Want to attach a listener to a preference, what am I doing wrong here?

SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(this);

SharedPreferences.OnSharedPreferenceChangeListener listener = new SharedPreferences.OnSharedPreferenceChangeListener() {
            @Override
            public void onSharedPreferenceChanged(SharedPreferences sharedPreferences, String key) {
                if (key == " /*the key I want*/ ") {
                   //do stuff here
            }
        };

prefs.registerOnSharedPreferenceChangeListener(listener);

Also, I learned that preferences all get automatically saved to a default SharedPreferences file which you access through getDefaultSharedPreferences(). Does that happen when app is first run or does it wait until a user interacts with a preference?