Arduino is attached to COM4 and is running a program that writes data back to Serial:

2. const int SENSOR=0;
3. int val=0;
4. 
   
5. void setup()
6. {
7.   Serial.begin(9600);
8. }
9. 
   
10. void loop()
11. {
12.   val = analogRead(SENSOR);
13.   Serial.println(val);
14.   delay(100); 
15. }

 When I try to read output from COM4 into processing I use the follow code from the Processing Serial manual:

2. import processing.serial.*;
3. 
   
4. Serial myPort;  // The serial port
5. 
   
6. void setup() {
7.   // List all the available serial ports
8.   println(Serial.list());
9.   // I know that the first port in the serial list on my mac
10.   // is always my  Keyspan adaptor, so I open Serial.list()[0].
11.   // Open whatever port is the one you're using.
12.   myPort = new Serial(this, Serial.list()[2], 9600);
13. }
14. 
    
15. void draw() {
16.   while (myPort.available() > 0) {
17.     int inByte = myPort.read();
18.     println(inByte);
19.   }

}

The output from Processing is as follows:

I do not receive this error if I change this line: 

1.  myPort = new Serial(this, Serial.list()[2], 9600);

to

1.  myPort = new Serial(this, Serial.list()[0], 9600);

or

1.  myPort = new Serial(this, Serial.list()[1], 9600);

but neither is my output displayed