Java Hello World Example Requirements

To run Java programs, you must have the following software or applications properly installed:

• Install the JDK. Download and install the JDK if it is not already installed.
  
• Sets the path of the jdk/bin directory. See how to set path of jdk directory
• Now, Writing first Java Programs.
  

Java Example

Let's have a quick look at Simple Java hello world program example

  /* This is first java program 

   This will print 'Hello Alok'  

   as the output */  

 class Simple{  

public static void main(String args[ ]){

System.out.println("Hello Alok" );

}

}

Hello Alok 

In the above Java program you have use following keyword

• class - class is a keyword is used to define a class in java program.
  
• public - The public keyword is an access modifier that set visibility and accessibility . public indicates it is visible to everyone.
  
• Static - Static is a keyword any method that is declared as static is known as a static method. The primary advantage of the static method is that it does not need the object to be invoked. Because the main() function is run by the JVM, As a result, memory is saved.
  
• void - The Void method returns nothing. It indicates it does not return any value. If it is int it will return value you will these things later
  
• String[] args or String args[] - For command line arguments, use String[] args or String args[].
  
• main - Main keywoed specify the program's starting point.
  
• System.out.println() -To print the sentence in output screen, we use System.out.println() . System is a class, 'out' is a PrintStream class object, and 'println()' is a PrintStream class function.
  

Internal detail how java program execute

Till now,We have learnt how to compile and run a java programme, as well as about the first java programme.
Here, we'll discover what occurs when Java applications are compiled and executed.

What happens at compile time?

At compile time, java file is compiled by java compiler and converts the java code or source code into bytecode i.e .class file

  /* This is my first java program 

   This will print 'Hello World!'  

   as the output */  

 class Simple{  

public static void main(String args[ ]){

System.out.println("Hello World!" );

}

}

Hello World! 

What happens at run time?

At the run time following steps are performed:

• Class File
  
• Classloader
  
• Bytecode verifier
  
• Interpreter
  
• Runtime
  
• Hardware
  

• Class file - Class file are the file that contain java byte code with .class extension.
  
• Classloader - Classloader is the part of jvm that is used to load the class file.
  
• Bytecode Verifier - Bytecode Verifier checks the code fragments for illegal code that can violate access right to objects.
  
• Interpreter - Interpreter read bytecode stream then execute the instruction.
  
• RunTime - Java Runtime is a software that java program require to run.
  

What is bytecode in java?

Bytecode in java is just a symbol and each symbol taking one byte in memory that is the reason is called bytecode. Bytecode i.e .class file is generated by the java compiler and after compilation of java source code bytecode is generated, now these bytecode are loaded into java virtual machine(JVM) and then interpreted and then execute. Java programming language is WORA(Write Once Run Anywhere) programming language i.e it is reusable and it save time too.

• Java bytecode is portable
• Java bytecode is platform independent

java file(source file)→compiler→.class file(bytecode)→machine code