Develop Reference

singleInstance launch mode: why is onCreate called after activity is brought to front?

I launch main activity, change some settings in UI, press back button and then reopen the activity. onCreate() is called again and activity is back to default state. Why is this?
I would expect only onResume() to be called since I have this in Manifest:
android:launchMode="singleInstance"

Try using moveTaskToBack(true); in onBackPressed() to make your current activity hide instead of destroying, so you can reopen it again
#Override
public void onBackPresses(){
moveTaskToBack(true);
}

I launch main activity, change some settings in UI, press back button and then reopen the activity. onCreate() is called again and activity is back to default state. Why is this?
This is expected behavior of activity life cycle. When you press back button then activity get destroyed, it will start from onCreate method.

This is because when you press back, it destroyes the activity.
I think that what you want is to keep the state when you come back to this activity, not really to have one instance of it. So you should save the information you need and then restore them in onCreate.
Single instance only means (if I remember it well) that if you launch multiple activities without finishing them, the ones that have singleInstance will go back to front when you will call startActivity() and will not call onCreate. But this means you can't have this behavior by pressing back.
Maybe you can override onBackPressed and start another activity (the one you should go back to) instead of calling super.onBackPressed(). this should do what you want but if think it will be kind of difficult to manage.
By the way remember that Activities might be killed by the system itself, so you should not rely on the fact that your activity as not been killed

How can I preserve previous android intent value when I press the back or up button?

I have a list view in the previous intent and when I tap on that it go to the next activity. Now when I press the back button or up button it comes back to the previous activity. But when I tap the list item again it give me run time Unexpectedly app closed error. Please help me.

It's hard to answer without code, but maybe it's because you don't finish your next activity when you press the back button

It's because of Activity life cycle...
When you go to the next activity then current activity is paused for you. when you press the back or up button then your previous activity resume.
That's why you have to define your code in onResume() method also where you got stuck. because when you come back onResume method is called

You can try override onBackPressed method in your activities :
#Override
public void onBackPressed() {
// put a breakpoint here and check your listview/activities states
//super.onBackPressed();
}

How to clear activity stack trace on back press event?

I have created an application that has multiple pages and navigation from one to another represents a crucial flow. I don't want the user to be able to press the back button and escape the activity without first warning him and then finally deleting all stack trace such that when the activity is launched again it starts afresh.
As of yet I have been using something similar to the function below :
#Override
public void onBackPressed()
{
this.finish();
Intent int1= new Intent(this, Home.class);
int1.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
startActivity(int1);
super.onBackPressed();
}
But sometimes when after quitting the application when I launch it again it restarts from some random page or the one from where I quit the application (basically not the home screen from where it is expected to start)
I cannot think of a cleaner way to quit the application other than clearing all the previous activity flags as described in the code.
Any help on the above is appreciated!
EDIT :
Anytime during the flow of my activity if the user presses the back button, I want the control to be thrown back to the main page (clearing all the previous activity stack traces). Such that in case someone re-lanches the application it will re start normally from the main page.

You don't need any of this custom code in onBackPressed(). All you need to do is add this to all of your <activity> definitions in the manifest (except the root activity):
android:noHistory="true"
This ensures that none of your activities (expect the root activity) is recorded in the back stack. When the user clicks the BACK key, it will just return to the root activity.
Another benefit of this is that if the user leaves your app (by clicking HOME or by pulling down the notification bar and clicking on a notification, when he returns to your app it will also just return to your root activity.

Anytime during the flow of my activity if the user presses the back
button, I want the control to be thrown back to the main page
(clearing all the previous activity stack traces).
This can be done just by finishing all the activities as they move forward, except the MainActivity.
Such that in case someone re-lanches the application it will re start
normally from the main page.
Is it like if user is in Activity_5 and uses Home Button and relaunches the app again, MainActicity must appear?
IF so, you can call finish() in onPause() of every Activity except MainActivity
EDIT:
Might not be the perfect solution, but this is what I did to achieve exactly the same(logout in my application):
OnBackPressed() in any activity updates a boolean shared preference say backPressed to true and in onResume() of all the Activities, except MainActivity check its value and finish if true.
#Override
protected void onResume() {
super.onResume();
SharedPreferences mSP = getSharedPreferences(
"your_preferences", 0);
if (mSP .getBoolean("backPressed", false)) {
finish();
}
}

Back Button is used to go back to the previous activity. So i would not override the back button to clear activity stack. I suggest you use a Action Bar for this purpose. Navigate to Home Screen of the application using the application icon.
http://developer.android.com/guide/topics/ui/actionbar.html
Also check this link and comments below the answer by warrenfaith
android - onBackPressed() not working for me

#Override
public void onBackPressed()
{
moveTaskToBack(true);
android.os.Process.killProcess(android.os.Process.myPid());
System.exit(1);
}
you can use that code, it's work for me!

How to close an Activity, not the whole app

I'd like to destroy an Activity when the user clicks on a button (not the back button, a different button). I've decided to just call super.onBackPressed. Is that okay? What's a better way to close the current Activity without closing the whole app?

for closing an Activity you can use finish() , but you have to aware of back stack, if you finish your last activity in the stack so there will be no activity in your stack and you must restart your app .
see more information on : Task and BackStack

The default implementation of super.onBackPressed finishes the activity.
Instead of using super.onBackPressed , Better way to close the current activity will be to call finish() method.

Also, When you press the back button, the onResume method is called.
Use onResume Method and refresh your stuff there.
Also, if it is listView refresh the data -> https://stackoverflow.com/a/12662994/2006412

problem with going back to last activity via custom back button

I have a custom back button that for the time being does nothing other than going to the previous activity on the back stack. Heres the code for the button :
backButton.setOnClickListener(new View.OnClickListener()
{
public void onClick(View v)
{
System.out.println("!!! BACK !!!");
finishActivity(0);
}
});
The problem is, its not working. There is simply no change.
Can anyone kindly tell me what I have done wrong here ? Thanks.

finishActivity finishes an activity that you had previously started with startActivityForResult(), it doesn't finish the activity you are currently in - finish() does that.

Use finish() instead of finishActivity().
finishActivity() forces an activity that you started via startActivityForResult() to quit, it doesn't finish the current activity. You can also use onBackPressed(), that just calls finish() internally (see the default implementation), so it has the same effect. Useful if you override the back button behaviour though.