Android: Make your Android apps unlockable through payment

check_paypal.java

if (isPackageAvailable("ch.pboos.android.SleepTimerPayPal")) {
    Intent intent = new Intent(
            "ch.pboos.android.SleepTimerPayPal.CHECK");
    startActivityForResult(intent, CHECK_PAYMENT);
}

detect.java

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);  
    setContentView(R.layout.unlock);  
    if(isAppPayed()){
        setAppToPayed();
        return;
    }  
    if (isPackageAvailable("ch.pboos.android.SleepTimerPayed")) {
        setAppToPayed();
        return;
    }  
    ...
}  
private boolean isAppPayed() {
    SharedPreferences settings = PreferenceManager.getDefaultSharedPreferences(this);
        return settings.getBoolean(getString(R.string.attr_ispayed), false);
}  
public boolean isPackageAvailable(String packageName) {
    int sigMatch = getPackageManager().checkSignatures(getPackageName(), packageName);
    return sigMatch == PackageManager.SIGNATURE_MATCH;
}  
protected void setAppToPayed() {
    SharedPreferences settings = PreferenceManager
            .getDefaultSharedPreferences(this);
    SharedPreferences.Editor editor = settings.edit();
    editor.putBoolean(getString(R.string.attr_ispayed), true);
    editor.commit();
    finish();
}

menu.java

@Override
public boolean onPrepareOptionsMenu(Menu menu) {
    if(isAppPayed())
        menu.findItem(R.id.menu_unlock).setVisible(false);
    return super.onPrepareOptionsMenu(menu);
}