The following code should get you working.
import processing.serial.*;
Serial myport; // Create object from Serial class
void setup() {
// The next 3 lines are needed to call the event
String arduinoPort = Serial.list()[0];
myport = new Serial(this, arduinoPort, 115200);
myport.bufferUntil('\n');
}
// This function must be called serialEvent for this to work!!!
// it is part of the processing.serial library and is defined this way
void serialEvent(Serial myport)
{
String inString = myport.readStringUntil( '\n' );
if(inString != null )
{
inString = trim(inString);
// process the code as you wish from here
}