const int ledPin = 13; // the pin that the LED is attached to
int incomingByte;      // a variable to read incoming serial data into

void setup() {
    // initialize serial communication:
    Serial.begin(9600);
    // initialize the LED pin as an output:
    pinMode(ledPin, OUTPUT);
}

void loop() {
    // see if there's incoming serial data:
    if (Serial.available() > 0) {
        // read the oldest byte in the serial buffer:
        incomingByte = Serial.read();
        // if it's a capital H (ASCII 72), turn on the LED:
        if (incomingByte == 'H') {
            digitalWrite(ledPin, HIGH);
        }
        // if it's an L (ASCII 76) turn off the LED:
        if (incomingByte == 'L') {
            digitalWrite(ledPin, LOW);
        }
    }
}

import processing.serial.*;

float boxX;
float boxY;
int boxSize = 20;
boolean mouseOverBox = false;

Serial port;

void setup() {
    size(200, 200);
    boxX = width/2.0;
    boxY = height/2.0;
    rectMode(RADIUS);

    // List all the available serial ports in the output pane.
    // You will need to choose the port that the Arduino board is
    // connected to from this list. The first port in the list is
    // port #0 and the third port in the list is port #2.
    // if using Processing 2.1 or later, use Serial.printArray()
    println(Serial.list());

    // Open the port that the Arduino board is connected to (in this case #0)
    // Make sure to open the port at the same speed Arduino is using (9600bps)
    port = new Serial(this, Serial.list()[0], 9600);

}

void draw(){
    background(0);

    // Test if the cursor is over the box
    if (mouseX > boxX-boxSize && mouseX < boxX+boxSize && mouseY > boxY-boxSize && mouseY < boxY+boxSize) {
        mouseOverBox = true;
        // draw a line around the box and change its color:
        stroke(255);
        fill(153);
        // send an 'H' to indicate mouse is over square:
        port.write('H');
    }else {
        // return the box to it's inactive state:
        stroke(153);
        fill(153);
        // send an 'L' to turn the LED off:
        port.write('L');
        mouseOverBox = false;
    }

    // Draw the box
    rect(boxX, boxY, boxSize, boxSize);
}