processing recursion error

import processing.serial.*;

Serial myPort;        // The serial port
int xPos = 1;         // horizontal position of the graph
float inByte = 0;

/* Intensity is a measurement of how many beams are interrupted over a set of time. It increases as beams get interrupted up to a maximum of 1 and normalizes to 0 over time.*/

float intensity = 0;
float increasePerInterrupt = .08;
float decreasePerFrame = .003;
int frameRate = 10;

void setup () {
  size(600, 400);
  setupMorseChars();
  frameRate(frameRate);

  // List all the available serial ports
  println(Serial.list());

  // Open whatever port is the one you're using. 1 on my desktop pc, will need to check for Raspberry Pi
  myPort = new Serial(this, Serial.list()[0], 19200);

  // don't generate a serialEvent() unless you get a newline character:
  myPort.bufferUntil('\n');

  // Set up the morse signal
  setupMorseSignal();

  // Set up the morse playback thread
  thread("morsePlayback");

  // Set initial background:
  background(0);
}

import processing.sound.*;
SinOsc morseSignal;

/* An array of morse code waiting to be played */
char[] character_queue = new char[0];

// Length of the current morse signal in ms
int signal_length = 0;

// Delay until the next morse signal in ms
int pause_length = 0;

// Debug to count recursion calls
int recursion_count = 0;

/* Duration of dots and dashes, using common morse practices */

final int dot = 110;
final int pause = dot;
final int dash = dot * 3;
final int separator_length = dot * 7;

/* Special characters: character separator and word separator */
final char char_separator = ' ';
final char word_separator = '/';

/* Hashmap of numbers 0-9 */
HashMap<Character, String> code = new HashMap<Character, String>();

void setupMorseSignal() {
  morseSignal = new SinOsc(this);
  morseSignal.amp(.25);
  morseSignal.freq(300);
  //morseSignal.play();
}

void setupMorseChars() {
  code.put('0', "-----");
  code.put('1', ".----");
  code.put('2', "..---");
  code.put('3', "...--");
  code.put('4', "....-");
  code.put('5', ".....");
  code.put('6', "-....");
  code.put('7', "--...");
  code.put('8', "---..");
  code.put('9', "----.");
}

// Plays back a number as morse code
void enqueueMorseCode(String number) {

  // Store the number in a char array
  char[] chars = number.toCharArray();

  // Loop through the digits of the number
  for (int i = 0; i < chars.length; i++) {
    String morse = code.get(chars[i]);
    //println(morse);

    // For each digit, loop through the morse representation and add it to the morse queue
    for (int j = 0; j < morse.length(); j++) {
      character_queue = append(character_queue, morse.charAt(j));
    }

    // Add a space at the end of each digit, except the last
    if (i < chars.length -1) {
      character_queue = append(character_queue, char_separator);
    }
  }

  // Add a separator at the end of each number
  character_queue = append(character_queue, word_separator);


  //println(character_queue);

  return;
}

void morsePlayback() {

  recursion_count++;
  println(recursion_count);

  // If there are no characters in the queue, abort and try again later.
  if(character_queue.length == 0) {
    delay(100);
    morsePlayback();
    return;
  }

  // The current character to be played
  // println(character_queue[0]);
  switch(character_queue[0]) {
  case '-':
    signal_length = dash;
    pause_length = pause;
    break;
  case '.':
    signal_length = dot;
    pause_length = pause;
    break;
  case ' ':
    signal_length = 0;
    pause_length = dash;
    break;
  case '/':
    signal_length = 0;
    pause_length = separator_length;
    break;
  }

  // Remove the element we just used via some reversing and shortening
  character_queue = reverse(character_queue);
  character_queue = shorten(character_queue);
  character_queue = reverse(character_queue);

  // Start signal for duration x, then stop
  if(signal_length > 0) {
    //println("Start signal playback");
    morseSignal.play();
  }

  delay(signal_length);
  //println("Stop signal playback");
  morseSignal.stop();

  // Wait the right amount of time until next signal
  delay(pause_length);
  morsePlayback();
}