Difference Between Content_Main.Xml and Activity_Main.Xml

why is there content_main.xml and activity_main.xml by default instead of just activity_main.xml?

The modern Android approach is based on Fragments, which are, in a way, "small Activities", which you can put in Activities, gaining lots of flexibility.

Therefore, activity_main.xml is simply the Activity layout containing a container (FrameLayout most probably) and content_main.xml is the layout for a Fragment put into this container somewhere within MainActivity.java. You should study the code in there to understand it better :)

What is the role of content_main.xml in android studio 1.4?

According to new design pattern in android studio activity_main.xml will determine how the global UI of the Activity should be. And on the other hand content_main.xml will determine the contents in the activity_main.xml.

That is content_main.xml will contain the textview, edittext, button etc component. And it will be included by the activity_main.xml.

So we can think of content_main.xml just like partial in HTML. activity_main.xml will contain your activity global design, and content_main.xml will contain the contents.

From view of what they contain:

activity_main : Co-ordinator layout, ViewPager etc

content_main : Developer choosen elements. textview, edittext, button etc.

What the android developer website says about them is

activity_main:

This XML layout file is for the activity you added when you created
the project with Android Studio. Following the New Project workflow,
Android Studio presents this file with both a text view and a preview
of the screen UI. The file contains some default interface elements
from the material design library, including the app bar and a floating
action button. It also includes a separate layout file with the main
content

content_main:

This XML layout file resides in activity_my.xml (activity_main) ,
and contains some settings and Textview(other) element.

What has to be used, content_main or Activity_main?

While setting the parameters for setContentView, which XML has to be
typed ? Content_main or Activity_main ?

You should use content_main.xml to put your views but pass
activity_main.xml as the parameter to you setContentView() method.

activity_main.xml is used to display coordinatorLayout, tabLayout, floatingActionButton, viewPager etc.

content_main.xml is used to display your stuff i.e. what you want to display to users.

reference

And if I want to add an Layout file (XML) do I have to create 2 files
with the same name ?

Totally depends on the nature of the activity you are creating the layout for. The difference above will help you decide whether you need two or one.

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