How to get the sender of an Intent?

Is there a way for an Activity to find out who (i.e. class name) has sent an Intent? I'm looking for a generic way for my Activity to respond to a received intent by sending one back to the sender, whoever that may be.

There may be another way, but the only solution I know of is having Activity A invoke Activity B via startActivityForResult(). Then Activity B can use getCallingActivity() to retrieve Activity A's identity.

Is it an external app you receive the intent from? You could use the getReferrer() method of the activity class

A simple example: I opened google map app to share some location with my app by using the share option of google maps. Then my app opens and this method call in the Activity:

 this.getReferrer().getHost()

will return:

 com.google.android.apps.maps

see documentation here: https://developer.android.com/reference/android/app/Activity.html#getReferrer()

Note that this requires API 22. For older Android versions see answer from ajwillliams