title: How to write a program that reads from and write to file
body: <p>I'm  reading a file with numbers checking if the number is a prime number then writing  next to the prime numbers "is a prime" and printing that out to a different file,
  I keep getting:</p>
  
  <blockquote>
    <p>Failed to open file in4.txt Exiting...</p>
  </blockquote>
  
  <p>This is my code:</p>
  
  <pre><code>import java.util.Scanner;
  import java.util.ArrayList;
  import java.io.File;
  import java.io.IOException;
  import java.io.PrintWriter;
  
  public class CheckPrimes {
  
      public static void checkPrimes(String in_file, String out_file) {
          File temp = new File(in_file);
          Scanner input; 
          try 
          {
             input = new Scanner(temp);
          } 
          catch (Exception e) 
          {
              System.out.printf("Failed to open file %s\n", in_file);
              return;
          }
          while (true) 
          {
            for (int i = 2; i &lt; input.nextInt(); i++) 
            {
             if (input.nextInt() % i != 0) 
             {
               try{
                 PrintWriter output = new PrintWriter(out_file);
                 output.print( input.nextInt() + " is prime");
                 output.close();
                  }
               catch(IOException ex)
               {
                   System.out.printf("Error : %s\n",ex);
               }
  
             }
  
            }
          }
    {
    public static void main(String[] args)
    {
      checkPrimes("in4.txt", "out4.txt");
      System.out.printf("Exiting...\n");
    }
    }
  </code></pre>