1. First Java Program

 

First Java Program

As when learning any new language, the place to start is with the classic Hello World! program. Our first Java program is as:

class FirstProgram{
    public static void main(String[] argc){
        System.out.print("Hello World!");
    }
}

To Compile:

javac FirstProgram.java

To Execute or interpret:

java FirstProgram

Output:

Hello World!

Let's break down the code to understand each line:

• class FirstProgram:
  • A Java program must contain at least one class.
  • class keyword is used to declare a class.
  • The file name should be same as the class name that contains main() function.
  • Here, our class name is FirstProgram.
  • All the code (i.e. variables and functions) should be inside our class whether in a single class or in different classes, but will be inside class only.
• public static void main(String[] args){..}
  • public keyword is an access modifier that represents visibility. public keyword means it is visible to all.
  • static is a keyword. In Java, we need to create an object to call a function or a method. But by declaring a method as static, we doesn't need to create an object to call our function.
  • void is the return type of the method or function. It means it doesn't return any value.
  • main is the function name and every Java program must contain the main() method.
  • String[] args or String args[] is a command line argument. It means an array of sequence of characters (i.e. String) that are passed to the main() function. This happens during the execution time of program.
• System.out.print("Hello World!")
  • System is an inbuilt class and out is the object of System class used to call the print() function.
  • System.out represents the output stream, where our output will go.
  • print is the function name.
  • The whole line is used to display the result on the screen.
  • Anything (except escape sequences) inside the inverted commas are treated as a string and will be printed on the screen as plain text.

Naming Conventions

• For classes, we use PascalConvention. The first subsequent characters from a word are capital letters. Example: FirstProgram, CodingBro.
• For functions and variables, we use camelCaseConvention. Here, the first character is lowercase, and the subsequent characters are uppercases like myArray, myFunction.

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