In my project i move from main activity to activity A and from activity A to activity B. From activity B using home menu on toolbar i jump back to main activity. Now when i press back button application should exit but it opens the activity A again.
You should make use of the launch flags to manage your activities in the back stack. As far as I understood your scenario, I think you need to use
FLAG_ACTIVITY_CLEAR_TOP for starting your main/home activity.
Read more about launch flags: https://developer.android.com/reference/android/content/Intent#FLAG_ACTIVITY_CLEAR_TOP.
Also, take a look this for more details on managing activity back stack on Android: https://developer.android.com/guide/components/activities/tasks-and-back-stack#ManagingTasks