiers for Class Variables and Class Methods

Below is a table showing the effects of access specifiers for class members (i.e. class variable and class methods).

◆ = Can Access. ◇ = No Access.

Specifier	class	subclass	package	world
private	◆	◇	◇	◇
protected	◆	◆	◆	◇
public	◆	◆	◆	◆
(none)	◆	◇	◆	◇