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Every time when i open class teacher's project to my system it shows this error.Error running app: Default activity not found

I'm a beginner in Android.
Every time when I open class teacher's project to my system it shows this error.
Error running app: Default activity not found.
What should I do to match the same project as my teacher's?
I've tried many solutions.
My Manifest file is
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.example.assigncheckbox">
<application
android:allowBackup="true"
android:icon="#mipmap/ic_launcher"
android:label="#string/app_name"
android:roundIcon="#mipmap/ic_launcher_round"
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>

maybe it happen because you didn't add this activity to your AndroidManifest.xml
<activity
android:name=".yourActivity"
/>

Gradle project sync failed error after invalidating caches/restart?

I am running android 3.4.1 and all I did was invalidate caches and restart now I get the error gradle sync issues and this
ERROR: Failed to parse XML in C:\Users\J.Robinson\AndroidStudioProjects\App\app\src\main\AndroidManifest.xml
ParseError at [row,col]:[17,64]
Message: expected start or end tag
I've google stack for an hour and a half now and nothing seems to work. How do I solve this?
Image of error in question
Manifest.XML
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.jstudios.curbyourmeme">
<application
android:allowBackup="true"
android:icon="#mipmap/ic_launcher"
android:label="App"
android:roundIcon="#mipmap/ic_launcher_round"
android:supportsRtl="true"
android:theme="#style/AppTheme">
<activity
android:name=".MainActivity"
android:label="app"
android:theme="#style/AppTheme.NoActionBar">
android:screenOrientation="portrait"
android:configChanges="orientation|keyboardHidden">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
</application>
</manifest>

Remove the > symbol in the line
android:theme="#style/AppTheme.NoActionBar">
It was causing the error

MainActivity unresolved class error with android studio

Im taking the udacity coarse and for some reason android studio wont recognize my MainActivity class. Ive tried cleaning the project, rebuilding and making sure that the names were correct but nothing has solved my problem.
Android Manifest code:
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="androidedx.example.activitylifecycle">
<application
android:allowBackup="true"
android:icon="#mipmap/ic_launcher"
android:label="#string/app_name"
android:roundIcon="#mipmap/ic_launcher_round"
android:supportsRtl="true"
android:theme="#style/AppTheme">
<activity android:name=".Activity1"></activity>
<activity android:name=".Activity2" />
<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>

Found the problem, this package name within the code I had to paste
package com.example.android.cookies;
Im not sure why specifically this caused the problem but atleast I solved it.

Android studio Default Activity not found

After i have created my own appplication class:
package com.example.bezzabarovaa.myapplication;
import android.app.Application;
import android.content.Context;
/**
* Created by bezzabarovaa on 04.08.2017.
*/
public class app extends Application {
public static Context context;
#Override public void onCreate() {
super.onCreate();
context = getApplicationContext();
}
}
and have changed manifest.xml (changed application to app class)
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.example.bezzabarovaa.myapplication">
<app
android:allowBackup="true"
android:icon="#mipmap/ic_launcher"
android:label="#string/app_name"
android:roundIcon="#mipmap/ic_launcher_round"
android:supportsRtl="true"
android:theme="#style/AppTheme">
<activity android:name=".MainActivity"
android:label = "#string/app_name">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
<category android:name="android.intent.category.DEFAULT"/>
</intent-filter>
</activity>
</app>
I`ve got a message Error running Unnamed: Default Activity not found
If I will change my classapp back to application in manifest.xml - everything is ok.
Where is a problem?
Thank you.

This <app is supposed to be <application you specify your custom app in here:
<application android:name=".app" ...
Check the documentation the name section says:
The fully qualified name of an Application subclass implemented for
the application. When the application process is started, this class
is instantiated before any of the application's components. The
subclass is optional; most applications won't need one. In the absence
of a subclass, Android uses an instance of the base Application class.
Full example (with your code):
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.example.bezzabarovaa.myapplication">
<application
android:name=".app"
android:allowBackup="true"
android:icon="#mipmap/ic_launcher"
android:label="#string/app_name"
android:roundIcon="#mipmap/ic_launcher_round"
android:supportsRtl="true"
android:theme="#style/AppTheme">
<activity android:name=".MainActivity"
android:label = "#string/app_name">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
<category android:name="android.intent.category.DEFAULT"/>
</intent-filter>
</activity>
</application>
</manifest>

Solved by clicking the "Sync project with gradle files"

I faced the same problem a while ago, after changing my default activity to another.
What solved it was invalidate caches and restart Android Studio:
File -> Invalidate Caches / Restart...
and then clicking on Invalidate and Restart.

Use below code snippet
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.example.bezzabarovaa.myapplication">
<application
android:name=".app"
android:allowBackup="true"
android:icon="#mipmap/ic_launcher"
android:label="#string/app_name"
android:roundIcon="#mipmap/ic_launcher_round"
android:supportsRtl="true"
android:theme="#style/AppTheme">
<activity android:name=".MainActivity"
android:label = "#string/app_name">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
</application>
</manifest>

You can get Android Studio not to complain by going to the "Edit Configurations" menu (tap "Shift" three times, type "Edit Configurations"), then change Launch Options > Launch to "Nothing".
I'll probably add a generic activity containing instructions, just to avoid any confusion.

Add below line in your manifest application tag
<application
android:name=".app"
...
</application>

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.example.android.bluetoothlegatt">
<uses-feature
android:name="android.hardware.bluetooth_le"
android:required="false" />
<uses-permission android:name="android.permission.BLUETOOTH" />
<uses-permission android:name="android.permission.BLUETOOTH_ADMIN" />
<uses-permission android:name="android.permission.ACCESS_COARSE_LOCATION" />
<uses-permission android:name="android.permission.ACCESS_FINE_LOCATION" />
<application
android:icon="#drawable/app_icon"
android:label="#string/app_name"
android:theme="#android:style/Theme.Holo.Light">
<activity
android:name="com.example.android.bluetoothlegatt.DeviceScanActivity"
android:icon="#drawable/app_icon"
android:label="#string/app_name">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<action android:name="android.intent.action.VIEW" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
<activity android:name="com.example.android.bluetoothlegatt.DeviceControlActivity" />
<service android:name="com.example.android.bluetoothlegatt.BluetoothLeService" />
</application>
</manifest>

I solved the very same issue by following steps
Close Android Studio
deleting from :
C:\Users\your user name.android/log and gradle folder
C:\Users\your user name.gradle
from C:\Users[your user name]
Restart Computer and run Android studio (it Will take some time to rebuild)

java.lang.classnotfound exception is coming while launching a simple helloworld app on mi4i android phone

java.lang.classnotfound exception is coming, while launching a simple helloworld app on mi4i android phone. it is running fine on Emulator. I am using android studio 2.3. I already checked the class name and path in mainfest file. Can anybody help, what wrong i am doing? Following is the error screenshot Mobile screenshot while launching app
Following is my manifest file
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.example.tarun.myapplication">
<application
android:allowBackup="true"
android:icon="#mipmap/ic_launcher"
android:label="#string/app_name"
android:roundIcon="#mipmap/ic_launcher_round"
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>