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Android Studio 3.1.3 - Unresolved reference: R - Kotlin

I am new to kotlin, i have converted some code from java but it seems like there's something wrong, The R in findViewById(R.id.my_id) is highlighted in red and it shows this message : "Unresolved reference: R".. I've been looking for a solution but i seem not to figure it out, So what should i do?
Here's a screenshot :

The issue can be caused by many factors,
as mentioned by martomstom in this Answer the issue is sometimes caused by com.android.tools.build:gradle version, changing it's version to a more stable one would solve the problem: for example: com.android.tools.build:gradle:3.4.0-alpha02 with com.android.tools.build:gradle:3.2.1
Also, having libraries from the same group, but with different versions may cause the problem or even more runtime errors. use the exclude group method like the following : implementation('com.squareup.picasso:picasso:2.71828') { exclude(group: 'com.android.support') } in this case, picasso library uses android.support components, the android library version used in picasso is different than the one you're currently using in your app, so in order to solve this issue, we have to exclude it completely from its sub library and class groups.
It can also happen by the mismatch of resources and code, including this importation line in your activity may solve the problem too : import com.package.name.R
Sometimes it can happen because of the IDE, performances or memory.. Cleaning the project from time to time may save you some time, on Android Studio it would be something like this : Build -> Clean Project / Rebuild Project - Cleaning IDE cash also helps with performance and memory, on Android Studio it would look like this : File-> Invalidate Chases/ Restart -> Invalidate Cashes and Restart
I noticed that this problem happens to me the most of the time when importing new resources, Using prohibited characters in their names would fire the error, such as . , , - , UpperCase or special Letters
And as a suggestion , if you're using Kotlin, i really recommend using Kotlin extensions in your activity such as : import kotlinx.android.synthetic.main.activity_page.* or if you're using a custom view : kotlinx.android.synthetic.main.view_layout.view.*
after that, in onCreat() method of an activity , you'll only have to call the id, for example : my_edit_text_ID.text = "Kotlin Dbest!", or from a custom view : mCostumView.my_edit_text_ID.text = "Kotlin Dbest!"
EDIT :
I have faced this issue againe and the problem was the '' R '' library was imported from 2 different sources :
com.android.R
com.example.package.R
You must only import the '' R '' library with your application package name,
in this case com.example.package.R
Sometimes the library is not imported at all, to import it, click on the
unresolved reference R and press Alt + Enter
EDIT:
As tobltobs mentioned in the comments section: " Most of the time the problem is caused by another error which prevents the build system from creating generated sources. To find the root cause look at the gradle log (the "toggle view" icon below of the green hammer in the Build output) and look for errors unrelated to R or BuildConfig (also generated). If there is no other error left and the problem with R persists then maybe something of this list might help. "
EDIT:
As Patrick Beagan mentioned, Kotlin extensions are now deprecated - I'd advise using ViewBinding instead

I used com.android.tools.build:gradle:3.3.0-alpha13 and had the same issue. Changing to stable Version 3.2.1 solved this problem for me.

So this is a misleading error.
The fastest way to get to the root cause is to run:
bash gradlew assembleDebug --debug
then scroll up and look for the real error happening.
However, if it still doesn't seem like you have the answer you are looking for, then read on.
I'm going to explain the 30,000 foot view of what is happening. This is not EXACT order or EXACT flow, it is just pretty damn close ;) so if you know more then I do of the exact order and care to make corrections with links, feel free I won't stop ya :).
The Process
The R file is generated code.
There is an order to the generation.
Gradle will do it's magic, pull it's dependencies and kick off it's
warning and error tree first,
then Android converts all Kotlin to Java behind the scenes. Yup that's
right, our beloved Kotlin still has to be Java to compile for our
beloved ART virtual machine.
Then it runs through and does the adapters that you have created for
JVM Statics and a few other tasks.
Next up it compiles all the xml databinding files first to create the
generated databinding files.
If everything succeeds it moves on to processing the assets and
resources. Which creates pointers or IDs for each resource that you
reference in code. Next it will run through and begin compiling the
code and packaging process after that.
Pretty straight forward process, but here in lies the problem.
The misleading Error
If any step fails before the R generation is complete, then the R does not get generated. Sometimes a simple rebuild is all you need to do, sometimes a simple File->Invalidate Cache and Restart is all you need. However, more often than not you have a code issue in your gradle, your xml, your databinding or your adapters that are preventing the compiler from even reaching the R generation stage.
So the next question is
"Well shoot, how do we troubleshoot it if the errors are worthless or
non-existent".
Well first let's talk about the many ways these errors present themselves.
Duplicate Databinding class found
xml Binding Error at line #
Couldn't find matching signature of bind:customAdapterMethod
Can't find R file of the correct project, only shows import options for sub modules or incorrect namespace R files.
Couldn't find DataBindingUtility or DataBinding for activity/fragment
And many other various ways as well, too many to list them all
Next, let's talk about potential candidates causing the problem. As there are sooo many lol.
Gradle Syncing issues
Bad versions of Gradle or Tools, you may have gone too far forward in your last gradle file modification. Try stepping back one version and "invalidate cache and restart" if that fixed it, great, if not, read on.
Caching Issues (File->Restart and Invalidate Cache)
xml elements with wrong namespace
xml elements with bad IDs or references IDs out of order (i.e. you say align to right of an element that is lower in the xml document then the sibling element that is trying to reference it)
xml data binding issues referencing namespace or member that doesn't exist or is not typed correctly
xml data binding issues in non-auto-filled spots like custom attributes using adapters as those are harder to spot. i.e. bind:myCustomMethod=#"myObject.mistypedProperty()"
JVM Static adapters with issues or duplicated signatures
Duplicated or bad character in the Strings or Dimens file or any other xml file for that matter
Private variable marked for #Binding without properties to access it
Member variable marked for #Binding that matches a parent class method causing duplications that manifests itself in almost impossible errors
Mismatch of types like using an adapter that takes (Int) but you are passing (Int?) via data binding and it isn't recognized with JVM Statics until compile time
You selected IMPORT on a popup to import R file of a sub module instead of the application file
Having bindable members in a child or parent class, but not giving fully qualified namespace to class cast in the XML usage of the parent or child class. As the databinding compiler is not smart enough to realize the variable provided for class Foo is also parentFoo baseclass, so you have to qualify it as android:text="#((com.path.parentFoo)foo).parentMethod"
Having a method name in a class, that matches a "generated property from #Binding member variable" i.e. firstName as a variable, but then having a method called getFirstName in a parent or child class, because you are now matching a method name that will get auto generated, thus causing dataBindingUtility duplication class errors.
There are more causes, but this should give you a series of places to look, but the list can go on and on seriously.
Unfortunately this happens a lot in bleeding edge technologies where the UI tools are not up to speed with the terminal based tools yet. So I suggest you run from the project root in a terminal with
bash gradlew assembleDebug --debug
When it fails, and it will. Start scrolling up through the logs until you find the red where you see what is actually failing and preventing the next stage from occurring.
You will find this especially useful when you start dealing with databinding.
TIP:
When you start dealing with databinding, make sure you compile and run often because the goal is to recognize right away before doing other files to make sure you didn't break generation and make your life MUCH easier to know code you just added caused the issue before getting too far.
Times to compile and run to confirm no issues before moving on.
If you add a few JVM statics compile and run
If you add variables to your XML to use
If you bind to properties of your model in 1 file
If you add a binding to a JVMStatic
If you add bindable members or properties to an model
If you refactor moving observable member variables or properties into children or base classes
Any other xml or binding related elements that can effect the generated code.
Like I mentioned above, the reason is to avoid getting so many changes, that it becomes a troubleshooting nightmare to find a generic vague, horrible error related to generated databinding code. I'm sure the tools will improve, but for now, do yourself a favor and compile and run often when changing Databinding related items.
Happy Coding

Use gradle commands.
In Android Studio, on the right menu:
Gradle -> :app -> Tasks -> build -> clean.
After that, Gradle -> :app -> Tasks -> build -> build

I had wrong import statement import android.R instead of import my.project.package.R. Fixing it solved the problem

I had the same problem, and I tried not to downgrade from gradle version 3.3 to gradle version 3.2.1. Instead I updated Android Studio to version 3.3, which made the trick for me ;-)

This worked for me. How much work it is depends on how big your project is. I started a new project, created the required modules (XML, Kotlin, colors, strings, etc.), then copied the code into the modules in the new project from the modules in the old project. Copying XML saves a lot of time compared to recreating the UI. All in all, it take a little while, but I have spent much more time tring to fix the unresolved reference error without it.

TRY THIS
Go to the content_main.xml file and there you need to change the
android:id="#+id/??????"> line of code to whatever id you have given to your file.
Replace question mark ?????? with the related file id name.(IF you dont know the id go to the design tab on the bottom and click on the related Asset.
On the right side below attributes, you can find the ID you have given to it.
If it is blank you can freshly name it and Android Studio will write the code.
Then restart Android Studio. Hope this will help. Happy coding.

I believe that I came across the real answer (though by accident).
I also, as the OP had my KT file fail to location R. as well as other classes that happen to be in java. What I noticed was that there was a case difference I the filenames. Once I corrected the import statements to match the case of the package (aka, folder) the errors resolved.

I had the same problem with R reference too.
Finally Android Studio 3.3 has been released and using 'com.android.tools.build:gradle:3.3.0' the problem has been fixed!

I update Android Studio to version 3.3.1 and solved this problem.

Downgrading gradle version worked for me.
I changed :
Gradle version from : 4.10.4 to 4.4.1
and Gradle Plugin version from : 3.3.1 to 3.1.3

If your are experiencing this issue in Kotlin because you cannot reference the elements of the xml layout by ids. (e.g. R.id.adView) then try removing the line import android.R from your kotlin file.

For me it was because I had created a new package and R wasn't available until I imported it from the package above

I faced the same issue. I restarted my Android Studio, invalidate caches, Sync Gradle but nothing was working. Then I looked into my file and there are 2 imports of my R. 1 import was related to my application package and the other was related to Android.
import com.example.myApp.R
import android.R // This import was added accidentally during the build.
Removing second import related to android solved this issue.

I had an issue because of this import:
import android.os.Build.VERSION_CODES.*
In the latest version it contains R

I had same problem while using auto-manifest plugin. Adding AndroidManifest.xml explicitly solved the problem to me.

I used to File --> Invalidate Caches... and issue resolved.

Just restarting Android Studio solved it for me.

I also had this problem, Gradle Sync, and Invalidate Cache, and Restarting Android Studio Didn't help. Upgrading and Downgrading Gradle were also not helpful.
What worked for me is: Make Project (Ctrl + F9) and then try to run the project.

I solved this error by following Android Studio's lint tools to upgrade the version of a dependency in the project-level gradle file. In this case, I upgraded androidx.navigation:navigation-safe-args-gradle-plugin from 2.3.2 to 2.5.2 (latest version), then synced the project.

Here is the solution,
File->Project Structure->Project, select Android Gradle Plugin Version as 3.2.1 from the drop-down. then click apply.

Java cannot access class, class file not found

When I try to make a project in IntelliJ I receive the following error on this line:
Sentence sent = new Sentence();
sent.emptySegments();
Error:
Error:(151, 10) java: cannot access javax.xml.bind.RootElement
class file for javax.xml.bind.RootElement not found
Sentence is a class which implements the RootElement interface
import javax.xml.bind.RootElement;
...
public class Sentence extends MarshallableRootElement implements RootElement {
All packages exist and I can jump to declaration of each interface or class but I don't know why IntellJ says it cannot access or find them? However RootElement is an interface and not a class
public interface RootElement extends Element {
void validate() throws StructureValidationException;
}
The above declaration is in a jar file named jaxb-rt-1.0-ea.jar and it exists in the Project librarians.

Try this
Go to File
Invalidate Caches/Restart
You can choose only Invalidate and restart
(See Invalidate caches on IntelliJ's manual)

The project contained several modules. While the library was added to the project libraries, some modules lacked it in their dependency part. So I solved the problem using the following steps in IntelliJ
Creating a module library and adding it to the module dependencies:
Open the Project Structure dialog (e.g. Ctrl+Shift+Alt+S).
In the left-hand pane of the dialog, select Modules.
In the pane to the right, select the module of interest.
In the right-hand part of the dialog, on the Module page, select the Dependencies tab.
On the Dependencies tab, click + (on the top right) and select Jars or directories.
In the dialog that opens, select the necessary files and folders. These may
be individual .class and .java files, directories and archives
(.jar and .zip) containing such files as well as directories with
Java native libraries (.dll, .so or .jnilib).
Click OK. If necessary, select the Export option and change the dependency scope.
Click OK in the Project Structure dialog.

File -> Invalidate Caches/ Restart
this worked for my after long hours of effectiveless

Rebuilding project worked for me.

Deleting the .idea folder and then running Invalidate Caches/Restart worked for me.

It also may be because you don't have dependencies in classpath, which used in dependencies.
For example: you use library A, but class you're using from A has superclass from library B. But you didn't add B to classpath.

If it is a single file, you can try deleting the file and undoing it. It seems to reindex that particular file alone, which is much faster than Invalidate Caches/Restart. As a precautionary measure, you can take a backup of the file before deleting, just in case if something goes awry.
Another reason might be different versions of same library with more/less methods. This happened for me with Gradle. Sometimes it compiles fine and sometimes, it doesn't. Just find and remove the unnecessary ones.

My Gradle/IntelliJ "big hammer"
(Optional, but preferred). Close all instances of IntelliJ or any other Java IDE.
delete the ".idea" folder (<< intellij specific, or whatever "workspace" folder your IDE uses)
..............
./gradlew --stop
OR
gradle --stop
(now delete the folders)
rm -rf $HOME/.gradle/caches/
rm -rf $HOME/.gradle/build-cache-tmp/
(now resume normal gradlew commands like:)
./gradlew clean build

For me just worked, turn off windows defender / add exclusion project folder / idea process.

Similar problem can happen if a library is imported with maven scope runtime.
In such case it isn't accessible by your classes located under src/main/java.
Only classes in src/test/java can directly use runtime dependencies.

I removed this location "amazonaws" file and clean install later run
/Users/testuser/.m2/repository/com/amazonaws

If you've made it this far because rebuilding or invalidating the cache didn't work work you, I found that deleting the class and adding a new one with the same code worked.

In my case there was an old .iml file in the module causing these problems. So if nothing else worked for you, try looking for one.

⬇ Download the last version of IntelliJ.
▶ Install it.
Worked for me. ✌

java.lang.NoClassDefFoundError: com.google.ads.AdView

I am attempting to incorporate admob ads in my app. So far I have added the following code
in the onCreate method of my app's main activity...
adView = new AdView(this,AdSize.BANNER,"my code number");
adView.loadAd(new AdRequest());
The program compiles without error but at run time I get the message java.lang.NoClassDefFoundError: com.google.ads.AdView. I have seen a supposed explanation of the problem and the cure here but I could not see how it was applicable to my project because I do not have either a "lib" or "libs" directory within my project.
According to eclipse's SDK manager, I have the most up to date version of everything that I use.

I had a similar problem, but did not have to add the libs folder; I tried, with no success, then removed them afterwards, when I realized that I had just forgotten to export the included Jar archived.
Fix:
Select project settings > Java Build Path, then Order and export.
Here, check the GoogleAdMobAdsSdk-package.
This should solve the NoClassDefFoundError-issue.

Just create manually the "libs" folder, and add your libraries there. By default, they doesn't exist

Project-> Clean
Project settings -> Java Build Path. Open "Order and export" tab.
Check GoogleAdMobsAdsSDK.
It is worked for me.

Was getting this error updating from android 16 to 20.
Solved by going to Java Build Path, Order and Export tab, checked and moved GoogleAdMobAdsSdk just under my source. For some reason that fixed it.
Changing the order back didn't break it. So some setting must have needed to be initialized.

I had this problem, and this problem too:
Multiple dex files define Lcom/google/ads/AdRequest$ErrorCode and Multiple dex files define Lcom/google/ads/AdRequest$ErrorCode
Despite many hours of following all solutions listed the only way I could get my app to work was to use the AdMob banner example here:
https://github.com/googleads/googleads-mobile-ios-examples/tree/master/admob/SmartBannerExample
And re-build my project around it, copying my files in to that project. If you are stuck, try it.
Don't know why it worked, but now it does. Something in my project was very wrong. Now I can carry on with my work. I love Google/Android, but really what a mess!

See this answer Android update 17 seems incompatible with external Jars.
You need to put all jars in libs folder insted of lib.

It's not enough only to add "GoogleAdMobAdsSdk-6.4.1.jar" in Java Build Path,then you should switch to table:Order and export.check the "GoogleAdMobAdsSdk-6.4.1.jar" here.

UPDATE:
Select project settings > Java Build Path, then Order and export.
Here, check the google-play-services.jar.
This should solve the NoClassDefFoundError-issue.

In my case I needed to click on the "Android Private Libraries" on the "Order and Export" Tab

Go to Properties - Java Build Path - Order and Export and check every Checkmark! That did the trick for my project.

Eclipse can't find / load main class

My Eclipse (Indigo) was running just fine. I created a simple class Hello. It is placed in package cont in the folder ch13. However, when I try to run it from Eclipse I get info from Java Virtual Machine Launcher:
Could not find the main class: cont.Hello.
I tried to run other classes from this package and they run just fine (that is the classes that existed there before). However any new class I create in this package has these problems. Actually any new class I create in Eclipse runs into this problems. I decided to check how it works from the command line. It seems that the problem still exist - I get same error. I checked the path and it is C:\Program Files\Java\jdk1.7.0_02\bin, which is correct (note the other classes are running from Eclipse just fine). I tried to run with java -cp . Hello and there are some Errors produced starting with java.lang.NoClassDefFoundError: Hello (wrong name: cont/Hello). Code itself is simple:
package cont;
public class Hello {
public static void main(String[] args){
System.out.println("Hello");
}
}
How can I fix it so that my classes still run under Eclipse?

.metadata is corrupted.
Steps:
Warning: Deleting .metadata will delete all your Eclipse configurations, plugins, project setups. Make a backup before you attempt this!
Stop eclipse, delete .metadata in workspace and restart eclipse
Import Project
Run again

Removing the Run Configuration
Sometimes I have a similar problems in some pre-release versions of eclipse. For fix the error, I delete the Run Configuration. You can find that in menu Run, Run Configurations...
Then I launch the app with Alt+Shift+X, then J. If this don't work, Ctrl+F11.
Deleting the .metadata directory
In another way, the configuration settings for your current workspace may are corrupted. Those settings are in the .metadata directory in your current workspace 1. In that case, there is no other choice than delete the directory:
Close eclipse.
Delete the .metadata directory.
Start eclipse.
Import the projects.
Run the project again.
Notes
You will see that directory with File > Switch Workspace > Other...

I have solved the issue following way:
Go to Run Configuration (Right Click on Java File->Run->Run Configuration).
Go to ClassPath Tab:
Click on Advanced -> Add Folders -> Add bin directory (which has class file in it for Java source code)
Re run the code, now it will solve the issue. It worked for me

Note: This worked in the past and I received many up votes. Perhaps this is not a solution anymore - but it once was - as the eclipse version was indicated.
Problem
This can also be caused by a Java Build Path Problem.
In my case, I had a an error:
A cycle was detected in the build path of project {project}. The cycle consists of projects {x, y, z}.
This can occur when you include other projects in the build path of the project you wish to run. In fact, all the projects will fail to run with the error
Could not find the main class: Example.class
Solution
Open
Windows -> Preferences -> Java-> Compiler -> Building -> Build Path Problems
Uncheck the Abort build when build path errors occur toggle
This seems like a can of worms if you end up with other build path errors I image. So use with caution.
Note: This only works if you have a "cycle error". This error message can be found in the "Markers" tab
I found the solution to this here
Info
Java 1.8.0_152
Eclipse Photon (June 2018)

Renaming the main class should be enough (and easiest):
- Go to your class and set cursor to your class name;
- ALT + Shift + R and rename the class (build if not done automatically);
- You should be able to run it now;
- Rename the class to the old name if you want;

You must have main function in your class. Like
public class MyDataBase {
public static void main(String args[]) {
}
}

I had this same problem in a Maven project. After creating the src/test/java folder within the project the error went away.

Another tip: I initialized static fields in a wrong order - surprisingly it didn't bring up a Problem (NullPointerException?), instead Eclipse complained with exactly the message OP posted. Correcting the static initialization order made the class run-able. Example:
private static ScriptEngineManager factory = null;
private static ScriptEngine engine = null;
static {
engine = factory.getEngineByName("JavaScript");
// factory is supposed to initialize FIRST
factory = new ScriptEngineManager();
}

I found the way to fix this problem was to rename the project. If you give it a name with strange characters (in my case, :), it will have trouble locating your class. I don't know if this rule applies to numbers, but try renaming the project or making a new one and copying the files. Name it without any numbers or special characters.

I solved this error by closing the project, removing it from eclipse and then importing it again.
Might be a little simpler than to redo the whole workspace setup.

I had this issue after upgrading to the Eclipse 2019-12 release. Somehow the command line to launch the JVM got too long and I had to enable the jar-classpath option in the run configuration (right click on file -> run as -> run configs).

I read so many blogs and tried so many tricks but my problem not resolved. I was able to run the code but not able to generate the jar file. :( Sad..
But I tried something which might be very silly but worked for me and bought eclipse on trace. What I did was..
Just deleted the main method from the class. Saved it. Did undo to bring the main class back. Tada... Issue resolved...
Just one think would like to say, keep your eclipse in "Build Autometically" mode.

Move your file into a subdirectory called cont

Standard troubleshooting steps for Eclipse should include deleting and re-importing the project at some point, which when I have dealt with this error has worked.

I solved my issue by doing this:
cut the entire main (CTRL X) out of the class (just for a few seconds),
save the class file (CTRL S)
paste the main back exactly at the same place (CTRL V)
Strangely it started working again after that.

It is possible to have 2 groovy-xxx-all.jar files by excample in lib directory. which makes that an app is not running

I had the same problem, this is my solution:
I manually deleted the bin folder of the project
Then I refreshed the project which recompiled the whole project and created a new bin with all .class files
I did it because when I performed Clean(project->clean) my .class files were not getting deleted. the above solution works for me hope its useful to others.

I had the same problem.I solved with following command maven:
mvn eclipse:eclipse -Dwtpversion=2.0
PS: My project is WTP plugin

If you are using a pre-defined run configuration, go to classpath and try "Restore Default Entries". This will reconfigure the classpath for that configuration.

This worked for me finally :
RUN -> RUN CONFIGURATIONS -> DELETE THE RUN CONFIGURATION
CLOSE ECLIPSE
REOPEN ECLIPSE
CREATE RUN CONFIGURATION AGAIN.
Tadaaaa !! It works

Could not find or load main class Swing Java

Error: Could not find or load main class jewelleryerpapplication.GUI.ERPMainMenu
Java Result: 1
Actually i am using Net Beans 7.1 and in that i am continuosly getting the same error but un able to get the solution for that even after installing the latest update for that. The solution which i got from some body that create new project and copy the source and library foldes in that Project from the previous which generate the above error. this solution works for me but i need some simpler solution for this problem.Lot of thanks in advance.
Don't Forget To Give UpVote It If It Helps.

Right click on project node, go to Set configuration, select the main class for your application. Then clean and build.
Even if this doesn't solve your problem, then delete the Netbeans cache by deleting the (index) folder
User\.netbeans\6.9\var\cache\index\

I think that the following is happening:
If this is a third party application, you have not included some jar files on your class path;
Or, (which in my opinion is the most likely) you do not have a method which has this signature: public static void main(String[] args). This method defines the main entry point for your application, not having it might cause that error when you try and run the project. To my knowledge, Netbeans creates a main class with such a method automatically whenever you create a new project. This might be the reason why you are not having this exception when you create a new Project and throw everything in it.
You might be having an issue with Netbeans itself. You can start by either checkin gout this previous SO thread in which a similar (I think) issue is being discussed, or else, as a final resort, you might want to remove Netbeans and all its files, and install an earlier version. You can also try and see if you get the same issue if you use a different IDE such as Eclipse.

the solution is,right click your package in netbeans,go to properties,source now go to the jre option and choose your version,the programme will now run.