SettingsFragment.java

    @Override
    public boolean onPreferenceChange(Preference preference, Object newValue) {
        Toast error = Toast.makeText(getContext(), R.string.plan_length_error_message, Toast.LENGTH_SHORT);

        String planKey = getString(R.string.plan_length_key);
        if (preference.getKey().equals(planKey)) {
            String stringSize = ((String) (newValue)).trim();
            if (stringSize.equals("")) stringSize = getString(R.string.plan_length_default_value);
            try {
                int size = Integer.parseInt(stringSize);
                if (size < 1 || size > 30) {
                    error.show();
                    return false;
                }
            } catch (NumberFormatException nfe) {
                error.show();
                return false;
            }
        }
        return true;
    }