Develop Reference

What is diffrence between back button and setDisplayHomeAsUpEnabled in an activity?

I have two activtiy. When a boolean variable in activity 1 is true then the UI in activity 2 should be update (activity 2 is with fragment). When I back to activity 2 with (setDisplayHomeAsUpEnabled) it works correctly but with back button it does not work. What is the difference between this ways and how I can solve this. I try update the activity 2 with this code but it does not work:
#Override
public void onBackPressed() {
super.onBackPressed();
//update();
}

The Home Button is not meant to behave like the Back Button. See this reference. The Home or "Up" Button should take the user up within the view hierarchy (i.e. if you have a number of multiple choice fragments, don't go back through each of them, but back to the activity that started the first fragment)
Each activity should specify what it's parent activity is in order to properly use this functionality. You can do this through the manifest, or by overriding onOptionsItemSelected() as outlined here
In terms of why your activity 2 may be updating when using the "Up" button but not the back button, this could be because both handle the back stack in a different way. Back will take you to the last thing on the stack and resume it, assuming you don't haven't tagged the activity with a launchmode that alters this behavior. Here's more info on the back stack. If you haven't designated your Activity 2 as "singletop" then when you use the "Up" Button, a new instance of that parent activity is created. See here. This may means that the information is updating after using the home button because it's creating a new instance of the activity, while for the back button - you are not creating a new instance but resuming a previous instance... make sure you implement an onResume() function to properly handle this and update the information.

What method call from Back button pressed of Android Activity?

I read along the documentation in this page. And obtaining clear picture of one activity starts in android. Just like this screenshot below:
Here is my case.
Activity -A called Intent to start Activity B-.
And then from Activity -B the user click Back button.
So then it returned to Activity -A.
my question is .... since now we're in that Activity -A.
Does onPaused() method called back or onResumed() method
that is called now?

When ActivityB takes the foreground, Activity A is put into the stopped state (onPause and onStop will be called). When ActivityA comes back, it will get onStart, followed by onResume.

Pop Activity From Stack onResume

When my program starts, it runs a mainActivity, which right away launches and loads data into another Activity. I do this because the data needs only be loaded once, in mainActivity, then sent along to the other activities!
So! With that being said, my problem is probably forseeable! When the user clicks back enough, he will get to this initial activity, and sit there... ungracefully, instead of going back to Android's main menu.
What I want to do is add something to the onResume() method of the mainActivity, so that when it is re-entered I "activate" the back button from within the code-- (a boolean flag makes sure I'm not doing this incorrectly, thus I know it was on a back click). So isn't this just popping the current Activity from the Activity Stack?
how do I pop an activity from the activity stack!
Thanks!

Instead of using the "noHistory" attribute,you can also call finish() as you launch your next activity.. It will not re-appear when you then hit the "back" button.This will close the first activity.. removing it from the stack..

Use no history option so that this activity never gets onto the backstack.
android:noHistory=["true" | "false"]

Android activity - Back not working properly after pressing home

If I have Activity A and Launch Activity B from it, then press home I go back to home screen. If I launch the activity again from the recent apps, it goes back to B as it should, but when back is pressed the Activity send me back to the Home screen not Activity A. Activity A and be are alive for the entire time. How can I make the back button function as expected? Thanks.

Don't call finish() on Activity A if you want the back button to go from Activity B to Activity A...

Pressing the Home button empties the Activity stack.
You can capture the Back button and make it do what you want, but you would have to implement your own stack of previous activities.

Navigate to previous Activity

I have two screens, in second screen i have one back button that shows the first screen(using intent). if i click that back button from second screen i navigated to first screen no probs in that.. now i want to navigate back to second screen automatically without clicking any button. Please help me how can i do this. Thanks for ur valuable time .

I would give a try to a ViewFlipper activity, with two screens. This, of course, if you can encapsulate your actions into one activity (which i'm sure is manageable with AsyncTasks / Threads, Services and Handlers).
This way you can use gestures, (soft/hard)buttons, dropdowns, -whatever-, to navigate between your screens.

You can automatically go back to a previous activity on the Activity stack by calling the finish() method.
If you want to do this without any user interaction, use a Handler that automatically calls finish() on your activity after a predefined timeout (see this article for more information on Handlers).

try with Thread.sleep([ms]) and then call finish or call intent to the first

The one solution is, when you press BACK button from second screen instead of call finish(); you should start first activity again, so the activity stack will be
FIRST
SECOND
FIRST
so after the delay of two second your activity will finish and eventually will come back to second screen, and state will be maintained also.