Java Program - Rules

Some important points to be remember before writing Java program are:

1. The code written in Java must be with in a class only.
2. Java is a case sensitive programming language, so be careful for small and capital letters.
3. While saving a program, the program name should be same as the main() class name followed by the java extension.
4. Java program processing starts from the main() method which is a mandatory part of every Java program.
5. Return type of main is always void.
6. Class Name – For naming a class, the first letter should be in Upper Case. If several words are used to form a name of the class, each inner word's first letter should be in Upper Case. Example: class MyFirstJavaClass
7. Method Names – All method names should start with a Lower Case letter.
   If several words are used to form the name of the method, then each inner word's first letter should be in Upper Case. Example: public void myMethodName().

Example 1: Write a Java first program to display a message "Hello World".

class First
{
    public static void main(String args[])
    {
        System.out.println("Hello World");
    }
}

Save the program with the name "First.java", Open 'Command Prompt', open directory in which you saved the program, and type the following commands:

• javac First.java - to compile the program.
• java First - to execute the program.

Code Explanation:

• class: class is a keyword used to declare a class. As stated earlier, Java is a pure object-oriented programming language therefore, everything must be placed inside a class. A class definition in C++ ends with a semicolon but not in Java.
• void main: This is similar to the main() function in C/C++. Every Java application program must include the main() method. This is the starting point to begin the execution of the program. A Java application can have any number of classes but only one main method.
• { }: Braces or curly brackets define the scope of the program.
• public: The keyword public is an access specifier that declares the main method accessible to all other classes.
• static: static is a keyword, it declare the main method as static means there is no need to create an object to invoke static method. The main must always be declared as static since the interpreter uses this method before any objects are created.
• String args[]: It declares a parameter named args, which contains an array of type String. String[] args is used for command line argument.
• System.out.println(): It is use to print statement.

Valid Java main method signature

public static void main(String[] args)  
public static void main(String []args)  
public static void main(String args[])
public static void main(String... args)
static public void main(String args[])
public static final void main(String[] args) 
final public static void main(String[] args) 

Invalid java main method signature

public void main(String[] args)  
static void main(String[] args)  
public void static main(String[] args)  
abstract public static void main(String[] args)

Example 2: Find Sum of 2 numbers.

class Sum
{
    public static void main( String args[] )
    {
        int a=10, b=20, c;
        c = a + b;
        System.out.println("Sum = " + c);
    }
}