Develop Reference

How to kill a particular activity?

When my android app is started, main activity is launched. It displays a full screen image for 5 seconds, and then it jumps to another activity using intent. What i want is to kill the main activity, so that when user presses the back button of navigation bar, instead of opening main activity, the app gets closed.
One more thing:- i don't want to keep on destroying previous activities. I just want to kill that one activity(namely main activity), just after the intent is sent to new activity, Because i will be adding more activities.
We can say that my true purpose is destruction of main activity, and making the next activity(out of all other activities) as a activity through which the app can be leaved using back button of navigation bar.
I am not able to properly explain my problem in words, but please try to figure out my problem what what all i have mentioned.

In your MainAcitivity ,call the second activity like this:
Intent intent=new Intent(this,<your second activity.class>;
intent.addCategory(Intent.CATEGORY_HOME);
intent.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
startActivity(intent);
finish();

I would add
android:noHistory="true"
to the AndroidManifest.xml, specifically in the MainActivity definition

To kill Activity you have to use method finish();
In MainActivity in code, where you are starting next activity add finish();:
Intent i = new Intent(MainActivity.this, NextActivity.class);
startActivity(i);
finish();

What you're trying to achieve is called a splash screen.
In your main activity start another activity and if the user presses back on another activity, simply call o finish on main activity. Destroying parent activities before child is sort of messy.
I'd recommend googling splash screens through cold app booting.

Using an Activity to Launch Either Login or Main Activities

I have an application which currently launches a simple LaunchActivity upon starting. Within this activity, there is a conditional in onCreate() to check for the existence of session data. If no session data exists, it starts the LoginActivity. Otherwise, it starts the MainActivity. In both cases, it finishes the LaunchActivity before starting either activity.
Using this approach, there is a brief flicker of the LaunchActivity before the start of either other activity. If this is an appropriate approach, what can be done to eliminate the flicker?
Is there another approach to this behavior which does not involve a LaunchActivity?

it finishes the LaunchActivity before starting either activity.
This sounds like you are destroying your Activity, then launching a new one. What you can do instead is open the new Activity using Intent then finish the launch Activity:
Intent intent = new Intent(this, MainActivity.class);//or LoginActivity.class
startActivity(intent);
finish();
Additionally, you can specify in your manifest that you do not want the launch activity to be included in the Back Stack. This will make it so that when the back button is pressed to exit the main or login activities, the launch activity will not be shown.
android:noHistory="true"

From within an android activity test, how do I finish another activity that my main activity started?

I am writing an activity test for an activity we wrote with 3 buttons. 2 of these buttons start other activities.
I can write a test that simulates a button push and then checks if the desired activity is running, but I can't move back from that second activity. The second activity stays at the front and prevents the other tests, that assume the first activity is running, from working properly. They just kind of freeze.
I have a reference to the first activity, but it is the second activity I need to I guess call finish() on. Is there a way to do this?
EDIT: I added some actual source code illustrating my problem in this gist: https://gist.github.com/3076103
It is specifically about testing activities. In the production code everything is fine.

You should probably use http://developer.android.com/reference/android/app/Instrumentation.ActivityMonitor.html to get a reference of the second activity or you can block the second activity from being launched(Still you are guranteed that the call to start the second activity infact had reached till the framework).

You need a way for your activities to communicate with one another, so that one activity can tell the other to finish. There are several ways you can accomplish this. One method is to create a service within my application; my "second" activities would connect to this service to register a way to receive messages, and my primary activity would connect in order to provide them.

In Activity1 add the following to start Activity2
Intent myIntent = new Intent(view.getContext(), Activity2.class);
startActivityForResult(myIntent, 0);
In Activity2 add the following to start Activity1 and finish Activity2
Intent intent = new Intent();
setResult(RESULT_OK, intent);
finish();
For more details: http://www.warriorpoint.com/blog/2009/05/24/android-how-to-switch-between-activities/

If you start the activity using startActivityForResult, then you can close the activity from parent using finishActivity(int requestCode).
To start the activity :
startActivityForResult(new Intent(...), 123123 /*requestCode*/);
And when you want to finish that activity (from caller), use :
finishActivity(123123 /*requestCode*/)
Also there is a way to find, whether child activity is finished or not. But you can track this only when child activity calls finish() for self. To receive the child finish request from the child, you need to override the finishFromChild() method in parent activity.

OnClick restartActivity?

How can I restart An Activity on click?
For exemple: I have got in my AndroidManifest.xml 2 activities the activity A and B and they start when application starts...
But what I want is when I click in a button that is on Activity A it must restart activity B.

when you are in activity A, and proceeding to activity B, then your activity B automatically starts/re-starts

Why do you need to start both Activities on starting your app? When you say "restart", do you actually need to stop Activity B and start Activity B again? Or do you just want to show it? To start an Activity from another Activity, you could call something like this:
startActivity(new (Intent(this, ActivityB.class)));
The Android documentation gives plenty of detail. However, I think you should consider why you are starting two activities at once, and whether you might want to use a Service instead (not knowing any details of your app, I can't say).

Intent intent = new Intent(CurrentActivity.this, ActivityToLaunch.class);
startActivity(intent);
call above piece of code on onClick of view method.

Update the first screen automatically even if it is declared as android:launchMode="singleTask"

I have 2 activities. Activity A and Activity B. Both calling each other though intent. Activity A calls Activity B. Activity B accesses database and sends it back to activity A through putExtra() and getExtra().
Now my activity A is in declared like this android:launchMode="singleTask"
When I go back to activity A I want my this activity A to be updated or refreshed automatically. But to my wonder what I understood on debugging that if I declare an activity as launchMode="singleTask" then it just brings forth the screen to top from the stack. It does not actually go inside the code.
Is the concept what I understood correct ?
The solution I see is have a refresh button and on click of that access the code and update screen. But I do not want to do that. Do you think there is any other alternative ? I do not want to change launchMode="singleTask"
Thanks in Advance.

Try startActivityForResult(intent); and while you have finished in B setResult(RESULT_OK); and finish(); the B activity, and in A onActivityResult(int,int,intent); catch the result code if it is RESULT_OK update your A.

No matter which launchMode, when you switch from Activity B to Activity A, the onResume() method must be invoked. You can put your refresh code there to get your activity A updated.

put your refresh part in the onResume method .Once Activity B completed your onResume method will be invoked in Activity A.