AndroidStudio doesn't recognize R - android-studio

Question

I am teaching android app programming at a highschool. I found Eclipse to fragile so we are now trying out Android Studio. But a couple of students get that Android Studio with R. in red font, showing that it is not recognized.
This happened for example when the student copied the contents of my files.

Answer 1

It sometimes happens when you open a proyect from other sources. Just Synchronize it or press Ctrl-Alt-Y.

Answer 2

R.java file issues : 1) check the naming convention of images in drawable folder with size.
2) check the style of your application
In your case it may be missing style .. So upadate the style in value folder as per application .

Answer 3

The most common reason for this error is bad XML coding. For example using android:src="#drawable#drawable/picture" in ImageView will give you R error as it will not let R.Java file to build. Its hard to trace such errors as XML file will not show any error.
Also take a look if your resources are of good quality!
Hope it helps! Cheers

Answer 4

Build -> Clean Project
Tools -> Android -> Sync Project with Gradle Files
and all OK!

Answer 5

Try adding the following:
android.enableAapt2=false
to gradle.properties and Refresh:
You can also do the following: Clean Project and Sync Project with Gradle Files.

Answer 6

try importing your project :
To migrate existing Android projects, simply import them using Android Studio:
In Android Studio, close any projects currently open. You should see the Welcome to Android Studio window.
Click Import Non-Android Studio project.
Locate the project you exported from Eclipse, expand it, select the build.gradle file and click OK.
In the following dialog, leave Use gradle wrapper selected and click OK. (You do not need to specify the Gradle home.)
Android Studio properly updates the project structure and creates the appropriate Gradle build file.

Answer 7

First if the existing project was developed in eclipse you need to import the project and then check what are the error. If your students are coping codes from your project then after coping the codes they need to clean and sync the project. If still your problem not solved try this in android studio :-
File/invalidate caches/restart.
Hope it helps you :)

Answer 8

Especially, for regeneration of R.Java file.
If you have tried all these options:
Clean Project
Rebuild Project
Invalidate Caches / Restart
deleted .gradle folder
deleted .idea folder
deleted app/build/generated folder
checked your xml files
checked your drawables and strings
and still face problem then check your classpath dependency in your Project Gradle Scripts and if it's, this:
classpath 'com.android.tools.build:gradle:3.3.2'
then downgrade it to, this:
classpath 'com.android.tools.build:gradle:3.2.1'
clean and rebuild it will sync your project.

Answer 9

I had the same issue, and the lint gave me a hand...
If you're using lint, it can help too.
In gradle.build(Module:app) add the lintOptions sub-element in android element as shown below, then lint will fix it for you!!.
android {
lintOptions {
abortOnError false
}
}
I hope it helps!!