Introduction to Java

Introduction to Java Programming

Java is a powerful object-oriented programming language introduced by Sun Micro systems in 1995, which has built-in support to create programs with a graphical user interface (GUI), utilize the Internet, create client-server solutions, and much more. Programs written in Java can run, without change, on any of the common computer operating systems Windows 95/NT, Macintosh, and Unix. A variant of Java programs called applets can be embedded inside a web page and execute on the computer that is viewing the page, automatically and in a secure environment.

As a language, Java is closely related to C++, which is also object-oriented but retains a lot of idiosyncrasies inherited from its predecessor language C. Java has removed the inconsistent elements from C++, is exclusively object-oriented, and can be considered a modern version of C++ Because of its logical structure Java has quickly become a popular choice as a teaching language, and because of its extensive Internet support and the promise of writing programs once and using them on every operating system Java is becoming more and more accepted in industry.


Basic Java Programming Guidelines

Every Java program must follow these guidelines:

    Java is case sensitive, i.e. the word Program is different from program.
    Curly brackets { and } are used to group statements together.
    An executable Java program must contain at least the following lines as a framework:

    package com.sample;

    public class MainClass {

    public static void main(String args[]){

     }

    }

           Every statement whose next statement is not a separate group must end in a semicolon.
    A Java program containing the above framework must be saved using the file name MainClass.java, where MainClass (including correct upper and lower cases) is the word that follows the keywords public class and the file extension is .java.



Source Code

A Java source code file is a text file that contains programming code written according to the Java language specifications, resembling a mixture of mathematical language and English. A computer cannot execute source code, but humans can read and understand it.

Java source code files should be saved as Name.java, where Name is the name that appears in the first line of the program: public class Name. That Name is referred to as the name of the class, or program. By convention its first letter is capitalized.


      package com.sample;

      public class Test {

           public static void main(String args[]) {

           System.out.println("Hi this is my first program");
       
    }
}

Output:

Hi this is my first program


This program, or class, is called Test and must be saved under the file name Test.java.


Compiling a Java Program or Class

A source code file, which is more or less readable in plain English, needs to be transformed into another format before the computer can act upon it. That translation process is called compiling and is accomplished using the Java compiler javac from the Java Developer's Kit (JDK), which could be invoked by an IDE such as BlueJ.

Definition For Compiling

Compiling is the process of transforming the source code file into a format that the computer can understand and process. The resulting file is called the byte-code, or class, file. The name of the class file is the same as the name of the program plus the extension .class. The program javac from the Java Developer's Kit is used to transform a source code file into a class file.

If a source code contains any errors, they are flagged by the compiler. You need to fix them and re-compile until there are no further errors.

Tip: In case of an error, the javac compiler shows the line number and position of where it thinks the error occurred in your source code.

    If the compiler points out an error, then there is an error at or before the indicated position.
    If the compiler reports a certain number of errors, than this is the least amount of errors.
    If one error is fixed, other errors may automatically disappear or new ones may appear.


Executing a Java Program or Class

The Java compiler does not produce an executable file, so Java programs can not execute under the operating system of your machine. Instead they execute inside a Java Virtual Machine, which is invoked using the java program of the JDK.

Definition For Executing a Class File

To execute a Java program the Java Developer's Kit provides a program called java. When executing that program with your class file as parameter the following happens:

    the Java Virtual Machine (JVM) is created inside your computer
    the JVM locates and reads your class files
    the JVM inspects your class file for any security violations
    the JVM executes, or interprets, your class file according to its instructions if possible


Default Program Entry Point

The default program entry point is that part of a class (or program) where execution begins. For every Java class (or program), the standard program entry point consists of the line:

   public static void main(String args[])


If that line is not present in your source code, the JVM can not execute your program and displays an error message.

At this point, we need to explain what the Java Virtual Machine is and how it relates to the operating system and to Java class files.


Java Virtual Machine (JVM)

The Java Virtual Machine (JVM) is a platform-independent engine used to run Java applets and applications. The JVM knows nothing of the Java programming language, but it does understand the particular file format of the platform and implementation independent class file produced by a Java compiler. Therefore, class files produced by a Java compiler on one system can execute without change on any system that can invoke a Java Virtual Machine.

When invoked with a particular class file, the JVM loads the file, goes through a verification process to ensure system security, and executes the instructions in that class file.