What Is Setcontentview(R.Layout.Main)

Android - setContentView(R.layout.main);

Change

setContentView(R.layout.main);

setContentView(R.layout.activity_main);

as you dont have an xml file named main.xml in your layout folder..

what does setContentView do?

There's one thing that must be clarified here.

It's true that setContentView(R.layout.acivity_main2); will link the layout file to your Activity. However, tools:context=".Main2Activity" does not link your Activity to your layout file in the way you're thinking it does.

First, you need to take note of the word tools in that line. tools is the name typically used for the tools namespace which provides a few xml attributes that only works during design-time. Take a look at this documentation: https://developer.android.com/studio/write/tool-attributes

Note how it said that when you're building your app, everything related to tools will be stripped out. Therefore, tools:context=".Main2Activity" plays zero role in linking your Activity to the layout.

Instead, it's only meant to link it during design, by allowing you to Preview your layout in a container that mimics the Activity you set in tools:context=. So if you have a certain Theme for that Activity, the Preview will reflect the theme in the Preview Window.

On the other hand, setContentView(R.layout.activity_main2); does play a major role in linking your xml layout to your Activity.

That statement is telling the Activity to inflate that xml layout and fetch all of it's Views and more. Essentially, by using setContentView(R.layout.activity_main2); you're telling the Activity that you want to use that layout file, so please create your layout using that file as the guidelines.

After you called setContentView(R.layout.activity_main2); your Activity will know which layout you want to use, therefore making it possible to find the Views you want to work with through methods like findViewById(). This wouldn't be possible if you never used setContentView().

Therefore, to answer your last two questions:

Switching Main2Activity to another Activity in your layout xml file will do nothing besides show you how your layout will look if it was placed inside the other Activity. It won't change the outcome of your app at all. All for designing purposes.
Switching to another layout file in setContentView() will completely change the layout used in the Activity. Be careful of doing this, since if you have code used only for Views in the original layout, this WILL cause a crash if that code runs for the 2nd layout, because those Views don't exist anymore.

Bottomline is:

tools:context=".Main2Activity" - This is unnecessary for connecting your Activity and layout.
setContentView(R.layout.acivity_main2); - This is necessary for connecting your Activity and layout.

Error: setContentView(R.layout.main) in Android Studio

I found the answer by my self. I thing the error is not because of wrong import as answered here. It was because of the use of wrong name and id of layout file and check box respectively. My .xml layout file name was activity_my_checkbox.xml. So I should use:

setContentView(R.layout.activity_my_checkbox);

And my check box id was c instead of check, so I should use

cb=(CheckBox)findViewById(R.id.c);

setContentView(R.layout.main); Gives error from start

this is quite a common mistake .
make sure that at the top of your java file of the activity , the import is not of android.R.layout (or whatever) ,since it should import the R of your own app (which is inside the gen folder).

if you still have problems with that , try to delete the gen folder , make sure the layout file is ok (meaning it's a correct android layout file, without any mistakes), make sure all of the resources (all images, sounds , xml files , all that are in /res) have only the legal characters in their files names (which means they are lowercase english ,optionally with "_" and digits, but no more) , clean the project , and then import the generated file .

you can also use CTRL+1 on the red underline of the R that it complains about , and choose the one of your project.

setContentView(R.layout.Main); Main cannot be resolved or is not a field

First just build your project. If it will not work, then delete from imports android.R.layout and use layout from your package.

setContentView(R.layout.main); error

Just take 2 steps and problem would be more likely to get solved:

Step 1:
Clean your project by clicking Project -> Clean.

Step 2:
Rebuild your project by clicking Project -> Build All.

Also make sure that your layout xml files are syntax error free and you don't have any image which has non-acceptable names (such as a "-" between image name).

Also I request you to have a look at problems window and let me know what errors are being shown there.