Develop Reference

How to cast AppCompatActivity to Activity

I am having an activity that extends appcompatactivity I want to use the methods of activity like getwindow(), setRequestedOrientation() and finish() etc. but not able to do so... It says cannot resolve method. so, is there any way cast AppCompatActivity to Activity.
I am using Android Studio 3.0 Canary 4

For a minimum API level of 15, you'd want to use AppCompatActivity. So for example, your MainActivity would look like this:
public class MainActivity extends AppCompatActivity {
....
....
}
To use the AppCompatActivity, make sure you have the Google Support Library downloaded (you can check this in your Tools -> Android -> SDK manager). Then just include the gradle dependency in your app's gradle.build file:
compile 'com.android.support:appcompat-v7:22:2.0'
You can use this AppCompat as your main Activity, which can then be used to launch Fragments or other Activities (this depends on what kind of app you're building).
The BigNerdRanch book is a good resource, but yeah, it's outdated. Read it for general information on how Android works, but don't expect the specific classes they use to be up to date.

You can cast AppCompatActivity to Activity:
((AppCompatActivity) getActivity()).someAction...

Why does MainActivity in Android extend different activity classes at different times?

Could you please help me with this question?
When I create a new project I see MainActivity extending the Activity class or AppCompatActivity. Why does this happens? What is the reason behind changing the default settings of android project everytime? What are the other classes that MainActivity can extend? I would appreciate any help.

Actvity has support for system ActionBar which was introduced in Android 3.0 (API level 11).
AppCompatActivity has interface to work with ActionBar from support library and can be used with API Level 7).

If you want to support some features like actionbar, to lower API level of android so use AppcompateActivity.For example, if you extend youractivity with Activity class then you can not use actionbar in api level 7 because actionbar feature added in api level 11.
AppCompatActivity provide backward compatibility for new features

I see MainActivity extending the Activity class or AppCompatActivity.
Well it depends on you what type Activity you uses in your project.when you creates a new project it asks for Activitytype. like FragmentActivity etc.

By default android creating new project as ActionBarActivity

I recently updated eclipse, SDK and my problems are
By default it takes fragment_main.xml
Extends default activity to ActionBarActivity
By default it is adding the library appcompat 7
I dont have any issues with adding the library, but when i am trying to run then it is not executing in the device(2.3 android). I noticed it and changed to Activity.
If i extends it to Activity and run it and again for the next time the same problem arises while creating new Activity like...
Creating 2 xml files with extending ActionBarActiity.
I just want the new class to extend Activity but not ActionBarActivity where my target device is 2.3 and above without downgrading it

ActionBarActivity extends FragmentActivity and provides a nice bar(hehe) above your layout. Your editor is doing it correctly. ActionBarActivity was added in newer version of the sdk and it was added in the support library for older versions using appcompact v7(api 8). So most probably your development enviroment is correct. My guess is you need to check your imports and import ActionBarActivity from the support package.

It comes in SDK v4.4 . It is a plus point for developers because developers don't need to manually add the ActionBarActivity into their project. There are two solutions for your problem.
Downgrade your SDK to 4.3 or lower.
or
Remove ActionBarActivity and extends Activity in your java file.
Both will work for you.

Difference between Activity and FragmentActivity

I was working on fragments and came across two things Activity and FragmentActivity which are used several times. I want to know that is there any difference between these two, because when I changed Activity with FragmentActivity, it had no effect on the app.

A FragmentActivity is a subclass of Activity that was built for the Android Support Package.
The FragmentActivity class adds a couple new methods to ensure compatibility with older versions of Android, but other than that, there really isn't much of a difference between the two. Just make sure you change all calls to getLoaderManager() and getFragmentManager() to getSupportLoaderManager() and getSupportFragmentManager() respectively.

FragmentActivity is part of the support library, while Activity is the framework's default class. They are functionally equivalent.
You should always use FragmentActivity and android.support.v4.app.Fragment instead of the platform default Activity and android.app.Fragment classes. Using the platform defaults mean that you are relying on whatever implementation of fragments is used in the device you are running on. These are often multiple years old, and contain bugs that have since been fixed in the support library.

Android tabbed Activity

I have created tabs using intents.MainPage.java is my class that extends TabActivity and other 5 activites that extends Activity which i have added as tabs. I need to get Strings from the 5 activities which i have added as tabs once a click is performed in the MainPage.java class

From the documentation:
The TabActivity class is deprecated. New applications should use
Fragments instead of this class; to continue to run on older devices,
you can use the v4 support library which provides a version of the
Fragment API that is compatible down to DONUT.
Here are some Fragment tutorials to help you get started.
Also, check out FragmentTabs.java on the developer site.