Arrays are not infinite in their size; in fact, in order to increase their size, you have to create a new array that is exactly the same in all ways except for the fact that it is one item longer. If you want a dynamically expandable array implementation, see ArrayList.

If that's too advanced for the time being, then the next best option is to use the append() function, which carries out the expansion process described above. The way that it must be used is slightly confusing, although there is an example of it in the above link:

1. arrayToExpand = append(arrayToExpand, elementToAdd);
   

That would work if you were using charAt(), because it returns a char value.
When dealing with Strings, however, comparison is not as easy; you need to use String. equals().
See the Processing Wiki article for more information.

In your code, that would be:

1. if(recCmds[0].substring(0,1).equals("S") ) {
2.   ...
3. }