I have processing running to an arduino currently.
The idea is, you press a button on the arduino, and processing writes the number 1, 2 or 3 to a file depending on what button is pressed.
This file is then read by javascript elsewhere that displays a picture related to that number.

What I'm currently stuck with is getting the file to be written consistently. It seems to work, but every now and again I get flash of no images while running my javascript. I've worked out its because when the file is written to each time, its being created with nothing in it for a split second. (ie, its an empty blank file)

Is there any way I can just edit the file, rather than re-creating a new one with the same name each time?

Thanks
Sam
Here's my code if it's of any help:

1. 
   import processing.serial.*;
   import cc.arduino.*;
   
   Arduino arduino;
   PrintWriter output;
   
   // constants won't change. They're used here to
   // set pin numbers:
   int greenbuttonPin = 4;     // the number of the pushbutton pin
   int greenledPin =  11;      // the number of the LED pin
   int amberbuttonPin = 3;     // the number of the pushbutton pin
   int amberledPin =  12;      // the number of the LED pin
   int redbuttonPin = 2;     // the number of the pushbutton pin
   int redledPin =  13;      // the number of the LED pin
   int LEDVal = 0;
   boolean red = false;
   boolean amber = false;
   boolean green = true;
   
   long time; //store time here for timing from time to time
   
   // variables will change:
   
   int redbuttonState = 0;         // variable for reading the pushbutton status
   int amberbuttonState = 0;         // variable for reading the pushbutton status
   int greenbuttonState = 0;         // variable for reading the pushbutton status
   
   void setup() {
    
     time = millis(); //set time = now
    
   arduino = new Arduino(this, Arduino.list()[0], 57600);
   
     arduino.pinMode(redledPin, Arduino.OUTPUT);     
    // initialize the pushbutton pin as an input:
     arduino.pinMode(redbuttonPin, Arduino.INPUT);
   // initialize the LED pin as an output:
     arduino.pinMode(amberledPin, Arduino.OUTPUT);     
     // initialize the pushbutton pin as an input:
     arduino.pinMode(amberbuttonPin, Arduino.INPUT);
   // initialize the LED pin as an output:
     arduino.pinMode(greenledPin, Arduino.OUTPUT);     
     // initialize the pushbutton pin as an input:
     arduino.pinMode(greenbuttonPin, Arduino.INPUT);  
   
    
   }
   
   void draw(){
    
    
     // read the state of the pushbutton value:
     redbuttonState = arduino.digitalRead(redbuttonPin);
     // read the state of the pushbutton value:
     amberbuttonState = arduino.digitalRead(amberbuttonPin);
      // read the state of the pushbutton value:
     greenbuttonState = arduino.digitalRead(greenbuttonPin);
    
   
     // check if the pushbutton is pressed.
     // if it is, the buttonState is HIGH:
     if (redbuttonState == arduino.HIGH) {
   
   
   
     red = true;
     amber = false;
     green = false;
     }
    
     
     else if (amberbuttonState == arduino.HIGH) {
    
    
     amber = true;
     red = false;
     green = false;
    
     }
    
     else if (greenbuttonState == arduino.HIGH) {
    
     green = true;
     amber = false;
     red = false;
    
    
     }
    
    if (green == true) { 
      arduino.digitalWrite(amberledPin, arduino.LOW);
      arduino.digitalWrite(redledPin, arduino.LOW);
     
       print("green true");
      
       try {
           output = createWriter("file.txt");
           output.println("1"); //Write "1" to the file
           output.flush(); // Write the remaining data
           output.close(); // Finish the file
          }
       
          catch(Exception e) {
          println("error: ");
          }
          
      while (millis() >= time + 5000)
     {
          
         // turn LED on/off:   
         LEDVal = 1 - LEDVal;
          if(LEDVal == 1) {
            arduino.digitalWrite(greenledPin, arduino.HIGH);
          }
          else if(LEDVal == 0) {
            arduino.digitalWrite(greenledPin, arduino.LOW);
          }
      
      
           
           time = millis(); //record current time
          // arduino.digitalWrite(greenledPin, arduino.LOW); 
         }
    
     }
    
     
    
    else if (amber == true) {
     arduino.digitalWrite(greenledPin, arduino.LOW);
     arduino.digitalWrite(redledPin, arduino.LOW);
     print("amber true");
    
     try {
           output = createWriter("file.txt");
           output.println("2"); //Write "2" to the file
           output.flush(); // Write the remaining data
           output.close(); // Finish the file
          }
          catch(Exception e) {
          println("error: ");
          }
           
      while (millis() >= time + 5000)
     {
          
         // turn LED on/off:   
         
             LEDVal = 1 - LEDVal;
          if(LEDVal == 1) {
            arduino.digitalWrite(amberledPin, arduino.HIGH);
          }
          else if(LEDVal == 0) {
            arduino.digitalWrite(amberledPin, arduino.LOW);
          }
      
           
           time = millis(); //record current time
         
         }
    
     }
      
    
    
      if (red == true) { 
        arduino.digitalWrite(greenledPin, arduino.LOW);
        arduino.digitalWrite(amberledPin, arduino.LOW);
        print("red true");
       
          try {
           output = createWriter("file.txt");
           output.println("3"); //Write "3" to the file
           output.flush(); // Write the remaining data
           output.close(); // Finish the file
          }
       
          catch(Exception e) {
          println("error: ");
          }
        
           
      while (millis() >= time + 5000)
     {
          
         // turn LED on/off:   
          LEDVal = 1 - LEDVal;
          if(LEDVal == 1) {
            arduino.digitalWrite(redledPin, arduino.HIGH);
          }
          else if(LEDVal == 0) {
            arduino.digitalWrite(redledPin, arduino.LOW);
          }
      
       
        
           time = millis(); //record current time
          
         }
    
     } 
     
   
     }