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Error in MainAcitivity.java file & no style are loading

I am new to android. I am creating an android application, but there are many errors showing in MainActivity.java. Here is a screenshot
Another problem is that no style in showing in activity_main.xml. Here is the screenshot
Although I have added appcompact_v7 jar file like this
But it's not recognizing it in android option of project propertise
I have installed Support library in SDK manager. Please tell me how to solve this problem?

It is really hard to say what is wrong there. here are some things that you should try:
first of all, there are errors in your xml file (activity_xml). Please check if xml is properly formatted and you are using right tags. For debugging purpose, please make your acitivity_xml an empty FrameLayout and see if it works.
Include android 6.0 into your build path. Clean the project and try building again. Make sure your layouts are error free before doing that.
Your eclipse is unable to detect referenced library at given location. Please make sure referenced library is at same place and ERROR-FREE. If library has error, your project won't include it.
Please solve all problems one by one. First make an error free xml, then a Hello world Activity. Run it on device or emulator. Once done, try to solve library and import problem now.
Best Suggestion: Please migrate to Android Studio and gradle for better support and learning.

Eclipse: what is the difference between android-support-v7-appcompat and appcompat_v7?

First off I'm soooooo frustrated trying to get android.appcompat....gridlayout to work. I've followed lots of tutorials and just keep getting that horrible "android.support.v7.widget.GridLayout failed to instantiate" message. So I'm doing some serious detective work to try and understand everything about the compat libraries.
So like the title asks, what is the difference? I'm building the libs/android-...jar files, adding them to the path, following all the directions on https://developer.android.com/tools/support-library/setup.html
Any insight? Should I just uninstall eclipse and start from scratch to try and do this? I'm working with a test project and nothing is working. I keep getting 1 instantiation errors along with an error saying "a resource already exists on disk 'C:......\android-supp..-gridlayout.settings' and '....prefs'.
Please please any insight. Have spent a long time on this. Any advice is much appreciated.

they both refer to the same "v7 appcompat library" discussed at the android developer site.
You cannot just copy the jar file, as this library has resources that are referred to by your project so it needs to be added as a library project with resources as discussed here.

unable to instantiate application - ClassNotFoundException

EDIT: This problem has not already been resolved in the other suggested SO question
I had a fully working app on the market for over a year, with very few crash reports. Then recently I changed my app into a library, so that it could be included within multiple different "wrapper" projects. This was so that I could easily make different version - free, paid, non-google markets, with/without in-app purchasing etc etc.
The new "library+wrapper" app appeared to work fine. I could run it multiple times, without error. But then a day later (when presumably the OS had closed some or all of the app's activities) I tried to run it and it reported
Unable to instantiate application com.mycompany.mygamelibrary.MyGameApplicationClass: java.lang.ClassNotFoundException: com.mycompany.mygamelibrary.MyGameApplicationClass
The class it failed to find is the first class that runs when the program starts up, MyGameApplicationClass - which extends Application. This class is part of the library.
I suspect something goofy in one of the two manifest files.
The manifest of the wrapper project contains the lines...
<application android:icon="#drawable/mygame_icon"
android:screenOrientation="portrait" android:label="My Game Name"
android:name="com.mycompany.mygamelibrary.MyGameApplicationClass">
Any ideas what could have gone wrong?
EDIT: The library was referenced "the correct way" as defined by yorkw's answer to this SO question.
EDIT: I can not repeat the crash at the moment :-( I don't know what it is the OS does when the app is not used for a day or two.

There are two possibilities. Either you, like me, have a spelling error in your manifest file. Have a co-worker or friend read it to make sure the name is correct. Or you have not referenced the project correctly.
The official document describes how to properly link projects in its documentation.
Why it would first seem to work and later stop working is a bit of a mystery. However, I guess that the VM might still have had the necessary references ready to resolve the classes in the library just fine. A restart of the VM removed all those references and trying to resolve them was unsuccessful.
Update: Regarding the edits in the OP: As you confirm that you have correctly referenced the other project, you can check if the project is included in APK, just to be sure. You can rename and open an APK as any other archive (.rar works fine for me). Sometimes, it happened to me, the project is not correctly included in the APK. A cleaning of your workspace usually remedies the problem and so could a restart of your IDE depending on what you are using. To manually conduct a clean in Eclipse for example, use Project->Clean... or try Android Tools->Fix Project Properties by right-clicking on your project.
As you seem to also have fixed the problem by restarting your device, it could be that the libraries were linked incorrectly. A problem that I have never seen myself but as a very common quote says: "Have you tried turning it off and on again?".

For Android Studio:
Build --> Clean Project
Fixed issue.

Have you tried to make a new subclass of MyGameApplicationClass in your 'main' project and set it in the manifest as Application class?

I had a slew of bugs with Android Studio 3.0 Canary 4 and the way I fixed them was by editing the AndroidManifest.xml by adding in some jibberish to the application name. Then, I clicked build. Obviously, a whole mess of new error messages appeared. I changed the name back to what it should be, built the app, and it just ran.
Sometimes, I just don't know...
EDIT: Just ran into this issue on Android Studio 3.0 Canary 4 on my laptop when switching over. I again went through the same process of changing AndroidManifest.xml file to contain a typo, building, and changing back. That didn't work.
I then noticed that instant run was still enabled. Going into settings (by clicking command + , (comma key)) and typing "instant run", I was able to disable instant run, built the app, and the error of class not found went away.
Summary of Steps to Fix [FOR ME]
Invalidate cache / restart
Clean the project
Manually delete the build folder (need to be in project view for this one)
Make an intentionally errant edit to your AndroidManifest.xml file, build the app, observe the errors, remove the errant edit and build again
Disable instant run
Again, I don't mean to insinuate that this will fix everyone's error, but I have now used some combination of these steps on two different machines (MacOS Sierra) and it has been resolved for me. Hope it helps.

In my case, application id and package were mismatched. This should be same as presented in following images...
AndroidManifest.xml
app/build.gradle
In this case you can see, applicationId and package both are same that is com.mycompany.mygamelibrary

May be its a Build Path Configuration problem.I did the following to solve the issue.
1.Right click on your project and go to Java Build Path.
2.Click on Order and Export tab.
3.Check Android Private Libraries and other 3rd part libraries if you have added.
4.Press ok and clean the project.
I hope it will solve the issue.

Once I had the same error message, but maybe the cause isn't the same.
I did a code and worked for a while, then I wanted to improve it and got the same error and I couldn't run it.
I could fix the problem with
the correct Build Path order (as I can see you've already did this)
I check on the Order and Export tab the android-suppor-v4.jar
and the key was the Android SDK Managert->Upgrade everything and (next) Eclipse->Help->Check for updates.
After I upgraded to the latest android plugin and SDK my app compiled and ran again.
I hope this will help you!

I'm not very sure about this but it might be that your system's debug.keystore license validity has expired as it is valid for only 365 days. You just need to delete the debug.keystore from your computer. The debug.keystore will be generated automatically by Eclipse when you compile your Android App.

Same message seen ... this time it turned out to be different output folders for MyApp/gen and MyApp/src in the Build Path (caused by Maven integration).
Unchecking "Allow output folders for source folders" solved the problem.

I had this issue in an Android application that needed an Application class which was created in wrong path inside the Android Studio project. When I moved the class file to the correct package, it was fixed.

This all Process work for me to solve application class Exception.
Step 1: Open Run(window+R) Search -> Prefetch Remove all file (Some file not Delete)
Step 2: Open Run(window+R) Search -> %temp% Remove all file (Some File not Delete)
Step 3: Open Android Studio -> Build -> Clean Project
OR
Select File > Invalidate Caches / Restart > Invalidate and Restart from Android Studio toolbar.
OR
Close and reopen Android project.
OR
Restart System

I ran into this issue several times and both times it seemed to be caused by some instant run feature.
In my case, deleting the application from the device and then installing it from Android Studio again resolved the issue.

I ran into this problem today. The project runs well for over a year but today it reports this issue, and cannot debug on my testing device.
I fixed it by updating to latest gradle version. Hope this can solve your problem.

Android Studio Gradle error

Everything is ok but when i try to run it gets one error
Gradle: : java.lang.NullPointerException
Please help me, because i wanna to throw all this stuff with my NTB down from the cliff, after about five hours of configuring git, android studio and copying my projects from Eclipse, still got this

I had the same problem. I solved it by going into 'Project Structure' and one of my modules had both 'Android' and 'Android - Gradle'. I deleted the 'Android - Gradle' and rebuilt the project and the error disappeared.
Edit: I had bigger problems than this and ended up creating a new project...

You need to make sure Gradle is installed and properly configured in the preferences.

I had the same problem. It was because the path to android studio contained a whitespace. Installing android studio to the recomended location solved my problem.

As easycheese underlined the problem is related to the presence of the android-gradle item below one or more modules in your project. Go to the "project structure" (from the File menu) and check which module has two sub-items, one of them must be the android-gradle, delete it and rebuild your project. It should fix the issue......as J.Romero says...Android Studio is far from being ready for professional use, but what it is really annoying is that Google, with all its resources, gives developers such "immature tools" that make rather waste time than anything else....shame

I wont re-write my whole answer, but check out the following page in which i replied to a Gradle common error with over 10 common solutions across the web including mine.
Long story short, for me it turned out to be system resources were getting low (Google Chrome was the greedy one)
If it works after a fresh reboot, but after a few minutes or hours it starts.. This would indicate this.
Gradle Error In Android Studio

Android: Lint says - Class referenced in the layout file, was not found in the project or the libraries

In my app, I am using Pull TO Refresh list view library. I had designed layouts for all screens respectively. When I ran lint on my project, in one of my layout file for xlarge screen, it gave me fatal error:
Class referenced in the layout file,
com.handmark.pulltorefresh.library.PullToRefreshListView, was not
found in the project or the libraries
While in same layout file for other screens, it is giving no error at all. Also, I had used same library in other layouts of my file, where it does not give any error. It is giving same error for Google Maps api as well. My project targets Google Api level 16 and I had checked, all libraries are added to it. But,the error doesn't go away even on cleaning project.

This is a lint bug and you can safely ignore it. Lint is meant to help, not to hinder your development time. Change the lint Errors to warnings and you can compile and run the app just fine. I don't recommend turning it off because it helps to keep your code cleaner.

Make a project Project -> Clean... this resets the bug errors for me.

I was also getting a similar error, for me, it was resolved by adding android.enableJetifier=true to gradle.properties.

You can suppress the lint error by, passing the error type to the ignore attribute within that view, for example:
<com.handmark.pulltorefresh.library.PullToRefreshListView
...
tools:ignore="MissingClass"
...
/>
Just make sure you have tools imported:
xmlns:tools="http://schemas.android.com/tools"
References:
https://googlesamples.github.io/android-custom-lint-rules/checks/MissingClass.md.html

It happened to me when i changed thé android studio IDE, To solve it:
-reate a new project;
-replace the content of dependencies in build.gradle of your project with the content of dependencies in build.gradle of the new project;
-sync project with gradle.
And it will work.