I found the example:
//import java.io.*; //import java.io.File; already imported by Processing!
File a = new File("d:/test"); // The folder must exist!
void setup() {
size(200, 200);
//selectFolder("Select a folder to process:", "folderSelected");
//selectFolder(prompt, callbackMethod, defaultSelection, callbackObject, parentFrame)
selectFolder("Select a folder to process:", "folderSelected", a);
}
void folderSelected(File selection) {
if (selection == null) {
println("Window was closed or the user hit cancel.");
} else {
println("User selected " + selection.getAbsolutePath());
}
}
void draw()
{
}
void keyPressed()
{
selectFolder("Select a folder to process:", "folderSelected", a);
}
:)