To the best of my knowledge, you would have to know beforehand what kind of instructions may be encoded within the string. If you know that, you would scan the string until you found a complete instruction. You would execute the instruction (this requires manually coding any possible incoming instruction) and then continue reading the string.

thx asimes for the answer. 

but im still convicted it's possible to do that : processing read strings to call function to the JVM

This would be better for PhiLho to explain, as he knows more about the inner workings of Java / Processing than me, but to the best of my knowledge, you cannot add new code to a Java / Processing program that is running. The code that you write is converted into a form that no longer resembles Processing code when it is run by a virtual machine (when you hit "run").

Processing is based off of Java, which is compiled. That means that every statement is processed before the application is run. In order for you to have a way to execute a statement from a String on the fly, the statements would have to be processed mid-execution, which cannot be done in Java. There are other languages, like Python, where each line is processed in a step-by-step basis, and these are called scripting languages.

You can gain access to a scripting interface from within a Java application through use of the javax.script package (I know that Mozilla and Google both provide their own scripting environments as well) and gain access to all of the functionality of your main program, but with the loss of a lot of resources required to maintain the environment.

In short, Processing isn't designed to be able to perform these kinds of tasks and, although they aren't impossible, they aren't nearly as elegant as what can be done with a scripting language such as Python.

I can only confirm what is said above. You can do that, indeed, from using JavaScript, Bean script, Lua or other exotic scripting languages, to compiling yourself some Java code. But the effort is quite big, for a result you can attain, perhaps, by other means.
Your question is valid if you want to obtain some code from a user, either in a text box or from a file.
But if that's an internal trick, part of your algorithm, it has to be questioned. Perhaps you want to use the Command pattern, or the Strategy pattern. Search them on Wikipedia, for example.
Java provides a Runnable interface, which can be used like this:

1. class Rect implements Runnable
2. {
3.   public void run() { rect(50, 50, 50, 50); }
4. }
5. class Circle implements Runnable
6. {
7.   public void run() { ellipse(50, 50, 50, 50); }
8. }
9. // Etc.
10. Runnable[] actions = new Runnable[10]; // Or use an array list
11. Rect rect = new Rect(): // You can define a constructor, have fields, etc.
12. actions[i++] = rect;
13. // Etc.
14. for (Runnable r : actions)
15. {
16.   r.run();
17. }

This kind of wrapping, a bit verbose (but hey, that's Java!) is the common practice to implement such behavior.