A basic program that gets input number (Integer) from the user and prints whether the number is prime or not. By this program, you can understand the usage of commandline arguments, use of arithmetic operators, printing variables, user-defined method and Prime number checking algorithm.

There are a lot of ways to check whether a number is prime or not. The simplest method is to divide a number x with the numbers from 2 to x, to find out its factors. If there are no factors then x is a prime number.

The logic used here is somewhat faster than dividing through all the numbers and finding the factors.

Suppose n is not a prime number (greater than 1). So there are numbers a and b such that

n = ab      (1 < a <= b < n)
By multiplying the relation a<=b by a and b we get:

a^2 <= ab
 ab <= b^2
Therefore: (note that n=ab)

a^2 <= n <= b^2
Hence: (Note that a and b are positive)

a <= sqrt(n) <= b
So if a number (greater than 1) is not prime and we test divisibility up to square root of the number, we will find one of the factors.

Task: Print whether a number is prime or not where the number is taken as input from the user.

  Eg: number = 43
  Final Output: 
          [x] 43 is a Prime Number.
          
  Eg: number = 121
  Final Output:
          [x] 121 is not a Prime Number.

The filename must have the same name as the public class name, This is to ensure that the Java Virtual Machine (JVM) understands the entry point of the program

To run the program, open your commandline and type the following:

• Without Commandline Arguments

 $ ..\JavaBasic> javac .\PrimeChecker\PrimeChecker.java
 $ ..\JavaBasic> java PrimeChecker.PrimeChecker

• With Commandline Arguments

 $ ..\JavaBasic> javac .\PrimeChecker\PrimeChecker.java
 $ ..\JavaBasic> java PrimeChecker.PrimeChecker 2

package PrimeChecker;

import java.util.Scanner;

public class PrimeChecker {
    public static void main(String[] args) {
        if (args.length > 0) {
            if (args.length == 1)
                primeCheck(Integer.parseInt(args[0]));
            else
                System.out.println("[!] Available arguments: [number] ");
        } else {
            Scanner scan = new Scanner(System.in);
            System.out.print("[->] Enter the number to check: ");
            int number = scan.nextInt();
            scan.close();
            primeCheck(number);
        }
    } // end of main method

    public static void primeCheck(int number) {
        boolean isPrime = true;
        if (number == 1)
            isPrime = false;
        else if (number < 1) {
            System.out.println("[!] Please enter a natural number.");
            return;
        }
        for (int i = 2; i <= Math.sqrt(number); i++) {
            if (number % i == 0) {
                isPrime = false;
                break;
            }
        }

        System.out.println("[x] " + number + (isPrime ? " is a Prime Number." : " is not a Prime Number."));
    }
}

import java.util.Scanner; : imports Scanner class from java > util package. Scanner is a class that lets you get input form the user. Scanner can take any primitive data type as input from the user.

public static void main(String[] args) : main method/function is the first method that will be executed in any java programs.

String[] args : args is a array of String type. It is used to get commandline arguments for your program.

args.length > 0 : checks whether args is empty or not

System.out.println(); : Prints the String and changes output cursor to next line

Scanner scan = new Scanner ( System.in ); : scan is an object(a special variable that stores the address in RAM/main memory) of the class Scanner. Here scan acts as the variable that has the access to System.in stream (An input stream that connects keyboard input to the program.

scan.nextInt() : gets Integer input from user.

scan.close(); : closes the input stream link/pipe.

public static void primeCheck(int number) : user defined method that prints whether a number is prime or not.

Math.sqrt(number) : A predefined method to find the Square Root of a number