The program won't crash if you use a try / catch exception handling around the code accessing the file. Unless it is already done internally by Processing (likely).

Check existence:
Code:

String path = "some path depending on system";
File f = new File(path);
if (f.exists()) print("I found the file."); 

(from memory, check Sun docs if it fails.)

Choosing a file: you have to use JFileChooser
Example of Processing code (tested) adapted from the example in the above page:
Code:

import javax.swing.JFileChooser;


JFileChooser chooser = new JFileChooser();
chooser.setFileFilter(chooser.getAcceptAllFileFilter());
int returnVal = chooser.showOpenDialog(null);
if (returnVal == JFileChooser.APPROVE_OPTION) 
{
  println("You chose to open this file: " + chooser.getSelectedFile().getName());
}
 

For the first example, to check the file's existence, the default path seems to be (on a mac): /Applications/Processing 0135/

How can I set the default path to be the program's sketch folder instead?

Thanks!

Thanks PhiLho,

To be more specific: what I need to do is to get the applet's (or the sketch's) current location - without specifying it through the file chooser.
There must be some function or method that returns the applet's absolute path.

Any ideas?
Thanks

probably safer, since dataPath may point inside a jar file:

Code:

File dataDir = new File(sketchPath, "data");
chooser.setSelectedFile(dataDir); 

or further, if you are dealing with a jar file, a "data" folder may not actually exist, in which case, you can create one:

Code:

File dataDir = new File(sketchPath, "data");
if (!dataDir.exists()) {
  dataDir.mkdirs();
}
chooser.setSelectedFile(dataDir); 

http://processing.org/discourse/yabb2/num_1140487204_seems_to_have_a_workable_solution.html