That's not really "executing" a string, but using it.

To check if a file exists:
Code:

boolean DoesFileExist(String fileName)
{
  File f = new File(dataPath(fileName + ".wav"));
  return f.exists();
}
 

dataPath() returns the path of the file in the data folder of the sketch.
You can do the same to play it.

I didn't write it isn't possible to execute strings, I wrote the explained usage isn't that.
Beside, I don't see an eval() function, you might be mixing with JavaScript or PHP...

Java has a number of interpreted languages running on the JVM, from Java-like syntaxes (BeanShell aka. bsh, DynamicJava, etc.) to any exotic language (Kahlua uses the simple syntax of Lua and is very lightweight).
Integration to Processing might not be simple, but is doable, and probably much simpler than doing parsing yourself.

Note that Processing is an example of Java program compiling Java code (after some pre-processing) and running it on the fly.

check out this
http://processing.org/learning/libraries/loadfile.html

and you will need a library

http://code.compartmental.net/tools/minim/
there are instructions on how to install the library
i haven't checked it but just search for
play sound in Processing you are not the
first one trying this

i do not have the time to explain step by step but if you still need help, after trying to figure it out, let me know.

if you alrady are playing sounds then just assign a new variable to the file you want to play


 File f_1 = new File(dataPath(fileName1+ ".wav"));
File f_2 = new File(dataPath(fileName2+ ".wav"));


play f_1
and play f_2

or
File f= new File(dataPath(fileName1+ ".wav"));
play f
File f = new File(dataPath(fileName2+ ".wav"));
play f