Develop Reference

Values in XML Attributes no longer accepted as valid after upgrading to Android Studio 3.0.1

After upgrading to Android Studio my styles.xml suddenly has lots of errors.
When trying to change the parent property Android Studio suggests 'TextAppearance.Widget.AppCompat.Toolbar' as a valid value when typing...
Also I had to prefix ?attr/... attributes with ?android:attr like below...
android:background="?android:attr/selectableItemBackground"
Note that my app will compile and run...
Are these bugs? (probably...)
If not; What is the reason for the change?
If so; What prefix is missing for my themes?
Thanks, Ove

It can be solved after Sync project with Gradle, Clean , Rebuild the Project. I think it is a bug of android studio 3.0.

I get that from time to time as well, I usually try the following:
Clean & Rebuild Project
Sync project with Gradle files
File -> Invalidate caches / Restart -> Invalidate and Restart
A combination of these in a random order usually solves it (I know, I know, really scientific method... but hey as long as it works :))

Intellij IDEA 14 - Package R does not exist

Well, I recognize that this has been asked before, but in browsing the other answers:
IntelliJ IDEA - Can't build anything, always get "package R does not exist"
Error: java: package R does not exist
etc.
none of the solutions work.
This is for a project that was building fine until at least the last couple days, so whatever is causing the problem must be new.
Symptoms:
Attempting to to a Build > Rebuild Project fails. I see the step for compiling generates sources go by in the status bar, but the next compilation step fails with the dreaded "java: package R does not exist."
R is found just fine in the IDE, so only compilation fails, suggesting there is a problem with the compilation of the generated sources. This seems borne out by the fact that in the out/production/[module] directory, only some resource files show up, no compiled classes, nothing in the "generated" sbupackage, a no apk file.
Interestingly, when I connect my phone to the computer and try running the application on the device, new changes are picked up, so something is being successfully built somewhere.
Things I've tried:
Per the other threads I've tried:
Doing a full rebuild.
Restarting IntelliJ (including with invalidating caches)
Restarting the computer.
Checking that the gen package is included under sources.
Checking for any Excludes (there are none)
Deleting the stub R file and forcing IntelliJ to regenerate it (which it does promptly)
Confirming that there is an Android facet on the module.
Double-checking that the manifest xml file has not changed and that the referenced activity is correct.
Checked that exclude output path is not checked (it's greyed as I'm inheriting from the project compile path, but I tried switching and manually unchecking it... no luck).
The only recent changes in xml were:
A new item in a menu file. I tried removing it to see if somehow that was an issue even though there were no errors reported, but the problem persisted.
I moved a View class to a subpackage of its prior package. That View class is referenced once in a layout xml and I confirmed that the xml was updated with the new package name.
So at this point I'm out of ideas. Sorry to rehash a somewhat common question.

I have the same problem and it definitely related to last update of the IDE. My version is IU-139.1117.1
Before the update Build -> 'Rebuild Project' was working ok in a windows and a mac installation of the IDE. I updated windows first and 'Rebuild Project' shows the error 'Package R does not exist' even for a new Android project. Same error apeared in mac installation imediately after the update. 'Make project', 'Make module' and 'Compile' commands on the Build menu are all working ok with no error only 'Rebuild project' raises the error.
For a new Gradle Android project everything works ok. It seems something is wrong with last update.

It is reported issue in IDEA 14.0.3 (http://youtrack.jetbrains.com/issue/IDEA-135234).
You can try to Compile Module (Ctrl-Shift-F9) for the module that contains missing R file. It works for me (but I have to do this every time i add/remove items in resources).

don't use Rebuild Project, use Make Project and refresh your project,maybe you should try it a few more times.

Try File | Invalidate Caches. This is solve my problem.

If you cannot wait for a officially stable build, install IDEA EAP 14.0.4 (IntelliJ IDEA 14.0.4 EAP build 139.1408, released on February 20th, 2015) from https://confluence.jetbrains.com/display/IDEADEV/IDEA+14+EAP
I've just solved all issues and rebuild project works again.

Solved with the 14.1 update of intelliJ IDEA, available now.

I had the same issue on a Windows 7 box today with a Java 7 Gradle project, using IntelliJ 14.1.5.
The problem was that somehow the classpath was "too long", but IntelliJ gave me only those strange "java: package foo does not exist" compile error's.
The usual tricks (see also above) did not help.
The workaround was to shorten the classpath a bit, in my case setting GRADLE_USER_HOME to c:/.gradle (was before c:/users/myusername/.gradle). After resyncing the gradle project everything worked fine.
The problem was therefor obviously IMO some kind of windows filesystem path length restriction.

Android Studio issue with R file

I was using Android studio now for a long time and it was fine. Recently, not sure what happened, but when I click on debug and have it perform make it complains about the R file and it seems like it doesn't generate it.
The only way it works is if I click on Build > Rebuild Project...problem is that it takes a long time for that to go through.
Is there anything I am missing? I updated the Android SDK tools, etc. as per some of the other suggestions?

Yes this is a known issue in the newest version of android studio. Google has suggested a temporary workaround until they can track down the problem and fix in next release. Turn off external builds in preferences > compiler. It might help.
Source: cannot resolve R into variable after Android studio update and R.java not updated after update to 0.1.6 .
Hope this helps.

Solution for android studio Users :
Goto : Menu tab - > Build - > Rebuild Project
after rebuild Project.R.Java file will be solve automatically ..

I was having the same problem, but it looks like a new update came out today. It seems to have solved the problem.

Try to import android.R, for every class you need R (of the package)
and then compile, it makes different error from before.
And remove all ('import android.R') , and rebuild it.
This trick is not for solving problem in proper way,
but makes the tool refresh.

And remove all "import android.R" , and rebuild it and import them again its working for me

Android studio suddenly cannot resolve symbol R

I was having a project open in Android studio. It was generated by the wizard and working fine.
I did some small changes to activity_main.xml and when I changed back to MainActivity.java i get the error in several places that it cannot resolve R. I might have done something to cause this. But I´m not sure what since it appered when I edited the xml.
Does anyone know what might be the solution to this?
I can find the R.java in r/release/packege/com/ and it looks fine.

Most of the time it is due to a bad XML file. XML files can be layout files, value files, or the Manifest file. Please check your xml files and try to rebuild the project. Sometimes cleaning the project and rebuilding it also works.

In addition, make sure you do not have a drawable with an invalid name. I had a drawable with a numeric filename and that didn't sit well with Android so it failed to compile R.java.

Downgrade Your Gradle Plugin Version
No amount of cleaning, rebuilding and restarting would do the trick for me.
The only thing that did the trick was downgrade our Gradle version from 3.4.0-alpha02 to 3.2.1.
So, instead of:
dependencies {
classpath 'com.android.tools.build:gradle:3.4.0-alpha02'
}
We used:
dependencies {
classpath 'com.android.tools.build:gradle:3.2.1'
}
After making that change and then doing a Gradle sync, everything worked.
Not sure if it's related to the alpha release or to that version or if changing your Gradle version at all just forces a refresh that otherwise cannot be done with the other commands, but there ya go.

Make sure in your AndroidManifest.xml the package name is correct. That fixed the problem for me when my R.whatever was marked red!

Try to clean the project by doing the following:
If you are using Windows, open CMD, cd to your project directory and run gradlew clean.

I just had this problem and the reason was that after editing the layout android studio automatically added an import to the source files:
import android.R
Of course the main R class has not all the symbols in your layouts, which made everything a red mess.
Removing that line solved for me.
EDIT:
Seems like eclipse had the same problem early on.

I sometimes get this after updating Android Studio. Not always straight away but after a while (perhaps after a restart of the program).
Here is what has worked for me: Tools - Android - Sync project with Gradle files.
For some reason just clicking the "Sync now" link that shows up after changing the build.gradle file has not done any change for me. Even though the messages says approximately the same thing.

I had the same problem because in my strings.xml I forgot to escape a apostrophe. Use " \' " never " ' " in xml file!

I had that same problem crop up, too (most recently while I was at lunch--came back and WTF?). And since this was a simple Hello, World program, there wasn't many places these errors could hide. The XML files looked fine--I hadn't even touched the manifest. A clean did nothing.
So I commented out a section of my layout.xml to try to narrow down the problem. Sure enough, the errors went away.
And then I simply UNcommented out exactly what I had commented...and everything was hunky-dory. Strange! Perhaps there was some lingering issues that needed to be touched in order to cause a rebuild of the layout file. You'd think basic stuff like that wouldn't happen these days, but they do.
One more thing! It's possible that something may happen to Java as well. It's very hard to know the status of the Java run-time environment; just easier to start again. So here's one more thing to try: restart your computer, restart Android Studio, AND THEN Invalidate Caches and Restart. Yep, the whole-hog.

When all else fails restarting android sometimes helps. During the start files are loaded afresh and this enables detection of the R

This problem occurs when you rename the package name.
After renaming the package name, you must change the older package
name to a newer one.
For me it solved the problem once I changed that.
After that, you might get errors in xml files, that you have to change
the package name there also.
That's it.

I just had the R problem and after a few hours found that the problem was my regional spelling as used in one of my xml files. The problem word was behaviour, which is spelled behavior by Americans.
I am Australian and we are taught ye olde English way of spelling certain words and so I looked past it a number of times before the penny dropped.
Its a small thing, but it can easily sneak up and bite you on the nether regions and is hard to spot and can waste a lot of time.

In my case I had just added some new image files to the drawable folder and accidentally put a GIF in there with my PNGs. I had not incorporated them into the project at all yet. I removed the GIF and then did a clean and rebuild. That fixed it for me.

I had this exact problem when importing a Udacity Example Program. The Message Pane in Android Studio said the error was failing to load a .PNG in a sub-directory deep in the Project Files.
Turns out the PNG with all of the sub-directory path names was greater than the name length limit defined by Windows 7 (260 characters). I gave the Project a shorter name - to free up some space - and then Android Studio happily compiled.

After you tried make project, clean project, rebuild project but still doesn't work.
Try restart Android studio

R.java malfunctioning

I created a new xml file, and had an errant ? in it that prevented R.java from regenerating. I tried Cleaning the Project, and Fixing the Project Properties but no luck.
Then I realized the XML was creating the R.java from recreating itself, so I deleted the XML file and the R.java was back.
Now though, I am getting an error on all the calls to R.. calls saying that :
cannot be resolved or is not a field
So, for instance I have
setContentView(R.layout.detectlayout);
ERROR: detectlayout cannot be resolved or is not a field
for all of my calls to 'R.'. Any ideas?
I have tried all of the suggestions on SO already to no luck =/

Look at the 'import' section of your code. Since you deleted your original R, chances is Eclipse help you to fill in the R as com.android.R instead of com.yourproject.R
I also sometimes have problem in Eclipse Resource stuff, sometimes I found turning off and on the "auto buiild" function may help, or simply restarting Eclipse may sometimes help. It's kind of buggy under Mac OS.

This can happen if .R is imported. Eclipse will automatically add it sometimes when you have difficulty with with R.java

first don't bother deleting the R file it not going to make things better only worst lol
as you said you some times need to clean the project
when you start modifying resources its good to select the root of your project and do a alt+shift+o to reload all ressources
then f5 to refresh the tree
then clean the project
you also have to check that eclipse is set to build automatically (Project->build automatically).