How are you getting the values in from MaxMSP?  You will definitely need to keep updating the values of the class variable as new values come in.  A simple option would be a class method to which you pass the new updated variable.  However, depending on how you're getting the data it may perhaps be possible to pass an instance of the data stream to the class constructor and access it directly from within the class.

Probably worth posting your class code and show how you're trying to implement it: that way it will be easier to give a more definite answer ;)

Thanks a lot for the responses guys.

The problem is you haven't assigned your link.declareInlet("bin#"); calls to your variables.  So the simplest solution would be to use the global variables you have declared, but not used, assign the declareInlet() calls to the variables and pass the global variables to each class instance.  i.e.:

1. // Note that the "bin0" parameter has nothing to do with the bin0 variable...
2. // it's a string parameter that presumably identifies the object sending a signal from MaxMSP
   
3. bin0 = link.declareInlet("bin0");
4. bar0 = new Bar(bin0);

It should also be possible to integrate this direct into your class.  I'm not sure what would happen if you passed the declareInlet() call direct to a class constructor; but I think you should at least be able to do the following (this is untested as I don't have MaxMSP, nor the library installed):

1. class Bar {
2.  
3.   public float fftBin;
4.   // constructor
5.   // pass a reference to the link object and the name of the MaxMSP object sending the signal
6.   Bar(MaxLink link, String InletName) {
7.      // assign the declareInlet call direct to the class property
8.      fftBin = link.declareInlet(InletName);
9.   }
10.  
11.   void display() {
12.    // in theory fftBin should be updated as and when a value is passed from MaxMSP
13.   }
14.    
15.    
16. }

Thank you very much, this is very helpful as always on the Processing forums so thanks. 

blindfish, given ndd501's answer, and the looks of declareInlet, I suppose the library uses reflection to update the variables.
Looking at the docs at the given link corroborates the idea.
I see also the link.declareInlet("xPosition", "setX"); form allowing a more proactive usage, the setX function being called when there is a change.
Alas, it seems that these variables and functions must be global, ie. that won't work in a class. Right now (it is late), I don't see an elegant and flexible way to do this with classes...