If you want to find all the files in the top folder of the SDCard you can do something like this:
public GetFileNames(Context context) {
File dir = Environment.getExternalStorageDirectory();
ArrayList<String> fileNames = new ArrayList<String>();
for (File f : dir.listFiles()) {
if (f.isFile()){
name = f.getName();
fileNames.add(name);
}
}
//Do whatever you want with fileNames array.
}
EDIT: If you want to get your apps private filenames you should do this:
public GetFileNames(Context context) {
File dir = context.getFilesDir();
ArrayList<String> fileNames = new ArrayList<String>();
for (File f : dir.listFiles()) {
if (f.isFile()){
name = f.getName();
fileNames.add(name);
}
}
//Do whatever you want with fileNames array.
}