Why did you delete your previous message and reposted it here, in the wrong section?
Please, read the Processing forum guidelines.
I moved your topic from Programming Questions because your code uses a contributed library and cannot be run / tested without (and without Midi hardware, likely...).
Your problem is apparently unrelated to the library? Then make a version working without it, and if you have the same issue, you can ask again: we can then test and tell if we see the same problem on our system, and help if we can.

Hi PhiLho,
Sorry about that.  I've pasted the code again below but this time without using the midibus library.  Instead, the keyboard keys 'a, 's,' and 'd' create the Note objects.  I also simplified it to one linked list of Note objects to try and isolate the problem.  Would you be able to move the post back to Programming Questions?

When running this program, hitting a, s, and d keys will create smoothly rotating squares that fade away most of the time.  It gets tripped up sometimes though.  The rotation looks jerky.  Just wondered if someone might have a suggestion.

Thanks,
Dave

1. /* A note object is created when the user hits an a, s, or d key on their keyboard. 
   A LinkedList stores the 5 most recent Note objects.  Each Note's display method draws a square
   and updates its angle (rotation) and opacity values for the next time display is called. */
   
   int numNotes = 5;  //Maximum number of notes shown at any given time
   LinkedList<Note> theNoteStream = new LinkedList<Note>();
   Note newNote = null;
    
   void setup() {
     size(854, 480);
     noStroke();
     smooth();
     rectMode(CENTER);
   }
   
   void draw() {
     background(0);
    
     for (Note y : theNoteStream) {
         y.display();  //display each Note and update its rotation and opacity
     }
    
     if (newNote != null) {  //if note events have happened since last time through draw, update the linked list
       theNoteStream.add(newNote);
       while (theNoteStream.size() > numNotes) {
         theNoteStream.remove();
       }
       newNote = null;
     }
   } 
   
   void keyPressed() {  //using keyboard keys a, s, and d in this version
     int channel = 0;
     int pitch = 0;
     int velocity = 90;
    
     switch (key) {
       case 'a':
         pitch = 30;
         newNote = new Note(pitch, velocity);
         break;
       case 's':
         pitch = 50;
         newNote = new Note(pitch, velocity);
         break;
       case 'd':
         pitch = 70;
         newNote = new Note(pitch, velocity);
         break;
     }
   }
   
   class Note {
     float opacity;
     float pitch, velocity;
     float xpos, ypos;
     float angle;
    
     Note(float pitchIn, float velocityIn) {
       opacity = 255;
       pitch = pitchIn;
       velocity = velocityIn;
       xpos = 400;
       ypos = pitch*5;
       angle = 0;
     }
    
     void display() {
       fill(255, opacity);
      
       pushMatrix();
       translate(xpos, ypos);
       rotate(radians(angle));  // not necessary for circles
       rect(0, 0, velocity, velocity);
       popMatrix();
      
       if (opacity > 0) {
         angle += (.04 * velocity) + 1;
         opacity -= 5;
       } else {
         opacity = 0;
       }
     }
   }
   

You have the floowing code to add a new Note

if (newNote != null) { 
  theNoteStream.add(newNote);
  while (theNoteStream.size() > numNotes) {
    theNoteStream.remove();
  }
  newNote = null;
}

The problem that I can see is yes you add a new Note but then if this exceeds the maximum number of notes you remove the note you just added. What would be better is to remove the oldest note. Since you are using a linked list this is easy to do change the code to this

if (newNote != null) { 
  theNoteStream.addLast(newNote);
  while (theNoteStream.size() > numNotes) {
    theNoteStream.removeFirst();
  }
  newNote = null;
}
Now the oldest notes are erased.

OK, thanks for simplifying the code, I was able to test it.
But I see no jerkiness. Either my eyes are bad, or my computer is too fast...
I am on Windows 7, BTW, with a relatively recent (less than 2 years) but medium level computer. AMD Athlon II X64 635 (2.9 GHz), Windows performance index 5.1.

Yeah I may just be too picky.  I'm running a pretty nice new imac and I don't really notice a problem today.  Glad you guys didn't spot any big coding errors though, it's been a while since I programmed something.  I think remove() and add() are the same as removeFirst() and addLast(), but I changed them in the code just to make sure.  Thanks.

I think remove() and add() are the same as removeFirst() and addLast()

You are right I just checked the Java documentation so my previous post was rubbish.

Personally I always use removeFirst() and addLast() because it makes it clearer what you intend to happen.

Out of interest I also looked up the source code and discovered that:

• remove() simply calles removeFirst() so adds an extra method call

    public E remove() {
        return removeFirst();
    }

• add and addLast do exactly the same thing but add also adds a return true statement

    public boolean add(E e) {
        linkLast(e);
        return true;
    }

    public void addLast(E e) {
        linkLast(e);
    }