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Best approach for handling same layouts for different activities

I have been working on application which has 3 types of accounts related to it. We create a single layout and view/hide items on it depending on, from which account you are currently logged in.
With this approach, we have activities/fragments doing a lot of different things, they handle all cases wrapped in if/else checks etc. With growing project, it is becoming hard to maintain these classes.
Say, if I have to hide a view in certain scenario, I have to look around many if/else checks just to hide a single button because if I hide it on one place other check will make it visible again, really hard to maintain.
I am looking forward for best advises on this issue from the experts.

If you are struggling with a lot of if/else scattered in the code, maybe you should use polymorphism in your code.
Create an abstract class for the Activity, then specialize it for each particular type.
Use the Factory method pattern for creating objects of this hierarchy. This method will use the parameters for deciding which concrete class to instantiate, and then it will initialize the instance being returned.
Use the Template Method pattern if there is an algorithm common to all sub-classes but that contains some open steps that should be implemented by each class.
Use the State/Strategy pattern if you need polymorphic code that may be modified at runtime.

If your separate apps require minor customization and theme changes, but are really the same base app, multiple flavors is definitely the way to go. However, if both apps require a lot of custom code differences, you might want to rethink using multiple flavors strategy.
Also, take notice of the difference between flavors and build types. Use flavors for situations where you might need different versions of the same app in the Play Store, for example, free and pro, or for situations where you are customizing the same app for multiple clients.
for details http://www.androidauthority.com/building-multiple-flavors-android-app-706436

you have create new xml files in which has common view's for your activity and fragment then need to use include tag in xml for adding those common view's into your activities & fragments xml.

Create different xml for same layout and use <include layout="#"/>
Tag to create the layout, it will reduce if/else and also provide you the code re-usability

I think you should create separate layout for all 3 types of account and you can create PickLayout static class/method to pick the layout by type
int getLayout(int type){
return layoutMap.get(type);
}
if you have re-usable layout then you should use include, merge or you can use ViewStub also.
if you have chain of if/else then you should use Map link that will be scale-able, error-prone free.
And try to follow android suggested design-pattern that will be helpful for writing test case also.

MainActivity is too big. How do I split it into multiple files

I'm building an Android App which has a complex screen with lots of logic.
It currently contains a listview, tabs, search box, and a panel for updating user stuff.
The probem is that the mainactivity code file became very big, although I'm seperating things to different layers, because there's a lot of UI components which affect things in the screen.
I'm trying to seperate it to several files but I don't seem to do it right.
As much as I understood Fragments is not what I need here. Are there any other ways?
Just need some directions please.
I'm asking mostly about the code, not the layout (Although I don't care changing the layout too).
Currently it's 616 lines and the biggest problem is that we are a team and the maintenance became hell...

Create utility class and put your listeners and adapters there. Use main activity only to initialise view instances and setting listeners and adapters.

Although 616 is not a particularly big file you could use Fragments as they just add another layer of abstraction. They also have a similar lifecycle to activities.
This tutorial shows how to add fragments to tabs
http://developer.android.com/training/implementing-navigation/lateral.html

You can create a base class put all the initializations and listerners there.
Likewise for function to be implemented later create a template in base class and override it in main activity. It works !

Converting Existing App to use Tabs

I have an existing app that is using a Dashboard style pattern where there's a main menu, and clicking icons on the main menu drive start different activities... In order to navigate to a different function, you need to go back to the Dashboard menu and select another icon.
I want to change my application to a tabbed format similar to the one below with Icons:
What type of View is being used below? Is this a FragmentActivity? Also, what is the best approach to go about conversion? I have a bunch of layouts with listviews in linear/relative layouts... Will I be able to reuse any existing code? I want also to be able to use the solution with Android 2.1 and up if possible.
Thanks!

In the image you provided, it looks to be a TabHost that is used (which can be within a normal Activity or a Fragment Activity) and will be available for Android 2.1 and beyond when using the Android Support library. Based upon your description, you most likely have an Activity per each of your items that you will probably want to convert into a different Fragment. This may take a little time, but a Fragment is very similar to a normal activity in many ways, so once you start getting used to it, converting over the old Activities should be a breeze.
If you plan on using these tabs and you follow the Android design UI guidelines, you may want to use the TabHost in conjunction with a ViewPager. There is a great tutorial for this online that also allows for backward compatibility (down to at least 2.1) found here: http://thepseudocoder.wordpress.com/2011/10/13/android-tabs-viewpager-swipe-able-tabs-ftw/
Support library for fragments/viewpager: http://developer.android.com/tools/extras/support-library.html
More info about a TabHost and using Tabs with fragments can be found here:
http://developer.android.com/reference/android/widget/TabHost.html
http://thepseudocoder.wordpress.com/2011/10/04/android-tabs-the-fragment-way/

You can use TabLayout and a TabActivity to achieve layout in picture. Bad news is these are deprecated as of Android 3.0. Google advises to use fragments instead of these.

Android : How to call activity inside running activity?

I am thinking to split the activity into two parts :
List view : show the files.
Second part will an activity contain the selected file.
its possible to do that, like frames in HTML ?

I think what you want to be doing is using activity fragments. The guide topic Fragments describes how to do pretty much exactly what you're talking about. To support pre-3.0 Android systems, you'll need to use the compatibility package.

Navbar Class in all activities

Ok, I am doing something. I have a navigation bar that contains all the buttons for my activities. I have tried the method of Extending the other activities to the "navbar" class.
Here is what I have done so far:
I have extended all the classes to navbar (Except those that will need multiple inheritance).
used an tag in every XML layout.
What I need:
I need classes to handle multiple inheritance
All my classes even those that do not need multiple inheritance, to extend my navbar class.
If multiple inheritance is not possible, I do not mind hardcoding those classes, but multiple inheritance would be very nice :)
thanks in advance.

Java does not support multiple inheritance, sorry.
Rather than roll your own "navbar", you might consider using the action bar on Honeycomb, perhaps then using ActionBarSherlock to support pre-Honeycomb devices from the same code base.

You could do what I did for my app.
Create a "base" activity that all other activities extend.
Fill your menu bar and set up all of the listeners with Intents in it, then allow each activity to handle everything else.
The other option would be to create a XML element with whatever you want in it (if it needs to be more complex than a menu for some reason). You should still use the "base" activity idea for this.
Hope this helps.