Best practice for displaying error messages

I have used the alertDialog.. refer the Images. futher google it

For user Attention.

for form Validation edit texts use editText.setError("Your error message") method

for internet connection failed

for internent connection failed with retry.

Update 1

For showing some auto terminate info/message we use Toast
for example notifying a user that your Email was sent Successfully. We can Use Toast like below

Toast.makeText(context, “Email was sent Successfully.”, duration).show()

Note: User can’t interact with Default toast, See also Custom Toast Layout

Another option is to use the new Snackbar

Hope this will be helpful