Creating Objects

Creating Objects

                 Creating objects means to allocate memory space for all the instance variables of the objects. Step to create an object are,

1. Declare an object.

2. Define the object using new operator. The new operator dynamically allocates memory for an object and returns a reference to it.

The general form is

1. Declare an object.

2. Define the object using new operator. The new operator dynamically allocates memory for an object and returns a reference to it.

The general form is

classname obj; obj=new classname();

Where

                classname           -              defined classname

                obj                         -              name of the object

                new                       -              keyword

Example

Student s1;  //this declares an object s1 of type student.

S1=new Student(); //this allocates memory space for object s1.

                 

The following figure shows the representation of student object in the memory.

Statement                                           Effect

Student s1;                                         s1

S1=new Student();                          

                The above two steps can be combined into one step as,

                Student s1=new Student();

Creating and Executing Java Program

Creating and Executing Java Program

     The following steps are follwed to create and execute a java program.

1 . Create the program using any text editor such as notepad or word pad etc. The general form for saving the created program in a  file is

                                                    

                                               filename.java

                       Where,  

                                     filename - Name of the class containing the main method

                                     java         - Keyword

     Ths file is called source file. If the program has more than one class, the file name must be the name of the main class.

 

Example

                                class Sample   // main class

                                { 

                                         public static void main(String args[])

                                         {

                                                 -------------------------

                                                 -------------------------

                                          }

                                  }

               Store this program as Sample.java

2 . Compile the created programs using java compiler. The general form of compile command is

                                                       

                                                      javac sourcefilename.java

                                 Where,

                                               javac                  - name of the java compiler

                                              sourcefilename - name of the already created source file.

   

     Java compiler creates a file called class file which cotains platform independent bytecodes of the program. The compiler automatically names the class file. The general form is

              

                                                      classname.class

                                   Where,

                                                classname - name of the main method class in our program

                                                class           - keyword

Example

                 Name of our source file - Sample.java

                 Compilation command   - javac Sample.java

          This create a class file as Sample.class

3 . Run the compiled program using java interpreter. The general form of run command is

                                                     java classname

                                    Where,

                                                 java           - name of the java interpreter

                                                classname - name of the main method class in our program

      Java Interpreter produces machine code from the bytecode and run the program.

Example

                 Name of our source file - Sample.java

                 Name of class file           - Sample.class

     To run the program give the run command at the command prompt as

                                                   c>java Sample

Java Virtual Machine

Java Virtual Machine

      Java compiler compiles the source code and produces a machine independent intermediate code called bytecode. This  code cannot be used dirctly by the computer. This intermediate codes are called java virtual machine (JVM).

      These intermediate codes can be used by any machine with the help of the correct interpreter. The interpreter produces the machine dependent code called machine code can be run by the computer.

       

--->sourcecode--->java compiler--->bytecode--->javainterpreter--->machinecode

     This shows the steps involved in executing a java pragram.

Charcteristics of Java Program

Charcteristics of Java Program

The following are the important characteristics of java program. They are

1 . Simple, small and familiar

2 . Object oriented

3 . Distributed

4 . Robust

5 . Secure

6 . Architecture neutral or platform independent

7 . Portable

8 . Compiled and interpreted

9 . High performance

10 . Multithreaded and interactive

11 . Dynamic and extensible

1 . SIMPLE, SMALL AND FAMILIAR

            Java is a simple small language. The syntax of java is just like C++ language. So it is very easy to learn. But programming in java is easier than C++ because

·         It does not use header files

·         It eliminates the use of pointers

·         It eliminates operator overloading and virtual base classes.

So java is a simple small object oriented language. Since java is a small language so we can write software that can run in small computers.

2 . OBJECT ORIENTED

            Java is a pure object oriented language. Everything in java is an object. All programs and data reside inside objects and classes. Object models in java is simple and easy to extend.

3 . DISTRIBUTED

            Java has strong networking facilities. So using java we can create applications on networks. Using java we can open and use the applications on the internet. This facility helps the users from different places to work together on a single application.

4 . ROBUST

            In many other languages attention is not given on memory management or exceptional behavior of a program in different situation. So most programs failed. But these issues are clearly dealt in java using the techniques called garbage collection and exception handling.

            In garbage collection java uses a thread to free the objects which are not in use. So the programmers do not have to worry about the memory management. In exception handling java uses exclusively written codes to correct the exception situation.

5 . SECURE

            Since java is used for programming on internet, security becomes an important issue. Before a java code from internet is interpreted, a security check is applied on it. This ensures that the java code does not contain any unwanted elements like viruses. Java has a facility to sign our java code before sending it. At the receiving end the receiver can tally the signature. If the signature matches, the codes reached correctly. If does not match the codes are not correct. This concept is called digitally signing.

6 . ARCHITECTURE NEUTRAL OR PLATFORM INDEPENDENT

            Java compiler generates an architecture neutral or platform independent code called byte code. These codes can be run in any type of system. The figure given below shows this.

                     source code --> java compiler --> byte code           

7 . PORTABLE

            Java compiler generates a code called byte code and this code can be used by any machine. In java the size of the primitive data types are machine independent. So java is a portable language.

8 . COMPILED AND INTERPRETED

            Generally computer languages are either compiled or interpreted. But java combines both compiler and interpreter. So java is a two stage as shown below.

  

source code --> compiler --> byte code --> interpreter --> machine code

        

         Java compiler generates a machine independent code called byte code. These codes are not machine code. But java interpreter generates machine code from byte code that can be directly executed by the machine that is running the java program.

9 . HIGH PERFORMANCE

            Since java interpreter uses byte codes, the performance is high. The speed is also comparable to other languages lake C, C++.

10 . MULTITHREADED AND INTERACTIVE

            Multithreaded means handling more than one job at a time. Java supports multithreading.  Java also supports constructing interactive programs.

11 . DYNAMIC AND EXTENSIBLE

            Java is a dynamic language. So it is capable of linking dynamically new classes, methods and objects. Java also supports functions written in other languages such as C and C++. These functions are called native methods. During run time native methods can be linked dynamically.

Introduction to Java

Introduction to Java

     Java is a platform independent object oriented language. It was developed by James Gosling and Patrick Naughton of Sun Microsystems, USA in 1991. 

     It was the first programming language which was not dependent on any particular hardware or operating system. It was initially named as Oak and later renamed as Java in 1995. 

      Java language is to nest suited for developing web based applications.

Getting Values of Variables

Getting Values of Variables

 The values of the variable are displayed on the VUD by using the following two methods

                     i.            Print ( )

                   ii.            Println ( )

Print ( ) method prints the output on a line until a newline character is encountered.

Println ( ) method prints the output on a line and the control comes to the new line.

The above two methods are invoked by using the object systems. Out as shown below.

                                                        System.out.print  ( );

                                                        System.out.println ( );

Example:

System.out.print (“Good”)

System.out.print (“Luck”)

The output is 

                       Good Luck

System.out.println (“Good”)

System.out.println (“Luck”)

The output is 

                    Good

                     Luck

System.out.print (“Good\n”)

System.out.print (“Luck”)

The output is 

                         Good

                         Luck

Java variables:

A variables refers to the name given to the memory location to store data. A particular memory location is given only one name. yet this name is called a variable because this memory location can store sequence of different values during the execution of the same program.

Rules:

1. A variable name is formed with alphabets, digits, underscore( _ ), dollar sign.

2 . The first character must be an alphabet.

3 . Variable name can be of any length.

4 . Both upper case and lower case letters are used. But they are not treated as same.

5 . It should not be a keyword word.

Example:

1.       The following are some valid variable names.

AO          BASIC_PAY         volume                 B12

2.       The following are some invalid variable names.

Variable	Reason for invalidity
AB 9A While	Period ( ) not allowed The first character should be an alphabet Reserved word

Declarations of Variables

Declarations of Variables

     All variables present in the problem must be declared before it is used. This is done with the help of declaration statement. The general form is

Data-type variable list;

                       Where,

                                  Data-type            -              valid data type such as int, char etc.

                                  Variable unit        -              list of variable separated by comma.

Example:              

               

          Int  a, b, c;

This declares a, b and c as integer variables and allocate memory space.

Use of declaration:

i.                     It gives name to memory location

ii.                   It allocates memory space

iii.                  It gives the types of data

Giving Values to variables:

               

        Giving values to already declared variables is called assigning values to variables.  this is done with the help of assignment operator (=). The general form is

Variable name  =             value;

Example:

1.                   X     =             20;

2.                   PI    =             3.14;

Initializing variable:

The process of assigning initial values to variables is called initializing variables. This is done at the time of declaration. the general form is

Data-type variable = initial value;

Example:

1.       Int  a11                 =             100;

2.       Char sex               =             ‘F’;

3.       Double pay         =             80.32;

4.       Int a, b                  =             20;