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Android, managing views, task and activities

I need general advice.
I'm building an app that can be started via the icon (normal way) but also with an intent triggered by a click in the notification area (the app places an icon in the notification area). The is awaken when a time event / alarm occurs.
The app has 5 to 10 views. Is it better to open a new activity for each view? How can I be sure not to have mutliple tasks open? Say if my app is Activity-A, Activity-B, Activity-C and can be started from Activity-A and Activity-B (depending on if it's opened via the icon or the notification area) ... how do I clean everything upon app close?
Or is it better to have one activity and just switch views (xml layouts)?

I'll start of with what you should watch and read. It should be clear that navigation in Android is very easy to do wrong. Google has a history of doing differently from app to app, but they are getting better. This said, if you are to do an app you should know how it is intended to work, and the best way to do that is to read the docs. As mentioned, there was an excellent talk at IO'12. There is also a very good section on the design site, and finally there is a good section in the API Guides.
To summarize: a full screen is an activity (which in turn can be constructed of fragments). The activities should in virtually all cases be structured as a tree with the root being the activity that are launched from home. For every activity you should have an "up" activity that takes you up in the hierarchy. Note that this is different from back which should take you to the last full screen state you were in. Also note that full screen means that for example tabs should not be recorded in the "back history", since they provide navigation within a screen.
When it comes to tapping a notification it should be equivalent to: pressing home, remove the task from the recents view (clearing the task), opening the task, and finally take the shortest path to get to the activity presenting the info that the notification told you about. Complicated, yes indeed... But at least there is a helper class in JB and in the support library called TaskStackBuilder.
The key to all this is a UI design that follow the Android design guidelines. Take your time to make the design for your app, and make sure to separate up (static) and back (temporal).

It might be as simple as using a different launchmode, which you can define in the manifest or I think in the inent you are using. It takes some experimenting but SingleTask or SingleInstance may be the right choices for you,
Your activity should support onNewIntent in this case, to reuse existing Activities if that is the desired effect.

In this case I better use one activity or use fragments.

Back button in Android

I have seen couple of Android applications when I came across one common practice. Navigation header does not have a Back button. Since then I was quite confused so as to place the same in the Navigation header of my application.
Should back button be placed in Navigation (Header) or we should leave keys to handle it? What is the best practice we should follow?
Thanks

My personal view is that each platform has its own way of handling certain functions. An apple device will have a back button on its navigation bar because that is the place where an iphone user will look for it.
As far as Android is concerned, because we are supplied with a physical back button, we must leave it at that because that is the first place an android user will look to if he wants to go back.
Therefore since user satisfaction and ease of use is the main concern, i would not play with the back button (unless i have to).

Similar to the responses above, however for the sake of simplicity I have taken a central approach in which I actually keep a back button in the navigation header however user can still press the hard key back button and the code actually performs the same functionality.
Essentially what it does is to cover both set of users, some really new ones who do not understand Android hard-keys yet, like people used to iOS (pun intended) and the other more suave Android users.

oI believe there could not be best approach. It depends on your custom UI and how much screen estate your navigation consume. The best approach is to buil two variants ant alllow to beta-testers decide.
Allthow back button on the screen could be as much handy, so much annoying.

What the "Back" button actually does, is to close your current activity and bring the previous one to the front.
You can do the same by calling finish() in your current activity. It will remove the current activity from activity Stack and take you to the previous one.
Asaf Pinhassi

Android, Unlimited Number of Activities in a Project and activity lifecycle !

Some days ago I have published my application in Android Marketplace and a question came into my mind during designing and developing application. This question is about limitation or unlimitation of number of activities.
For example, my application includes 37 activities. Is it so much? I want to know for a game such as "Angry Birds" which has more than 200 levels, it has 200 activities?!!
My next question is, assume that I am designing an application which includes 100 activities. User starts application and gos into last activity (such as reading a book). If user wants to close the application what should he do? 100 times press back key on his devise?!
If I want to put close button(to help user) in options menu, do I have to repeat it for each activity? because as far as I know each activity has its own menu.
If i put close button in last activity and user clicks on it, program will close. What will happen to other 99 activities? are they still exist in stack? or When I close an application, all activities related to that package will delete from stack.
I have read activities page published by Android but I couldn't find my answers.
Sorry if above questions are stupid questions :)
Thank you

In the case of Anry Bairds, I doubt they have 100s of activities, I think they just have one (or possibly a couple) for levels and another for the welcome screen - different levels are probably loaded depending on some parameter that was passed to the activity.
As for how to handle 100s of loaded activities, first of all, it is very likely that the system will recycle the old ones at the bottom of the stack before you get to the 100th activity. If not, then yes, your user will have to click back through a 100 activities.
If you want to change the behaviour of what the back button, you can override onKeyDown().
Or if you want to change the default behaviour of how activities are launched, then take a look at activities launch mode.

Please read Tasks and Back Stack and Activity and Task Design Guidelines

Well, first, regarding angrybirds, I guess it is written nearly entirely in OpenGL ES, so it consists of only a few activities.
And second, if you're really having 200 activities, you should probably think of an activity which can be reused multiple times. e.g. why would you need one activity for every page of a book, if every page should have the same look and functions? Just fire an intent with extra data and it should work. And, to close the app, you can simply press the home button :-)

An application like Angry Birds probably has one activity with an OpenGL ES view, with all of the interfacing done with a RelativeLayout
that being said, you could have an unlimited number of activities, the only concern is how much space your app will take, why any user would want to navigate through such a robust program on their mobile device, and how much RAM you are using and are you managing your activities efficiently

Moving through classes without creating a new instance

Does anyone know of a way to show another class without creating a new instance?
It seems a bit crazy from a memory management point of view that each time you want to display a different form / page you need to use StartActivity which then creates a new instances of the class instead of reusing instances previously created.
Thanks in advance
I guess from what has been said - there is no real way to do it which won't hinder the "Back" functionality of the OS?
I'm building an app which is linear except on each screen it has a home button which then makes it possible to countermand this functionality and end in a loop - is there anyway you know of to destroy all over views and reset back to the main class? (IE prevent a memory leak from becoming a problem but also not damaging OS functionality)
Consider it a "clear history" without restarting the app

Not sure if this would work for Android (coming from a MS/C# background), but conceptually one option is to iterate through open forms looking for one with a specific handle. Then, once you find it, simply call the method to show that form. This would depend on there being a Java equivalent to the Application.OpenForms property in .NET.

It seems a bit crazy from a memory management point of view that each time you want to display a different form / page you need to use StartActivity which then creates a new instances of the class instead of reusing instances previously created.
Tactically, you are welcome to add FLAG_REORDER_TO_FRONT to bring an existing activity back to the foreground, so long as you understand the ramifications from a navigation standpoint.
However, your question is rather curious. How are you accessing StackOverflow?
Clearly it's not via a Web browser. Web browsers use the exact mechanism that you feel is "crazy", rendering Web pages even if that Web page had been viewed previously. They have been doing so for over 15 years, and we've been doing OK by it.
The Android navigation model is designed to approximately mirror that of the Web:
Users click on things to move forward
Users click on a BACK button to move to the previous thing they were looking at
Users click on a HOME button when they want to switch to some other major thing to go look at
By "reusing instances previously created", you're circumventing that navigational model. For example, let's suppose your activity stack were A-B-C-D, and you call startActivity() with an Intent for B and FLAG_REORDER_TO_FRONT. Now, the activity stack is A-C-D-B. When the user presses BACK times, they no longer are on the B they were looking at originally, but are back at A. In a browser, this would be rather strange behavior.
There are other flags on Intent, or attributes on <activity> in the manifest, that offer "reusing instances previously created". However, they are not there "from a memory management point of view". They are there where the traditional Web BACK-heavy navigation pattern does not fit your needs.
Assuming you aren't screwing up anywhere, Android will destroy under-utilized activities, garbage collect that memory, and even return that memory to the OS.

An example of when to have multiple "activities" in an Android application

I seem to be missing something obvious here, why would I want more than one activity per application in Android? Does somebody have some solid examples?

Suppose you are creating a game. You need to have at least two activities - a welcome screen, and the actual game screen. The third activity in this example might be a settings page of the game.
Another example.
Suppose you are developing an application and you need to pop up a dialog, i.e. asking user to set username and password (Standard login screen). You might choose to create and activity and apply a dialog theme to it.
Think about it as the form of desktop application. you don't put everything on one form do you? :)

Sorantis' answer is spot on. Here are other thoughts as well:
Most Web applications, even AJAX-y ones, don't try to have everything in one single page. Some do, and those tend to be the ones that are slow as molasses to load (Evernote, I'm looking at you), have code that looks like a heaping mound of spaghetti, etc. Android is no different.
Also, state management for a super-complex Activity will be nasty, causing you problems with screen rotations and supporting being kicked out of RAM because you screw up onSaveInstanceState(). Memory management in Android assumes lots of cheap activities, not fewer massive ones. Intelligently handling the BACK button requires gobs of your own logic. If you want multiple entry points (e.g., Launcher icon and a MIME type handler and something some other app can call with startActivityForResult() and a search results handler), doing that in one activity will be a nightmare. And so on.

One very basic thing that makes having multiple activities in your program desirable is the use of the back button. I have a form in app after the user clicks search he is presented with another activity showing the results of the search. If he wants to change the search parameters he just can press back and without me doing anything particular he gets back to the search form. Doing this with one activity would be a lot of work for you.
The next thing is the memory management. Android will trigger the garbage collection automaticaly after changing activities that means my whole search form leaves the memory and doesn't take any resources away from the user.