Java random file extension generator

import java.util.Scanner;
import static java.lang.System.out;
import java.lang.String;
import java.util.Random;

public class mainclass {
    public static void main (String[] args){
        Scanner myScanner = new Scanner(System.in);
        Random myRandom = new Random();

        // Declaration of variables
        String input;
        int randomNumber;
        String extension = null;

        // Begin program run
        System.out.println("Welcome to the random file extension generator.");
        System.out.println("Please enter your string of text to have an extension added.");
        // User input
        input = myScanner.nextLine();
        // End user input
        randomNumber = myRandom.nextInt(10);
        System.out.println("Thank you. Please wait.");
        if(randomNumber == 0){
            randomNumber = randomNumber + 1;
        }

        // Random extension generation

        if (randomNumber == 1){
            extension = ".png";
        }

        if (randomNumber == 2){
            extension = ".txt";
        }

        if (randomNumber == 3){
            extension = ".avi";
        }

        if (randomNumber == 4){
            extension = ".docx";
        }

        if (randomNumber == 5){
            extension = ".jpg";
        }

        if (randomNumber == 5){
            extension = ".html";
        }

        if (randomNumber == 6){
            extension = ".class";
        }

        if (randomNumber == 7){
            extension = ".wmv";
        }

        if (randomNumber == 8){
            extension = ".z64";
        }

        if (randomNumber == 9){
            extension = ".exe";
        }

        if (randomNumber == 10){
            extension = ".sys";
        }

        // End random extension generation

        System.out.println("Thank you. Your string with the random extension added is:");
        System.out.print(input + extension);
    }
}