First Class

First Class

Concepts Involved With Classes In Java

Concepts Invoved with Classes

Any thing in java revolves around a class,Any concept we wish to implement in java program must be encapsulated with in a class .

• Ø      class and object

Class is used to define new data type, once defined this new type can be used to create objects.

Thus a class is a template for an object and an object is an instance of a class.

• Ø      General form of a class:

A class is declared by use of the class keyword.

class  

{

   type InstanceVariable1;

   type InstanceVariable2;

   type InstanceVariable3;

    //

   type InstanceVariableN;

  

   type MethodName1(ParameterList)

   {

      //body of method

   }

   type MethodName2(ParameterList)

   {

      //body of method

   }

      //

   type MethodNameN(ParameterList)

   {

      //body of method

   }

}

The data variables, defined with in a class are called instance variables. Code is contained in methods.

Collectively, the methods and variables are called members of the class.

• Ø      A Simple Class

class Box

{

double width;

double length;

double height; 

}

a class declaration only creates a template.

To create a Box object. This statement is used is

            Box MyBox = new Box(); // creates Box object called MyBox

• Ø      dot operator

The methods and the instance variables associated with an object can be accessed with the dot operator.

For example to assign values to width, length and height  variables with in MyBox object.

 

MyBox.width = 100;

MyBox.length = 100;

MyBox.height = 100;

• Ø      new operator

The new operator dynamically allocates memory for an object and returns reference to it. i.e; a new object is created with the operator new. For example

Box MyBox = new Box ();

1) Lets examine a program that uses the Box class

  /* A simple program demonstrating the ability to use classes     */

  class Box

  {

            double width;

            double height;

            double length;

  }

  class BoxDemo

  {

            public static void main(String args[])

            {

            Box MyBox;

            MyBox = new Box();

            MyBox.width  = 23.33;

            MyBox.height = 33.33;

            MyBox.length = 43.33;

System.out.println("length of MyBox object is "+ MyBox.length);

System.out.println("hight of MyBox object is "+MyBox.height);

System.out.println("width of MyBox object is "+MyBox.width);

            }         

 }

2) Lets re write the above program with a slight difference

/*  This program BoxDemo1.java calculates the volume of each box object*/

class Box

            {

              double length;

              double width;

              double height;

            }

            class BoxDemo1

            {

 

              public static void main(String args[])

              {

                Box MyBox1 = new Box ();

                Box MyBox2 = new Box ();

                Box MyBox3 = new Box ();

            double vol1,vol2,vol3;

//initializing objects MyBox1's variables

                MyBox1.width =23.33;

                MyBox1.height = 33.33;

                MyBox1.length =43.33;

//initializing objects MyBox2's variables

                MyBox2.width =33.33;

                MyBox2.height = 43.33;

                MyBox2.length =53.33;

//initializing objects MyBox3's variables

                MyBox3.width =43.33;

                MyBox3.height = 53.33;

                MyBox3.length =63.33;

//calculating MyBox1 volume

            Vol1 = (MyBox1.width*MyBox1.height*MyBox1.length);

//calculating MyBox1 volume

            Vol2 = (MyBox2.width*MyBox2.height*MyBox2.length);

//calculating MyBox1 volume

            Vol3 = (MyBox3.width*MyBox3.height*MyBox3.length);

//displaying the volume of MyBox1

                System.out.println ("volume of MyBox1 is  "+vol1);

//displaying the volume of MyBox2

                System.out.println ("volume of MyBox2 is  "+vol2);

//displaying the volume of MyBox3

                System.out.println ("volume of MyBox3 is  "+vol3);

 

              }

            }

• Ø      Declaring Objects

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, it is simply just a variable that can refer to an object. For example to declare a variable of Box class use the following syntax.

                 Box My Box; // My Box is just a variable of type Box, My Box contains NULL value.

• ·        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 memory for an object. The following code best illustrates this

MyBox = new Box() ; // allocate a Box object.

          Note object can also be created using the below statement

           Box MyBox = new Box();

• Ø      Assigning object reference variable

        When we assign one object reference variable to another object reference variable, we don't create  

       a copy of the object, but we only make a copy of the reference.

       consider the example below

   

       Box  MyBox1 = new Box();

       Box MyBox2 = MyBox1;

       Here MyBox1 and MyBox2 refer to the same object. as shown in the below picture

Width            

    

 

Height         

 

                 

Depth

 

MyBox1

MyBox2

 

Box Object

    So any changes made in width, height and depth in MyBox2, will be reflected in MyBox1 object also.

• Ø      Methods

If we observe the above program 1.3.4, we find that the instance variables are being accessed directly by the object MyBox1, MyBox2, MyBox3 which is not a favored situation in oops. As one of the oops principle, data abstraction and encapsulation is not implemented.Thus best way to achieve data abstraction and encapsulation is through methods.

3)// the program includes a method setValues() and another method volume()

class Box

{

 double  width;

 double  length;

 double  height;

 void setValues(double width,double length,double height)

 {

  this.width   = width;

  this.length = length;

  this.height = height;

 }

 void volume()

{

  System.out.println();

  System.out.println("the volume of the box is :    "+ (width*height*length));

}

}

class BoxDemo3

{

  public static void main(String args[])

 {

   Box MyBox1 = new Box();

   MyBox1.setValues(10,20,30);

   MyBox1.volume();

  }

 

}

You will observe that while assigning values to variables of class or while calculating volumes of each box, the objects are not allowed to access the data directly. (data abstraction and encapsulation)

Now consider if we wanted to calculate the cost of a box, depending on its volume. Observe the above program, the volume() method, just simply displays volume of a box on the screen, if we want to calculate the cost of the box, then volume calculated by the volume() method should be multiplied with the cost , that means the volume() method should return a value, let us see how this is achieved.

4) // this program introduces a method volume() that returns a value and this value is used to calculate the cost of the box.

class Box

{

double width,length,height,cost;

void setValue(double width,double length,double height,double cost)

{

this.width   = width;

this.length  = length;

this.height  = height;

this.cost     =  cost;

}

double volume()

{

return length * height * width;

}

void cost()

{

  System.out.println( "Cost of the box is   "+cost * volume() );

}

}

class BoxDemo4

{

public static void main(String args[])

{

  Box  MyBox1  = new Box();

  Box  MyBox2  = new Box();

MyBox1.setValue(10,20,30,4.5);

MyBox2.setValue(30,40,50,5.5);

MyBox1.cost();

MyBox2.cost();

}

}

In the above two examples, we have seen two types of methods: method that takes parameters,setValue() and

method with out parameters, volume().

• Ø      Constructor   

We know that ,when an object is created, it's  variables have to be initialized, Instead of initializing the class instance variables, after an instance is created, with convenient functions like setValues(), as in the previous example. It could be more convenient to have the initialization done, at the time the object is first created. Fortunately, java allows objects to initialize themselves when they are created. This automatic initialization is performed through the use of a constructor.

                               i.            A constructor initializes an object immediately upon creation.

                             ii.            A constructor has the same name as the class in which it resides and is syntactically similar to a method.

                           iii.            Constructors don't have return type not even void, this is because the implicit return type is class type itself.

Default constructor:

Lets us take a closer look at  the new operator in the below statement

class_var = new class_name();

Now you can understand why the parenthesis are needed after the class_name().

What is actually happening is that the constructor for the class is being called.

This is in the line Box MyBox1 = new Box ();

new Box () is actually is calling a default constructor.Java creates a default constructor, because the constructor is not defined in the class,

The default constructor automatically initializes all instance variables to zero.

Parameterized Constructor's:

The default constructor discussed above, is of no use, though it initializes the instance variables of Box to zero because a box will have dimensions other than zero. So to construct Box objects of various dimensions, the easy solution is to add parameters to the constructor. As shown in the example below

 5)  /* In this program, here Box uses a parameterized constructor which sets the dimensions of a box as  Specified by those parameters     */

class Box{

double length,height,width;

 

 Box(double length,double height,double width )

   {

     this.length = length;

     this.height = height;

     this.width = width;

    }

double volume( ){

    return length * height * width;

    }

 }

class BoxDemo5{

public static void main(String args[]){

Box MyBox1 = new Box(10,20,30);

Box MyBox2 = new Box(40,50,60);

double vol;

vol = MyBox1.volume();

System.out.println("volume of MyBox1 is  "+ MyBox1.volume());

System.out.println("volume of MyBox2 is  "+ MyBox2.volume());

}

}

• Ø      This Key Word

 Sometimes a method will need to refer to the object that invoked it. Java defines the "this" key  

 Word, for this purpose. "This"key word can be used inside any method to refer to the current

 Object.  

6)    //this program demonstrates the ability and usage of this key word

 class Box

  {

            double width;

            double height;

            double depth;

                double cost;

     Box (double width, double height, double depth,double cost)

     {

             this.width = width ;

             this.height = height;

             this.depth = depth;

             this.cost = cost;

     }

    double volume(){

    return width*height*depth;

    }

   double cost()

  {

   return this.volume()*cost;

  }

   }

  class BoxDemo6

  {

   public static void main (String args[])

   {

   Box MyBox1 = new Box (23.33,33.33,43.33,5.5);

   Box MyBox2 = new Box (20,30,40,5);

   System.out.println("cost of MyBox1   "+MyBox1.cost());

   System.out.println("cost of MyBox2   "+MyBox2.cost());

  

   }                  

 }

• Ø      Instance Variable Hiding

 

It is illegal to declare two variables with the same name, with in the same scope but Interestingly, you can have local variables, including formal parameters to methods, which overlap with the names of the class's instance variable.

However when the local variable has the same name as the instance variable, it hides the instance variable. This was the reason, name's width, height & depth were not used as the names of the parameters' to the Box() Constructor. if they had been then width, height and depth would have referred to the formal parameter's, there by  hiding the instance variable's (width, height and depth).

To handle such a situation considers the program below.

 

/*    Technique to resolve when local variables' names'

       are same as names' of instance variables of class  */

  /*       A simple program demonstrating the ability to use classes     */

 

  class Box

  {

            double width;

            double height;

            double length;

 

     Box(double length,double width,double height)

     {

         this.width = width;

             this.height = height;

         this.length = length;

     }

  }

 

 

  class BoxDemo8

  {

            public static void main(String args[])

            {

              Box MyBox = new Box(23.33,33.33,43.33);

System.out.println("length of MyBox object is "+ MyBox.length);

System.out.println("length of MyBox object is "+MyBox.height);

System.out.println("length of MyBox object is "+MyBox.width);

            }         

 }

• Ø      Garbage Collection

An object is assumed to be no longer needed, when no references exist to an object, hence such objects have to be destroyed, so that the memory is reclaimed. In java, there is no explicit need to destroy objects as in c++.

Garbage collector will act, automatically, when ever one or more unused objects exist.

Java run time environment will decide when the garbage collection be done. However, we can only request the java run time environment to invoke the garbage collector. Again it is at the decision of Java run time environment, to decide when to invoke the garbage collector. We can invoke by calling, static method  gc() of System class.

Lets us understand it by an example shown below

     //this program demonstrates the ability and usage of this key word

 class Box

  {

            double width;

            double height;

            double depth;

                double cost;

     Box (double width, double height, double depth,double cost)

     {

             this.width = width ;

             this.height = height;

             this.depth = depth;

             this.cost = cost;

     }

    double volume(){

    return width*height*depth;

    }

   double cost()

  {

   return this.volume()*cost;

  }

   }

  class BoxDemo7

  {

   public static void main (String args[])

   {

   System.gc();

   Box MyBox1 = new Box (23.33,33.33,43.33,5.5);

   Box MyBox2 = new Box (20,30,40,5);

   System.gc();

   System.out.println("cost of MyBox1   "+MyBox1.cost());

   System.out.println("cost of MyBox2   "+MyBox2.cost());

   System.gc();

   }                  

 }

• Ø      Finalization

By using finalization mechanism, we can define specific actions that should occur, when an object is just about to be destroyed, by the garbage collector.

an object will need to perform some action when it is destroyed.

 

finalize() method

Some times, an object might be holding non java resource, such as a file handle  or window character font. So the object needs to free these resources, before it is destroyed. Java provides a mechanism called finalization, So the object performs some action before it is finally destroyed.

To achieve finalization, we simply define the finalize() method, in the class.Inside the finalize() we will specify those actions that must be performed before the object of that class ,is destroyed.

The general form is

protected void finalize()

{

// finalization code here

}

here, the key word protected is a specified that prevents access to finalize() method, defined outside its class.

 

 

About the Author

the author is trainer and developer in oracle and java technolgies. you can reach me on raju.allu@yahoo.com

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