This is an example of how to connect to an Arduino automatically in Processing

auto-arduino.pde

import processing.serial.*;
Serial arduino;
 
void setup() {
  // Create a canvas
  size( 800, 600 );
 
  // Get a list of serial ports
  String ports[] = Serial.list();
  int id = 0;
 
  // Find an Arduino tty.usb* port
  for ( int i = 0; i < ports.length; i++ ) {
    if ( ports[i].indexOf( "tty.usb" ) != -1 )
      id = i;
  }
 
  // Connect to the Arduino
  println( "Connecting to: " + ports[id] );
  arduino = new Serial( this, ports[id], 9600 );
}
 
void draw() {
  
}
 
// New Serial Data Event
void serialEvent(Serial p) {
  println( p.readChar() );
}