Checking Integer

Question

Hello. I have just recently begun to learn java. I was attempting to create a function which tests if a string is and integer. I incorporated it into a prime number program that I wrote. The idea on how it (isInt function) works is that it goes through each substring of the string and compares it to each substing of the variable "numbers" which is the cantonation of all decimal number system numbers. If the substing of the string does not match any of the 10 (kept track by counter 'c') 'numbers' then it returns a 0 meaning false that it is not a integer. Otherwise it returns a 1 for true. The prime number part is not the problem, just the isInt part. It doesn't seem to work how I thought it would. Here it is:
// Prime.java
// Tests integers if prime
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
public class Prime extends JApplet
     implements ActionListener {
     JLabel numberLabel, resultLabel;
     JTextField numberField, resultField;
     public void init()
          Container container = getContentPane();
               container.setLayout( new FlowLayout() );
          numberLabel = new JLabel( "Enter Integer:" );
               container.add( numberLabel );
          numberField = new JTextField( 10 );
               container.add( numberField );
               numberField.addActionListener( this );
          resultLabel = new JLabel( "Prime?" );
               container.add( resultLabel );
          resultField = new JTextField( 10 );
               resultField.setEditable( false );
               container.add( resultField );
     public void actionPerformed( ActionEvent e )
          String s = numberField.getText();
          if ( testInt( s ) == 0 )
               showStatus( "Not an Integer." );
          else {
               double n = Double.parseDouble( s );
               showStatus( "Calculating..." );
               if ( isPrime( n ) == 0 )
                    resultField.setText( "False" );
               else
                    resultField.setText( "True" );
               showStatus( "Done." );
     int isPrime( double n ) {
          if ( n == 1 )
               return 0;
          if ( n % 2 == 0 )
               if ( n == 2 )
                    return 1;
               else
                    return 0;
          for ( int i = 2; i <= Math.floor( Math.sqrt( n ) ); i++ )
               if ( n % i == 0 )
                    return 0;
          return 1;
     int testInt( String s ) {
          String numbers = "0123456789";
          int c = 0;
          for ( int i = 0; i < s.length(); i++ ) {
               for ( int n = 0; n < numbers.length(); n++ ) {
                    if ( numbers.substring( n, n+1 ) != s.substring( i, i+1 ) )                                   c++;
               if ( c == 10 )
                    return 0;
               c = 0;
          return 1;
}

Answer 1

Hey-
A quick way to check (if you're not tied to your testInt method) is this:
boolean testInt(String s) {
    boolean retVal = true; // assume we have a number
    try {
        int n = Integer.parseInt(s);
    catch (NumberFormatException nfe) {
        // this was not a number
        retVal = false;
    return retVal;
}Hope that helped
Lee