String your app_selected = "your package name";
final PackageManager pm = getPackageManager();
//get a list of installed apps.
List<ApplicationInfo> packages = pm.getInstalledApplications(
        PackageManager.GET_META_DATA);
int UID;
//loop through the list of installed packages and see if the selected
//app is in the list
for (ApplicationInfo packageInfo : packages) {
    if(packageInfo.packageName.equals(app_selected)){
        //get the UID for the selected app
        UID = packageInfo.uid;

    }
}