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archive for required library could not be read or is not a valid ZIP file

Before coming to the problem let me explain what I did that has landed me in the problem.
I created an account on github and made a repository named Android.
Then I installed github client in my windows 7.
Then I opened this client, provided my authentication and cloned the repository to a local directory C:\Users\Aniket\Documents\GitHub\Android(This folder has the .git folder in it).
Then I went to my Eclipse ADT and installed EGit plugin as described here. Also I
Then in Eclipse I right click on my project TicTacToe go to Team->Share Project and provide my repository path i.e C:\Users\Aniket\Documents\GitHub\Android.
My Project was added to the local repository and in my github client it shows me all option to commit file in the actual repository on the github site.
But my project is suddenly showing error with a red '!' sign on it.
Description --> Archive for required library: 'C:/Users/Aniket/AndroidWorkspace/TicTacToe
/libs/android-support-v4.jar' in project 'TicTacToe' cannot be read or is not a valid ZIP file
Resource --> TicTacToe
Path Location --> Build path
Type --> Build Path Problem
Note : the Error was a single line displayed in error console on Eclipse. I just split it up for readability.
Even after detaching repository it shows that error.
Has anyone encountered this scenario before. What is the solution or workaround? I googled and first few links suggest it is an Eclipse bug. Please suggest what can be done to bring my project back to executable state?

It is an Eclipse bug. I have faced the similar problem several times. closing and reopening the project works sometime. if it doesn't work try restarting Eclipse.

I've seen the same issue. I removed that jar file, and then rightclick on the project, select maven, and do "update project...". The jar was downloaded again, and the problem was gone.

There is other case to display error Archive for required library
Right click on project and open --> project project properties --> java build path --> Android private libs
if there are two jar files with same name then remove one from libs.
jar file may the hidden some times then
open you libs folder in window and check if any hidden files are exist
Organize --> folders and search options --> view --> check show hidden files
and Delete the hidden jar file , The same cass delete if hidden java fies exist in src and packages if any

Looks like everyone has different story to tell! For me, I had to delete the error from eclipse Markers tab, and then cleaned the project again. Before that, I closed and re-opened the project/eclipse several times as suggested by #rachit, which did not work for me.

In my case, i'd downloaded project from internet, after unzipping project some files had been blocked, just unblock files and try again, then problem was solved.

In my case I tried all the tips suggested but the error remained. I solved changing version with a more recent one and writing that in the pom.xml. After this everything is now ok.

In my case restarting Eclipse did not solve the problem. I restarted the computer and did a Project -> Clean in eclipse

A solution that worked for me:
go to *.classpath and delete the line :
<classpathentry kind="lib" path="the_problematic_class.java"/>

Could be due to corrupted jar files as well. Better to check that first as that was the reason in my case:
jar tf myjar.jar
should list the content inside.

i was facing same problem in Eclipse Mars. I downloaded the jars from [https://oss.jfrog.org/webapp/#/artifacts/browse/tree/General/repo/org/ethereum/ethereumj-core/1.0.0-SNAPSHOT][1]
and replaced it in the directory.
I got error while adding ethereum.jar using pom.xml at C:\Users{machineName}.m2\repository\org\ethereum\ethereumj-core\1.0.0-SNAPSHOT\ethereum-core-1.0.0-SNAPSHOT.jar , i added the downloaded jar(ethereum-core-1.0.0-SNAPSHOT.jar) here and the problem was solved.

I will say that it can be that some answers work for some cases, but for me it was necessary to go an extra mile. So I will try to make a summary of what can be done:
Verify that the jars are intact:
jar tf myjar.jar
Restart eclipse and update projects setting over right click on project -> Maven -> Update project
The option which has work for me was to navigate in the workspace folder and then delete the files:
.metadata/.plugins/org.eclipse.jdt.core/invalidArchivesCache
.metadata/.plugins/org.eclipse.jdt.core/nonChainingJarsCache
After that restart eclipse and rebuild project.

I found no duplicates anywhere and tried restarting eclipse, rebuild project. I was getting error for only one libary. So I just tried to rename the jar file from ksoap2-android-assembly-2.5.8-jar-with-dependencies.jar to ksoap2-2.5.8.jar
So if none of the problems solves from above you can also try this one

In my case, the java installation was not proper. So, I uninstalled Java and reinstalled a new version. Now, it works.

For us, the problem seemed to be the size of the .jar file. I would recommend doing the following test to see whether this is the case or at least rule it out. First have a look at other jars in your Eclipse project and compare them to the problemetic jar. Is it a lot bigger than the others? If so, try the following workaround. Else, this answer probably won't help you. Before starting, configure Windows to treat .jar files as .zip files with the following command line command:
assoc .jar=CompressedFolder (see https://superuser.com/questions/121540/can-you-configure-windows-to-open-jar-files-like-zip-files-without-a-3rd-party-t)
Optional: A simple Test
Before trying the workaround, here is a test to see if it is indeed the jar file size that's tripping you up.
Create a large .jar full of random .class files that aren't in your problematic jar. You can do this by making a folder full of files, zipping it, and renaming the .zip extension to a .jar. Windows might warn you about changing the extension. Ignore this. Make sure you have enough files in the folder so that it's a bit larger than the problematic jar.
Try to import this artificially created jar. If it fails with the same error, then size is probably the issue.
The Workaround
If it is indeed size that's the problem, which you might have identified by doing the optional simple test above, you can try the following workaround.
Decompress your .jar file. You should be able to do this easily in Windows
Split the folder you get from step 1 into smaller sub-folders. It could be just two folders, or more, depending on how big the original is. You want the folders to be smaller than some of your existing "good" jars so that size won't be an issue. Make sure to keep the package structure intact when you're doing this.
Zip up all the folders you created in step 2, and rename the .zip to .jar.
Import all of the .jars from step 3 individually.
The steps might be a bit unclear, so here's an example. Imagine your jar file was called mylib.jar. You unzip this to a folder called mylib. Inside, let's say there are three sub-folders called package1, package2 and package3. Create 3 folders called mylib1, mylib2 and mylib3, and put in package1, package2 and package3 respectively. Then zip these up and rename extension to .jar. You'll then be importing mylib1.jar, mylib2.jar and mylib3.jar.

For Java web application, web.xml added as jar in the project. In project explorer, try to find web.xml as in jar icon and remove from the project. Error will go away!!

I was stacked on this problem for days despite searching for a solution on the net. I first moved my project to Netbeans and all worked fine. I then returned to eclipse->properties->java compile-> Building and changed "Incompartible build path" option from error to warning and then did maven update and it worked.

I realize this is an old question, but I have yet another solution if none of these work for other people coming across this thread.
I had this same problem, and spent alot of time trying to figure it out, only to find that my Eclipse classpath/buildpath has a .json file in it, it doesn't like that, so it wouldn't compile.
So, check to make sure no text files or json files, etc are in your build path.

I had accidentally put the web.xml into the build path :x

R.java was modified manually! Reverting to generated version

I added one xml file in layout. After that this error is coming. But the problem is I haven't touched R.java file.
Now in my new activity i m going to set its content view as my newly created xml file, but when i m doing R.layout., the newly created xml is not coming in suggestion. Do anybody have any idea how to correct this.
I have cleaned my project several times but it is not working.

Delete R.java and let eclipse regenerate it, then project --> clean will fix it.

Check if you have any existing errors in your code, if yes correct it and then try refreshing and building your Project.If your Project is properly build,you will get the new xml file in suggestion

It happened to one of my Eclipse projects today. This is what fixed it mysteriously. Don't know why or how.
Changed Properties->Android->Project Build Target to a higher (different) version of Android and hit the Apply button. Then did a clean build. The problem went away.
After the above steps, I could even go back to the original Project Build Target without the problem re-appearing.

May this Help you...
I think your xml file name might be having some capital letters or some special characters..
Delete them and then try again to run the project..

Try cleaning the project this will do the work.

make sure your not importing any thing that ends with com.R
make sure your naming convention of the layout is correct

R.java file have all the resources IDs provided to resources(drawables, layouts, styles etc) either by Programmer, or generated through sdk. So check your xml files as well as your drawable folders. If any of them including wrong format it will show an error in R.java file like sometime R.java is not generating or our class file shows R.id.xxx is not found or R.class is not defined error.Then clean your project.

There must be some mistake in one of your xml files. Eclipse does not always show error messages if there is something wrong with your xml. Mostly code gets errors, in this case 'R', so you will need to go through your xml manually, it could just be a simple mistake. Good luck

I had similar problem. Cleaning project didn't solve the issue and there was no error in the code or the uppercase or spaces in file names.
The solution was going to a popup window which came when you choose PROJECT-->CLEAN and then UNcheck the "Start a build immediately" option. Clean the project. Close and reopen Eclipse and then build the project (PROJECT-->BUILD).
The question remains why simple clean and build without closing Eclipse didn't worked.

Right Click on Project->Bulid Path->Configure Build Path
ther you see error android-support-v4.jar remove this library and add library again this works fine for me.
Else
Try
Go to gen Folder delete R.java and from Project option choose Clean Project.

Very late on the scene...
I had this line mysteriously appear at the start of my imports:
import android.R;
I deleted it and shazam error was gone.

I had similar error, but when I import new project to my eclipse. and the solution was, I should check the option (copy project to work space) specially when our project has library.

R.java disappeared

I recently downloaded the Android Support Package and have been using it trying to get my application, running at SDK 10, to use a GridLayout.
Anyway, I cleaned my project a few times working on this error in my XML files, and then when I went back to my classes in /src, every single reference to R.* was an error.
This has happened to me before, but it usually fixes itself, and it has not yet. Does anyone know what to do?
The R.java file is no longer in my /gen folder.

It sometimes happens when you import another project First i suggest you to build it, it will create automatically .
If it's not working then try to create new project from existing source and specify the downloaded project . It worked for me
Hope this works for you let me know..

Sometimes if there is an error in your XML files R.java won't build. Go through your XML files to make sure that everything is in order (typo, forgetting to close a bracket, etc). When you fix that it should build itself again.

You can also try Adnroid Tools>Fix Project. That may help you to create gen/R.java file

Follow following steps :
In Eclipse,
=> Right Click on Project > Android Tools > Fix Project
By this way, you can fix many common problems which can help generate R.java in gen/R.java file.

There can be several reasons why this occurs. The following I have found happen to me and I have included the steps I took to resolve them.
First step is to try and clean the project (from the Project menu option I believe)
Check there are no errors with your XML files. Sometimes this is obvious from the red 'x' that appears next to an XML filename in the Project exlporer window and sometimes it is not. If it is not obvious check the 'Errors' window (whic is founf in the 'Windows' Menu option
Failing that I have found that by disabling the automatic build option, adding some arbitrary comment to an arbitrary source file, saving the source file, clean the project, remove the comment and then re-selcting automatic build can work too.
Apologies for the approximate locations for some of the Eclipse functions but I am not currently in front of Eclipse to check.

This happens when you have an error in the res folder.
Fix the error first, then do :
Project --> Clean --> Ok
Et voila !

Developing for Android in Eclipse: R.java not regenerating

I've found out that my R.java is never updated, so it doesn't contain information about my new resources, so I decided to delete it and thought that Eclipse would generate a new one. But that didn't happen, and I don't have R.java now. How can I regenerate one?
I'm using Windows 7.
From one of the comments: "Doing Project -> Clean is what caused the problem for me. Cleaning deletes R.java...and for whatever reason the plugin is not regenerating the file."

I found this happening to me with a broken layout and everything blows up. Relax, it's like that old mistake when you first learned programming C where you forget one semicolon and it generates a hundred errors. Many panic, press all the buttons, and makes things worse.
Solution
Make sure that anything the R. links to is not broken. Fix all errors in your XML files. If anything in the ADKs are broken, R will not regenerate.
If you somehow hit something and created import android.R in your activity, remove it.
Run Project -> Clean. This will delete and regenerate R and BuildConfig.
Make sure Project -> Build Automatically is ticked. If not, build it manually via Menu -> Project -> Build Project .
Wait a few seconds for the errors to disappear.
If it doesn't work, delete everything inside the /gen/ folder
If it still doesn't work, try right-clicking your project -> Android Tools -> Fix Project Properties.
Check your *.properties files (in the root folder of your app folder) and make sure that the links in there are not broken.
Right-click your project > properties > Android. Look at the Project Build Target and Library sections on the right side of the page. Your Build Target should match the target in your AndroidManifest.xml. So if it's set to target 17 in AndroidManifest, make sure that the Target Name is Android 4.2. If your Library has an X under the reference, remove and re-add the library until there's a green tick. This might happen if you've moved a few files and folders around.
What to do if R doesn't regenerate
This usually happens when you have a broken XML file.
Check errors inside your XML files, mainly within the /res/ folder
Common places are /layout/ and /values/, especially if you've changed one of them recently
Check AndroidManifest.xml. I find that often I change a string and forget to change the string name from AndroidManifest.xml.
Check that Android SDK Build-tools is installed. Window -> Android SDK Manager -> Tools -> Android SDK Build-tools
Make sure when you update the Android SDK Tools, you also update the Android SDK Platform-tools and Android ADK Build-tools. Build fails silently if they don't match.
If you can't find the issue, right click /gen/ -> Restore from local history... -> tick R.java -> click Restore. Even if it doesn't solve the problem, it will clear out the extra errors to make the problem easier to find.

This site suggests:
if you run a clean on the project it should regenerate all the generated Java files, namely R.
...and...
In Eclipse, under the Project menu, is an option build automatically.
That would help you build the R.java file everytime modifications are
made. The Clean... option is also there under Project.
This site suggests another solution.

I had the same issue. When I checked it out I found that the name of the XML resource under layout was not having the correct naming convention. It had some capital letters. So I renamed it to make all letters lowercase and the magic worked.

One reason the R.class can go missing suddenly is when there are errors in you XML files. For instance, when you add an XML file with uppercase letters in the name like myCoolLayout.xml which is not allowed. Or when you have references that don't point to existing files, etc.

In Eclipse, simply use Project --> clean to clean the project. The
R.java is going to be automaticly (re)-created.
If for some reason that dosn't work: Make sure your layout.xml files don't contains errors. Eclipse seems to be a bit buggy here: sometimes it doesn't mark the errors within the XML nor the package explorer. In such a case: Take a look at the "console" or "problems" view after using "clean". All errors should be displayed there. Fix them and redo a clean.
NOTE: It is NOT neccessary to fix the errors you get because of a missing R file! Just fix the XML files and other project errors and use clean!

Quick fix:
The package name in the manifest needs to be the same as the one in the /src folder, the /gen folder package will be automatically reproduced.
Detailed observation:
Observe the name of package in the /gen folder. In my case it was different than the one in the /src folder.
The package referenced in the manifest was that of the /gen folder.
I attempted to add a package with the name of the /src folder to the /gen folder too see what would've happened, but this did not solve the issue. I proceeded then to remove the package name that was not the same as the package name of the /src folder. This package with the name that did not correspond to the /src folder was recreated as soon as I removed it.
Since the package reference in the manifest corresponded to the one that was being senselessly recreated and that did not correspond with the package in the /src folder, this prompted to rename package = "oldPackage" with the package = "srcFolderPackage".

I had the same issue, and I finally found the problem: In the strings.xml, I did not keep the line <string name="app_name">program-name</string> which is present by default when creating a new Android project. I thought it was only used for the program HelloWord, which is the original program.
However, the AndroidManifest.xml refers to this value, hence:
=> A compile error;
=> the R.java isn't updated;
=> ID are not recognized...

Android has added in SDK build tool ADT 22 for the building mechanism. You just need to do the following steps.
Update Android SDK Tool
Update Android SDK Platform Tool
Update Android SDK Build Tool
Add the path of your build tool to the path variable. (path up to---- YOUR DIRECTORY-PATH\android-sdk\build-tools)
This will solve the issue.

If your R.java isn't getting generated, one of the solutions is to delete the layout file named "blabla.out.xml".
After deleting this file, try cleaning the project from menu Project -> Clean.

As a generalization of Glaux's answer, if you have any errors in the res directory, then R.java may not generate - even if you clean and rebuild. Resolve those errors first.
As an example: when you add an image file of say, "myimage-2.jpg", the system will consider this an error, since file names are limited to alphanumeric values. Do a refresh on your 'res' directory after adding any files and watch the output in your console window for any file name warnings.

If your OS is Ubuntu, I can provide some suggestion:
Install or upgrade ia32-lib:
sudo apt-get upgrade ia32-libs
Check if you have the right permission on the aapt folder:
cd ANDROID/adt-bundle-linux-x86_64-20130522/sdk/build-tools/android-4.2.2
chmod 777 aapt
Start Eclipse:
sudo eclipse
Run Project -> Clean in Eclipse

You 100% have an error in an XML-file, but the XML verification does not show you the error. This is the reason why you need to check your XML files first!

It is ALWAYS helpful to take a look at the Problems Tab in Eclipse. In my case, I was getting a "android unable to resolve target 'android-8'" error message that kept the R.java from being generated. So, I corrected the imported target to the one I was using in the default.properties file, then I performed a clean via Projects->Clean and voila! R.java is automatically generated! Hope it helps!

Cleaning project, closing Eclipse, re-opening it and launching my project finally made my R.java to reappear... Hope it helps

Make sure you are not importing
android.R;

R.java will never be generated if there are any errors in the res folder. For example, in the drawable subfolder there are two files which have the same name, one is icon.png and the other is icon.html.
You can see some error in the Eclipse console log window which is saying "Resource entry icon is already defined.". After deleting icon.html, you can clean or just delete the gen folder. You will find that R.java is created.

I've found that any file that has capital letters in the res folder will create this error. This happened to me with a PNG file I added and forgot about.

After reading through many posts and YouTube videos, I found that each of us have R.java missing for different reasons.
Here's how I fixed this in Eclipse:
Create R.java in gen folder manually and save.
After that go to Project and click "Clean"
The following message will display and your file will automatically be rewritten:
R.java was modified manually! Reverting to generated version!

For me, I had linked v7 appcompat twice. Anyhow, in Eclipse, right click the project name in Package Explorer or Navigator, go to Properties, Android, and uncheck any duplicates in the Library section. You may need to Build --> Clean afterwards.

There's obviously no one final answer to this, but here's another one I don't see here already:
I've had R.java disappear after (attempting) a Team Synchronize, and there were conflict resolution files in the project (i.e., thingy.xml.mine,thingy.xml.r35, etc.). Deleting them/resolving the conflicts regenerated R.

I had this problem. Accidentally I deleted this
xmlns:tools="http://schemas.android.com/tools"
which started causing build errors all over the project in my XML files as well as my Java files. As soon as I retyped what I deleted, it worked again :)

OK so it's clear that there can be a lot of causes for this problem. If you're on a 64 bit linux machine and you are just setting up the ADT for the first time, you may get this problem where R is not automatically generating. Check the console tab and you may see an error similar to:
'No such file or directory' while attempting to get adb version from '/home/patrick/code/android-sdks/platform-tools/adb'
If that's the case you need to install ia32-libs, using something like:
sudo apt-get install ia32-libs
See here for details: Android adb not found

I've came across this problem a few times. I found that if I didn't import the package R through my application's name, for example, if my application had the package name example.test then I found that I had to import example.test.R in order to access any of the resources.
If this wasn't imported then the resources that where getting returned were the default resources with none of my own included.
With that said if you find that you are only getting a list of default resources then just check to make sure that you're importing application_package_name.R and not android.R.

My problem was that appcompat was not in the same directory as my project.
I found this out when compiling my app with Ant.

Also make sure that you have the latest version of the ADT plugin, and open the Android SDK manager to update all SDK tools to the latest version. Check for updates in Eclipse itself as well. Non-matching versions caused the problem for me.

My problem was inside a menu file.
The compiler doesn't seem to warn you if strings which do not exist inside strings.xml are referenced in menu files. Check if your items in your menu XML files reference any strings which do not exist in your strings.xml. Usually in android:title.

For me, this error was caused when I tried to duplicate an existing res/menu instead of creating one with the wizard.

All of these answers could not work if you use Maven. The solution for me was to add
<genDirectory>${project.basedir}/gen</genDirectory>
to the configuration section of android-maven-plugin.

I found a solution why R.class is not made by Eclipse after making it again - 2 clean, build, etc.
The problem is here in strings.xml:
<string name="hello">Hello World, HelloAutoComplete!</string>
<string name="app_name">HelloAutoComplete</string>
These are by default created by Eclipse when you create projects.
Definitely you are changing the strings.xml for your own requirement. Sometimes you clear the string.xmls these two lines from your code:
It is making a problem in the AndroidManifest.xml file:
<application android:icon="#drawable/icon" android:label="#string/app_name">
<activity android:name=".HelloAutoComplete" android:label="#string/app_name">
So it can't communicate with strings.xml.

I changed my layout XML file name and found out later that of the XML file (widget provider in this case) still refers to the old layout XML which doesn't exist, and that prevented the auto generation/correction of R class.

Android - How to regenerate R class? [duplicate]

This question already has answers here:
Closed 10 years ago.
Possible Duplicate:
Developing for Android in Eclipse: R.java not generating
I have imported a project into my Eclipse IDE, but it's giving me an error since R file is not generated automatically.
How can I edit the R file so that it matches my project requirements?

In another stackoverflow question, somebody answered this : you must get rid of warnings and errors (syntax and links) in your xml files. This plus doing a clean / build project.
Project -> Clean, and choose the option to build the project.

I found this happening to me with a broken layout and everything blows up. Relax, it's like that old mistake when you first learned programming where you forget one semicolon and it generates a hundred errors. Many panic, press all the buttons, and makes things worse.
Solution
Make sure that anything the R. links to is not broken. If it's broken, ADK may not regenerate R. Fix all errors in your XML files.
If you somehow hit something and created import android.R in your activity, remove it.
Run Project -> Clean. This will delete and regenerate R and BuildConfig.
Make sure Project -> Build Automatically is ticked.
Wait a few seconds for the errors to disappear.
If it doesn't work, delete everything inside the /gen/ folder
If it still doesn't work, try right-clicking your project -> Android Tools -> Fix Project Properties.
What to do if R doesn't regenerate
This usually happens when you have a broken xml file.
Check errors inside your XML files, mainly within the /res/ folder
Common places are /layout/ and /values/ especially if you've changed one of them recently
Check AndroidManifest.xml, I find that often I change a string, and forget to change the string name from AndroidManifest.xml.
If you can't find the issue. right click /gen/ -> Restore from local history... -> tick R.java -> click Restore. This doesn't solve the problem, but it will clear out the extra errors to make the problem easier to find.
I know there's already a lot of answers here, but this is the first link on Google, so I'm compiling all the advice here and hope it helps someone else new to this :)

Ok, I fixed it:
When I changed manifest to target 1.5 version, million errors appeared and only one wasn't related to inexistance of R class - in manifest file attributes "targetSdkVersion" and "maxSdkVersion" did not exist in sdk 1.5
Because of this R class was not able to generate.

I had a similar problem where the R.java file was not being regenerated after I'd added some icons and layout files. I tried cleaning and rebuilding and things got worse since then the original R.java file was gone and still wasn't regenerated. Eventually it dawned on me that there must be compiler errors for the compiler that generates R.java and we aren't being shown those in the console. Fortunately, I had noticed earlier that
it's an error if one .xml file refers to another which is non-existent and
it's an error to have upper case letters in your file names (why is that forbidden, I LOVE camel case).
Anyways, after renaming my icons and .xml files to all lowercase and then fixing all the references inside the .xml files, voila, R.java was regenerated. Just in time to feel good when I stopped for lunch :-)

To generate the R file again:
Modify something in layout->activity_main.xml
Save the file
Project->Clean...
Just delete the R file located in gen directory, and then try Project->Clean.
Then build your app again.

You can also right click on the project -> Android Tools -> Fix Project Properties. That should cause the R.java class to be re-generated.

I reopen eclipse, edit one of my xml files and then discover that my activy class was refering android.R instead of my package "R" :-)

My problem was in Manifiest file.
I have deleted launcher icons and did not reference the new ones on Manifiest :)

In order to regenerate the R.class, you will have to try multiple techniques.
Remove any R imports from your java source files: import R.android
Make sure to open the "Problems" panel by going to Window->Show view->Problems
If you do have some errors in your xml resources, you will need to correct those errors. In my case I had to change my id attributes to include the "+" sign as follows: android:id="#+id/grp_txt"
Now proceed to do Project->Clean
Finally do Project->Close, Project->Open
The "+" sign indicates a new resource id and the forces the aapt tool to create a new resource integer in the R.java class.

If you use Intellij IDEA you can click Build - Rebuild Project

If you accidently deleted your R in eclipse by "project clean" (don´t do this)
Rightclick on your Projectfile -> Restore from local History...
recreate your R
My Case:
Broken xml caused bug in R. (because of ä, ö, ü, letters in id)
clean project deleted my R.
What I Did:
recreated R from local History
checked for wrong R imports in my classes (like import android.R) and removed them
fixed that xml file and saved its content in an external txt file
deleted the xml completely from my project.
ran my project one time
put that fixed xml back at its place
and...
gave my girlfriend a big hug without a reason after freaking out like a Hulk.

My problem was errors in my XML files. Errors aren't always automatically detected in the Android Common XML Editor or Android Common Manifest Editor, so try using the regular text editor instead. For example, somehow some random gibberish from a .java file had been inserted into my manifest and strings.xml files.
Right click on any XML files (most likely in the /res folder or AndroidManifest.xml) and click "Open With," then select "Text editor." Make sure everything looks good and then save. Also, make sure to enable "Build Automatically" for your project. I had deleted my R.java file and it was rebuilt from scratch, so no need to restore it from local history.

Try to add a new "Android XML file" to, for example, the /res/layout folder. This might cause the plugin to to regenerate the R class.

You can just modify any xml files in /res folder and even just add a space and save, it will be regenerated.

I had the same problem - "R" missing. And none of the advices given here helped. Fortunately, I've found the reason - somehow Eclipse got lost the Project-Properties-target. It was unset. I don't know till now, what had done it, but setting it back helped.
So, attention - missing R can mean the missing target, too!

I have tried all the solutions above. But they unfortunately it did not solve my problem.
My R.java was gone as soon as I pasted a new picture to my drawable folder.
After reading the answers for this question I tried to delete the icon I have previously pasted (while Build Automatically Enabled) and everything was fine afetr. Hope this will help someone!

I had that problem and I just right-clicked on packet of my project and Build Project. Instantly the class R appeared in packet gen.

In case someone else finds this thread when Googling, like I did:
I had what appeared to be the same problem for a while, and after some digging I noticed that the R.java file got correctly built, but was ignored for only one of my source files. The problem for this particular file was that Eclipse had "helpfully" added the following line at the top:
import android.R;
That effectively hides the project R stuff. Just remove it and everything will be fine again...

You can use Eclipse's "Restore from local history" to restore your old R file if it has been deleted. After that you possibly see what keeps Eclipse from building your files (I've seen other errors than before which really helped fixing the problem).

May be you can go to project properties and select Android option and then define the target for the project. Once you apply the selected target API for this project, it will automatically generate R.java.

Mostly a problem with an invalid layout file.
Open the 'Problems' view and look for erros in XML layout files. Fix them and the R class will be recreated.
In my case there wasn't a real error. The problem view showed an error in a xml layout. The editor showed the xml error too. Just reformatting the xml using the auto formatter with 'ctrl-f' 'fixed' the xml 'error' and the R class was recreated.

You can just delete the R.java file from your project and it will regenerate assuming you have build automatically enabled. If you do not have build automatically enabled simply goto project->build all

For me, it was a String value Resource not being defined … after I added it by hand to Strings.xml, the R class was automagically regenerated!

This problem normally happens when importing from an existing project. Try to fix all errors from all .xml files in res folder, as well as Manifest file. Most likely the errors are shown at "match_parent" parameter, change it to "fill_parent". When all errors are clear, R.java will be appeared.
If there is no error on any .xml file. Try edit any xml file (like delete a letter and type it back) and then save the file, this will cause to generate the R.java.

I have a problem that the findViewById(R.id.edit_text) but return a TextView not a EditText. I think the problem is the R.id class has wrong id number, then, i clean the project then its works fine.

In my case it was an improperly named XML file that caused my R file not to rebuild. This did not show up in the package explorer, but it was shown in the LogCat. Look out for any warnings there about your res files. No matter how many times you clean your project, the R file will not rebuild until those errors are taken care of.