[Novice Help]what's wrong of my prime number programme !!~?

Question

import javax.swing.*;
public class prime1 {
     public static void main (String args[])
          String number;
          int n;
          number = JOptionPane.showInputDialog( "Enter" );
          n = Integer.parseInt( number );
          for ( int counter = 2 ; counter < n ; counter++ ){
               if ( n % counter != 0 )
                    System.out.println(
                         " It is a prime number ");
               else
                    System.out.println(
                         " It is not a prime number ");
               System.exit(0);
Why n only mod 2,but not mod all number which is smaller then n?
when i input 9 ,it is wrong ~
i discover that when 9 is mod 2 ,it will not continue to mod 3,4,5,6,7,8

Answer 1

The else only applied to the lien directly after it as there were no brackets to collect the two statements in an else block.
This means that the program was going into the for loop, checking you if/else then always running System.exit(0) which stopped the program before the loop had finnished.
Your statement "it is prime" is also in the wrong place. This really should say "it is no divisible by " + counter
as there are still numbers to check.
Try this revised version and see if you can understand the difference:
import javax.swing.*;
public class prime1 {
     public static void main (String args[]) {
          String number;
          int n;
          number = JOptionPane.showInputDialog( "Enter" );
          n = Integer.parseInt( number );
          for ( int counter = 2 ; counter < n ; counter++ ){
               if ( n % counter != 0 )
                    System.out.println("It is not divisible by " + counter);
               else{
                    System.out.println("It is not a prime number");
                    System.exit(0);
          System.out.println("It is prime");