Permission denied while using runtime in android application -

when trying execute shell commands on lollipop getting permission denied error.

runtime.getruntime().exec("su")

i have super user permission in manifest

<uses-permission android:name="android.permission.access_superuser"/> 

the device rooted.

since device rooted, can try installing apk in system directory:

system/app

certain 'serious' permissions granted apks in directory. (on rooted device, browse directory in file browser, , find pre-installed apks here).