Styjun

I got this error when moving a lot of files, to fix just resync your gradle files.

File->Sync Project with Gradle Files

Sometimes when moving or renaming files, you can have broken XML files. My case that the ic_launcher.xml was corrupt. I regenerated that and worked just fine.

Check every XML in your proyect.

You are most likely just missing the below from your AndroidManifest:

<activity
 android:name=".MainActivity"
 android:label="@string/app_name"

Here is a full example of an AndroidManifest:

<?xml version="1.0" encoding="utf-8"?>
<manifest
 xmlns:android="http://schemas.android.com/apk/res/android"
 package="com.example.pejner.myapplication">

<application
 android:allowBackup="true"
 android:icon="@mipmap/ic_launcher"
 android:label="@string/app_name"
 android:supportsRtl="true"
 android:theme="@style/AppTheme">
 <activity
 android:name=".MainActivity"
 android:label="@string/app_name"
 android:theme="@style/AppTheme.NoActionBar">
 <intent-filter>
 <action android:name="android.intent.action.MAIN"/>
 <category android:name="android.intent.category.LAUNCHER"/>
 </intent-filter>
 </activity>
</application>
</manifest>

Try to go to File->Invalidate Caches / Restard and choose invaliadate and restart ,it worked for me

This can be only two reason,

the bold one, missing extend statement

class MainActvity extends AppCompatActivity 

or

you are using a wrong package name to register your activity so just in this case use ALT+Space then studio will show the options itself.

replace in your AndroidManifest.xml

 <?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
 package="com.example.android.justjava">

 <application
 android:allowBackup="true"
 android:icon="@mipmap/ic_launcher"
 android:label="@string/app_name"
 android:supportsRtl="true"
 android:theme="@style/AppTheme">
 <activity android:name=".MainActivity">
 <intent-filter>
 <action android:name="android.intent.action.MAIN" />

 <category android:name="android.intent.category.LAUNCHER" />
 </intent-filter>
 </activity>
 </application>

</manifest>

You should try this

<activity
 android:name="com.example.stockquote.StockInfoActivity"
 android:label="@string/app_name"
 />

For more detail you can check hereActivity declare

Thanks everyone for taking the time out to help a beginner.
I found out that I had somehow misspelled the Package in MainActivity.java
When I corrected it the error was gone.
Thank you all.

I was getting an error "The activity 'MainActivity' is not declared in AndroidManifest.xml", even though it was correct in the manifest file.

My problem was that when I created the project I had to mark the item "Use AndroidX artifacts".

If you see that error occur after upgrading versions of IntelliJ IDEA or Android Studio, or after Generating a new APK, you may need to refresh the IDE's cache.

File -> Invalidate Caches / Restart...

All of my XML file were cut in half for some reason, I'm not sure when it happened (I use Git VCS, so maybe that). I had to go through each of them and restore the code.

It have three ways to resolve

1:-
Go File->Invalidate Caches / choose invalidate and restart ,it works

2:-
If you make MainActivity make sure you have onCreate method in it and extend with AppCompatActivity .

3:- And last option sync project with Gradle file