Create a File object, passing the directory path to the constructor. Use the listFiles() to retrieve an array of File objects for each file in the directory, and then call the getName() method to get the filename.

List<String> results = new ArrayList<String>();


File[] files = new File("/path/to/the/directory").listFiles();
//If this pathname does not denote a directory, then listFiles() returns null. 

for (File file : files) {
    if (file.isFile()) {
        results.add(file.getName());
    }
}

Based on the documentation for File.isDirectory(), it sounds like you may have a security manager enabled. isDirectory requires the checkRead permission to be enabled, and you can test this yourself by using the something like the following (edit as needed):

SecurityManager security = System.getSecurityManager();
if(security)
    if(security.checkRead(FILE_PATH_HERE))
        // we have permission
    else
        // no permission

Adding the directory to the list of allowed directories may or may not be on your list of things to do. There are dozens of Google links with instructions if this is the case. The same goes if you would like to disable the SM completely.