Java programming with Hello World Example

We can write a simple hello java program easily after installing the JDK. To create a simple java program, you need to create a class that contains the main method. Let's understand the requirement first.

• The requirement for Java Hello World Example For executing any java program, you need to Install the JDK if you don't have installed it, download the JDK and install it.
• Set path of the jdk/bin directory.
• Create the java program
• Compile and run the java program

Creating Hello World Program Example

1. class Simple{
2.     public static void main(String args[]){
3.      System.out.println("Hello, Java");
4.     }
5. }

Output:

Hello, Java Compilation Flow: When we compile Java program using javac tool, java compiler converts the source code into byte code  

Parameters used in First Java Program

Let's see what is the meaning of class, public, static, void, main, String[], System.out.println().

• class keyword is used to declare a class in java.
• public keyword is an access modifier which represents visibility. It means it is visible to all.
• static is a keyword. If we declare any method as static, it is known as the static method. The core advantage of the static method is that there is no need to create an object to invoke the static method. The main method is executed by the JVM, so it doesn't require to create an object to invoke the main method. So it saves memory.
• void is the return type of the method. It means it doesn't return any value.
• main represents the starting point of the program.
• String[] args is used for command line argument. We will learn it later.
• System.out.println() is used to print statement. Here, System is a class, out is the object of PrintStream class, println() is the method of PrintStream class. We will learn about the internal working of System.out.println statement later.

The “Hello, Java!” program consists of three primary components: the Hello, Java class definition, the main method and source code comments. Following explanation will provide you with a basic understanding of the code:

1. Class definition:This line uses the keyword class to declare that a new class is being defined.

   class Simple
   

   HelloWorld is an identifier that is the name of the class. The entire class definition, including all of its members, will be between the opening curly brace  {  and the closing curly brace  } .
2. main method: In Java programming language, every application must contain a main method whose signature is:

   public static void main(String[] args)
   
   public: So that JVM can execute the method from anywhere.
   static: Main method is to be called without object. 
   The modifiers public and static can be written in either order.
   void: The main method doesn't return anything.
   main(): Name configured in the JVM.
   String[]: The main method accepts a single argument: 
             an array of elements of type String.

   Like in C/C++, main method is the entry point for your application and will subsequently invoke all the other methods required by your program.
3. The next line of code is shown here. Notice that it occurs inside main( ).

   System.out.println("Hello, Java");
   

   This line outputs the string “Hello, World” followed by a new line on the screen. Output is actually accomplished by the built-in println( ) method. System is a predefined class that provides access to the system, and out is the variable of type output stream that is connected to the console.
4. Comments: They can either be multi-line or single line comments.

   /* This is a simple Java program. 
   Call this file "HelloWorld.java". */
   

   This is a multiline comment. This type of comment must begin with /* and end with */. For single line you may directly use // as in C/C++.

How many ways can we write a Java program

There are many ways to write a Java program. The modifications that can be done in a Java program are given below: 1) By changing the sequence of the modifiers, method prototype is not changed in Java. Let's see the simple code of the main method. 2) The subscript notation in Java array can be used after type, before the variable or after the variable. Let's see the different codes to write the main method. 3) You can provide var-args support to the main method by passing 3 ellipses (dots) Let's see the simple code of using var-args in the main method. We will learn about var-args later in Java New Features chapter. 4) Having a semicolon at the end of class is optional in Java. Let's see the simple code.

class A{ static public void main(String... args){ System.out.println("hello java4"); } }

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Valid java main method signature

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Invalid java main method signature

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