Develop Reference

'Could not find or load main class' in Eclipse. Again and again

I'm asking this question even though it has already been asked many times because I didn't find a solution after hours of searching.
I'm using Eclipse to learn Java. Last week everything worked properly. I could run whatever I wrote. Then today, I opened Eclipse, wrote some lines and when I tried to run it, I got an error:
Error: Could not find or load main class test.ArrayTest
Caused by: java.lang.ClassNotFoundException: test.ArrayTest
But that's not all : every project in my workspace is having the same issue. I can't run anything. At first I thought it was because I made a mistake at Eclipse start, so I closed it and re-opened it, but I was in the correct workspace.
I've tried to clean/rebuild it, doesn't work.
I've tried to mess with Properties > Java Build Path, doesn't work.
I've tried to create a new project with a simple 'Hello World', doesn't work.
package Hello;
public class Main
{
public static void main(String[] args)
{
System.out.println("Hello World");
}
}
What I have in package explorer.
Console display.
The thing that suprises me the most is that I don't think I've done anything that can mess with Eclipse between the moment it was working and now.
Does someone have another idea ?

I think I found the root of the problem !
The path to my workspace was containing a letter with an accent ('Bibliothèque' by default with Windows 7+ in french...), so I tried to create a new workspace outside of it, and it worked !
I'm not a 100% sure it was really it, but since it worked, it's worth sharing.

Probably you have disconected JDK in project.
Try open conextual menu on project (right mouse button)
properties/Java Build Path/Libraries
check is correct JDK or remove
then: [Add Library] / JRE System Library/ Alternate JRE/ [Installed JREs] / [Add] / Standard VM / [directory]
select path to your JDK (not JRE) and confirm, select checkbox your new jdk and [Apply and close]
on select list choose your new jdk and [Finish] and [Apply and Close]
should works. I have sometimes this problems. It's Eclipse

Exception in thread "main" java.net.UnknownHostException: services.gradle.org

I am getting the below error when executing gradle.bat. I have checked the URL path and it looks fine. The URL is defined like this in the wrapper properties:
distributionUrl=https\://services.gradle.org/distributions/gradle-2.1-bin.zip
Here's the error message:
Downloading https://services.gradle.org/distributions/gradle-2.1-bin.zip
Exception in thread "main" java.net.UnknownHostException: services.gradle.org at java.net.PlainSocketImpl.connect(PlainSocketImpl.java:195)
Could someone help me on this?

Go to "C:\Users\[yourusername]\.gradle\" folder (hidden folder - Go to folder option and enable show hidden files if folder is not showing up)
Look for file "gradle.properties" if DOES NOT (most likely this would be the case) exists, Create it under "C:\Users\[yourusername]\.gradle\
Copy and paste following code (More detail ... https://docs.gradle.org/current/userguide/build_environment.html#sec:gradle_configuration_properties ... look for "20.3 Accessing the web via proxy" for https and user/pwd variables)
systemProp.http.proxyHost=[PROXY SERVER]
systemProp.http.proxyPort=[PROXY PORT]
save and try again

look for gradle.properties file open it and change from 'distributionUrl=https\' to 'distributionUrl=http\' that will fix it.

I was facing the same problem when I was connected to WiFi with a download speed of 80 Mbps which is good enough, I guess. After spending hours while still being clueless, I just changed the network. Connected to hotspot from my mobile data with a much lower download speed of around 4 Mbps. The error vanished! Maybe something was wrong with the WiFi.

1) Go to your project directory
2) Find below file on your project directory:
gradlew
gradlew.bat
3) Edit gradlew file and change DEFAULT_JVM_OPTS variable value as below:
DEFAULT_JVM_OPTS="-Dhttp.proxyHost=XXXX -Dhttp.proxyPort=XXXX -Dhttp.proxyUser=XXXX -Dhttp.proxyPassword=XXXX -Dhttps.proxyHost=XXXX -Dhttps.proxyPort=XXXX -Dhttps.proxyUser=XXXX-Dhttps.proxyPassword=XXXX"
save it.
4) Edit gradlew.bat file and change DEFAULT_JVM_OPTS variable value as below:
set DEFAULT_JVM_OPTS=-Dhttp.proxyHost=XXXX -Dhttp.proxyPort=XXXX -Dhttp.proxyUser=XXXX -Dhttp.proxyPassword=XXXX -Dhttps.proxyHost=XXXX -Dhttps.proxyPort=XXXX -Dhttps.proxyUser=XXXX-Dhttps.proxyPassword=XXXX
save it.
5) Now run your "gradlew clean build" and check.
Please set your appropriate value on the place of XXXX
Note: gradlew is use for linux system user and gradlew.bat is user for window system user

Make sure you have enough bandwidth for the download.
I had the same problem just 15 minutes ago (that's why I'm on this page in the first place). Same error message --- and then I noticed that my uTorrent client was occupying the whole datalink. So I killed it, and run gradlew.bat again. This time everything went just fine...

I had similar issue, fixed by following steps:
Created: C:\Users\<username>\.gradle\gradle.properties
Values entered in the property file as below:
systemProp.https.proxyHost="company proxy server"
systemProp.https.proxyPort="company proxy port "
systemProp.https.proxyUser="user login "
systemProp.https.proxyPassword="password "
systemProp.https.nonProxyHosts=localhost
You can try HTTP instead of HTTPS.

I had this problem for a flutter project and what I did was create a new flutter project with the same name as my problem flutter project. I then copied the files from the "android" directory from the new project into the existing flutter project. Basically some set up file was messed up. I don't know which one, but this did the trick

Maybe it's a bit late but you might have to try to UPDATE your ANDROID STUDIO. Had a similar problem, working on Linux pop_os (Ubuntu derivative). So I had Android SDK on my $PATH and everything set well but Ionic v3 build was having a Gradle issues while trying to download.
I simply updated my Android Studio and tried again and it worked. Good luck

Go to your command prompt and set the HTTP and HTTPS proxy
Execute the below in your cmd -
set HTTP_PROXY=http://<proxyservername>:<port>
set HTTPS_PROXY=http://<proxyservername>:<port>?
Once you have set your proxy variables , try rebuilding or re-assembling your application / project

I've got an issue with Android studio. The value of the distributionUrl field in the graddle-wrapper.properties has been changed during package name refactoring.

If error is under flutter
Just hoping this might help someone in the future...
When i encountered that error in flutter, I just had to change network from the current one which seemed unstable, am sure build.gradle was trying to get more resources from internet.
try disconnecting and reconnecting to the network
try changing networks(wifi- date or data - wifi)

In my case disconnecting from my office VPN fixed the problem

Just remove your .android and .gradle folder from C:\Users\[Your user name]\
Uninstall android studio and reinstall it
Open any simple android studio project and run it. It will automatically set up the Gradle and other things
Close the project and run your flutter project. It works for me!!!

Jenkins SVN plugin failed when removing files (Checksum mismatch)

We encounter a strange error with Jenkin today. After investigating, we find that it's due to the fact that we delete some files in SVN, which cause a problem in Jenkin SVN plugin.
As I investigate, it seems this bug was known but it hasn't been fixed yet. So that upgrading to a newer SVN plugin version is not a solution(we are using Jenkin 1.474).
Our temporary fix is that "always checkout before building". But this is clearly very slow and takes much time for big projects. So that I'm looking for a way to (at least) detect the SVN problem in post-build scripts when it happens, and probably send an email to notify the developers.
Does anyone have any experience with this? I'm not too familiar with Jenkin, hence any help or pointer would be greatly appreciated.
Here's the error log:
Building in workspace /workspace-directory/workspace
Cleaning up /var/lib/jenkins/jobs/aaa/workspace/.
Deleting /var/lib/jenkins/jobs/aaa/workspace/logs
Deleting /var/lib/jenkins/jobs/aaa/workspace/target
Updating svn://address/trunk#HEAD
U src/main/some_file.java
D src/main/another_file.java
U src/main/other_files.java
ERROR: Failed to update svn://address/trunk#HEAD
org.tmatesoft.svn.core.SVNException: svn: E155017: Checksum mismatch while updating '/var/lib/jenkins/jobs/aaa/workspace/src/main/address/.svn/text-base/StudioSignUpController.java.svn-base'; expected: '39fc987bbeb8cd332e6b94abfb934720', actual: 'e9fa300ee28a2b1e15b2273f4b14ae18'
at org.tmatesoft.svn.core.internal.wc.SVNErrorManager.error(SVNErrorManager.java:85)
at org.tmatesoft.svn.core.internal.wc.SVNErrorManager.error(SVNErrorManager.java:69)
at org.tmatesoft.svn.core.internal.io.svn.SVNEditModeReader.driveEditor(SVNEditModeReader.java:250)
at org.tmatesoft.svn.core.internal.io.svn.SVNRepositoryImpl.update(SVNRepositoryImpl.java:1503)
at org.tmatesoft.svn.core.internal.wc16.SVNUpdateClient16.update(SVNUpdateClient16.java:557)
at org.tmatesoft.svn.core.internal.wc16.SVNUpdateClient16.doUpdate(SVNUpdateClient16.java:414)
at org.tmatesoft.svn.core.internal.wc16.SVNUpdateClient16.doUpdate(SVNUpdateClient16.java:324)
at org.tmatesoft.svn.core.internal.wc2.old.SvnOldUpdate.run(SvnOldUpdate.java:27)
at org.tmatesoft.svn.core.internal.wc2.old.SvnOldUpdate.run(SvnOldUpdate.java:11)
at org.tmatesoft.svn.core.internal.wc2.SvnOperationRunner.run(SvnOperationRunner.java:20)
at org.tmatesoft.svn.core.wc2.SvnOperationFactory.run(SvnOperationFactory.java:1221)
at org.tmatesoft.svn.core.wc2.SvnOperation.run(SvnOperation.java:292)
at org.tmatesoft.svn.core.wc.SVNUpdateClient.doUpdate(SVNUpdateClient.java:315)
at org.tmatesoft.svn.core.wc.SVNUpdateClient.doUpdate(SVNUpdateClient.java:295)
at org.tmatesoft.svn.core.wc.SVNUpdateClient.doUpdate(SVNUpdateClient.java:391)
at hudson.scm.subversion.UpdateUpdater$TaskImpl.perform(UpdateUpdater.java:136)
at hudson.scm.subversion.WorkspaceUpdater$UpdateTask.delegateTo(WorkspaceUpdater.java:144)
at hudson.scm.SubversionSCM$CheckOutTask.perform(SubversionSCM.java:789)
at hudson.scm.SubversionSCM$CheckOutTask.invoke(SubversionSCM.java:770)
at hudson.scm.SubversionSCM$CheckOutTask.invoke(SubversionSCM.java:753)
at hudson.FilePath.act(FilePath.java:842)
at hudson.FilePath.act(FilePath.java:824)
at hudson.scm.SubversionSCM.checkout(SubversionSCM.java:743)
at hudson.scm.SubversionSCM.checkout(SubversionSCM.java:685)
at hudson.model.AbstractProject.checkout(AbstractProject.java:1245)
at hudson.model.AbstractBuild$AbstractBuildExecution.defaultCheckout(AbstractBuild.java:589)
at jenkins.scm.SCMCheckoutStrategy.checkout(SCMCheckoutStrategy.java:88)
at hudson.model.AbstractBuild$AbstractBuildExecution.run(AbstractBuild.java:494)
at hudson.model.Run.execute(Run.java:1488)
at hudson.maven.MavenModuleSetBuild.run(MavenModuleSetBuild.java:477)
at hudson.model.ResourceController.execute(ResourceController.java:88)
at hudson.model.Executor.run(Executor.java:236)
Caused by: org.tmatesoft.svn.core.SVNException: svn: E155017: Checksum mismatch while updating '/xxxxx/.svn/text-base/SignUpController.java.svn-base'; expected: '39fc987bbeb8cd332e6b94abfb934720', actual: 'e9fa300ee28a2b1e15b2273f4b14ae18'
at org.tmatesoft.svn.core.internal.wc.SVNErrorManager.error(SVNErrorManager.java:64)
at org.tmatesoft.svn.core.internal.wc.SVNErrorManager.error(SVNErrorManager.java:51)
at org.tmatesoft.svn.core.internal.wc.SVNUpdateEditor15.textDeltaEnd(SVNUpdateEditor15.java:637)
at org.tmatesoft.svn.core.internal.wc.SVNAmbientDepthFilterEditor.textDeltaEnd(SVNAmbientDepthFilterEditor.java:221)
at org.tmatesoft.svn.core.internal.wc.SVNCancellableEditor.textDeltaEnd(SVNCancellableEditor.java:130)
at org.tmatesoft.svn.core.internal.io.svn.SVNEditModeReader.processCommand(SVNEditModeReader.java:176)
at org.tmatesoft.svn.core.internal.io.svn.SVNEditModeReader.driveEditor(SVNEditModeReader.java:232)
... 29 more
Caused by: svn: E155017: Checksum mismatch while updating '/xxxx/.svn/text-base/SignUpController.java.svn-base'; expected: '39fc987bbeb8cd332e6b94abfb934720', actual: 'e9fa300ee28a2b1e15b2273f4b14ae18'
at org.tmatesoft.svn.core.SVNErrorMessage.create(SVNErrorMessage.java:208)
at org.tmatesoft.svn.core.SVNErrorMessage.create(SVNErrorMessage.java:189)
at org.tmatesoft.svn.core.SVNErrorMessage.create(SVNErrorMessage.java:141)
at org.tmatesoft.svn.core.internal.wc.SVNUpdateEditor15.textDeltaEnd(SVNUpdateEditor15.java:634)
... 33 more
no change for svn://some_address/trunk since the previous build
No emails were triggered.

We have occassionally seen the problem with some builds, but we normally just clear the workspace and then rebuild. (From Jenkins, Navigate to the job, then "Workspace".)
There are a couple of different plugins that may help with detecting and possibly actually taking some actions based on the issue.
Warnings Plugin - parses the console output and looks for built in and user defined patterns that it reports as Warnings.
Log Trigger Plugin - triggers downstream jobs if a specific trigger is found in the console log. (Perhaps have an SVN "clean-up" job that is triggered?)
Of these, I have only used the Warnings Plugin myself.

These threads from Jenkins JIRA and JetBrains are interesting:
https://issues.jenkins-ci.org/browse/JENKINS-14550
http://youtrack.jetbrains.com/issue/IDEA-83673#comment=27-379397
As you mentioned, the issue remains open in Jenkins. JetBrains believe they have corrected IDEA with the updated svnkit: svnkit 1.7.5-v1.
Also noted in the jetbrains issue are some strategies to fix the issue using the commercial tool SmartSvn, an svn client which has a special feature "Validate Admin Area".
I am going to try to update the Jenkins JIRA issue with a note about the svnkit to see if that results in a fix.
EDIT:
Actually, it looks like the January Jenkins Subversion plugin, 1.4.5, takes care of this issue. After installing and manually re-starting, the mismatch went away.

JUnit4 + Eclipse "An internal error occurred during Launching"

I'm trying to run JUnit4 test cases on Eclipse 3.4.2 but it's not even starting for me. I have the junit-4.7.jar in my build path and the test application.
Here is a simple example that illustrates my problem
package test;
import org.junit.Before;
import org.junit.Test;
public class UTest {
#Test
public void test() {
}
#Before
public void setUp() throws Exception {
}
}
This compiles fine
Then I do "Run JUnit Test case" from Eclipse and I get an error dialog with this message
"Launching UTest' has encountered a problem
An internal error occurred during: "Launching UTest".
java.lang.NullPointerException
What causes these NullPointerExceptions? What am I doing wrong?

What worked for me after trying everything:
Go to help
Install New Software
Work with: Juno
Programming languages (expand it)
Install Java Development Tools
Restart
It works :)

I was able to fix this just by deleting the workspace and the Eclipse directory and starting over.

This worked for me:
create another copy of the test class (CopyOfUTest.java)
run the copy to make sure it passes
go into Run > Run Configurations
under JUnit, find the run configurations for the original class and the copied class
right click and delete the configuration of the original class
rename the configuration of the copied class to the original configuration name
delete the copied class from the project

None of the given answers here worked for me, so I ended up just installing and using InfiniTest instead. It doesn't have this problem, and it also runs the tests automatically so I can focus on my work.

Have you looked in the Eclipse error log? You can see it by opening the "Error Log" view.
http://help.eclipse.org/help32/topic/org.eclipse.pde.doc.user/guide/tools/views/error_log.htm

This error In eclipse can be caused if you are also using the Android Development Kit plugins:
"Launching UTest' has encountered a problem
An internal error occurred during: "Launching UTest".
java.lang.NullPointerException
Can be caused if you are loading a normal Java project into an Eclipse instance with android ADT plugins installed and enabled. In this situation, Eclipse looks for "Android" project files, and doesn't find any. So it says: "NullPointerException".
So to fix it, re-download Eclipse without the ADT Plugin: https://www.eclipse.org/downloads/
Then re-import your project fresh. And the junit tests run without a problem.
Many people hate eclipse for it's enigmatic error messages. It's like we are back in the 1950's punch card world, where there are no error messages. The program just halts and undefined behavior occurs.

Thanks that solved my problem too.
The problem started when i removed an old simulator, and created a new one.
Fix: Like the OP says remove the workspace, make sure to keep the projects inside it :)
then import them back to eclipse
"Sound like a lot of work" ?
Took me less than half a minute !!!

If you are using Android and its associated plugins, then Android only supports JUnit 3.
I resolved the problem by selecting Test Runner as JUnit 3.
In my class, JUnit 4 is added in the build path->libraries.
Then to run the test file, go to: Run As -> Run Configurations then select the corresponding test.java file and select Test Runner accordingly(whether it is JUnit 3 or 4).

Your code works fine for me.
Eclipse
Version: 3.4.1
Build id: M20080911-1700
I right click on the .java file RunAs JUnit Test. This would indicate the problem is caused by an Eclipse configuration problem, not a code problem.

I encountered a similar problem but I am using Python. This is what I did to solve/avoid it:
Removed my .project file and the project from Eclipse.
Created the project again.
Everything was working.
The problem seemed to be in the .project file where there were some references to CDT Builder and were not there in the new .project file.

Class file name must end with .class exception in Java Search

I was hoping someone could help me out with a problem I'm having using the java search function in Eclipse on a particular project.
When using the java search on one particular project, I get an error message saying Class file name must end with .class (see stack trace below). This does not seem to be happening on all projects, just one particular one, so perhaps there's something I should try to get rebuilt?
I have already tried Project -> Clean... and Closing Eclipse, deleting all the built class files and restarting Eclipse to no avail.
The only reference I've been able to find on Google for the problem is at http://www.crazysquirrel.com/computing/java/eclipse/error-during-java-search.jspx, but unfortunately his solution (closing, deleting class files, restarting) did not work for me.
If anyone can suggest something to try, or there's any more info I can gather which might help track it's down, I'd greatly appreciate the pointers.
Version: 3.4.0
Build id: I20080617-2000
Also just found this thread - http://www.myeclipseide.com/PNphpBB2-viewtopic-t-20067.html - which indicates the same problem may occur when the project name contains a period. Unfortunately, that's not the case in my setup, so I'm still stuck.
Caused by: java.lang.IllegalArgumentException: Class file name must end with .class
at org.eclipse.jdt.internal.core.PackageFragment.getClassFile(PackageFragment.java:182)
at org.eclipse.jdt.internal.core.util.HandleFactory.createOpenable(HandleFactory.java:109)
at org.eclipse.jdt.internal.core.search.matching.MatchLocator.locateMatches(MatchLocator.java:1177)
at org.eclipse.jdt.internal.core.search.JavaSearchParticipant.locateMatches(JavaSearchParticipant.java:94)
at org.eclipse.jdt.internal.core.search.BasicSearchEngine.findMatches(BasicSearchEngine.java:223)
at org.eclipse.jdt.internal.core.search.BasicSearchEngine.search(BasicSearchEngine.java:506)
at org.eclipse.jdt.core.search.SearchEngine.search(SearchEngine.java:551)
at org.eclipse.jdt.internal.corext.refactoring.RefactoringSearchEngine.internalSearch(RefactoringSearchEngine.java:142)
at org.eclipse.jdt.internal.corext.refactoring.RefactoringSearchEngine.search(RefactoringSearchEngine.java:129)
at org.eclipse.jdt.internal.corext.refactoring.rename.RenameTypeProcessor.initializeReferences(RenameTypeProcessor.java:594)
at org.eclipse.jdt.internal.corext.refactoring.rename.RenameTypeProcessor.doCheckFinalConditions(RenameTypeProcessor.java:522)
at org.eclipse.jdt.internal.corext.refactoring.rename.JavaRenameProcessor.checkFinalConditions(JavaRenameProcessor.java:45)
at org.eclipse.ltk.core.refactoring.participants.ProcessorBasedRefactoring.checkFinalConditions(ProcessorBasedRefactoring.java:225)
at org.eclipse.ltk.core.refactoring.Refactoring.checkAllConditions(Refactoring.java:160)
at org.eclipse.jdt.internal.ui.refactoring.RefactoringExecutionHelper$Operation.run(RefactoringExecutionHelper.java:77)
at org.eclipse.jdt.internal.core.BatchOperation.executeOperation(BatchOperation.java:39)
at org.eclipse.jdt.internal.core.JavaModelOperation.run(JavaModelOperation.java:709)
at org.eclipse.core.internal.resources.Workspace.run(Workspace.java:1800)
at org.eclipse.jdt.core.JavaCore.run(JavaCore.java:4650)
at org.eclipse.jdt.internal.ui.actions.WorkbenchRunnableAdapter.run(WorkbenchRunnableAdapter.java:92)
at org.eclipse.jface.operation.ModalContext$ModalContextThread.run(ModalContext.java:121)
Thanks McDowell, closing and opening the project seems to have fixed it (at least for now).

Comment #9 to bug 269820 explains how to delete the search index, which appears to be the solution to a corrupt index whose symptoms are the dreaded
An internal error occurred during: "Items filtering".
Class file name must end with .class
message box.
How to delete the search index:
Close Eclipse
Delete <workspace>/.metadata/.plugins/org.eclipse.jdt.core/*.index
Delete <workspace>/.metadata/.plugins/org.eclipse.jdt.core/savedIndexNames.txt
Start Eclipse again

Two more general-purpose mechanisms for fixing some of Eclipse's idiosyncrasies:
Close and open the project
Delete the project (but not from disk!) and reimport it as an existing project
Failing that, bugs.eclipse.org might provide the answer.
If the workspace is caching something broken, you may be able to delete it by poking around in workspace/.metadata/.plugins. Most of that stuff is fairly transient (though backup and watch for deleted preferences).

Got this error to the other day. Tried deleting the all .class-files and resources from my output folder manually. Didn't work. Restarted my computer (WinXP). Didn't work. Closed my project in Eclipse and opened it again. Worked!!! Hopes this solves someones problem out there. The search facilities and truly essential to Eclipse.

I also encountered this issue recently, the below scenario worked for me.
Close Eclipse
Delete <workspace>/.metadata/.plugins/org.eclipse.jdt.core/*.index
Delete <workspace>/.metadata/.plugins/org.eclipse.jdt.core/savedIndexNames.txt
Start Eclipse again

Closing the projects didn't do the trick for me. I started eclipse with the -clean flag and that worked for some reason.

Just
Close project
Clear manually output folder(s)
Open project
(Eclipse 3.5 SR2, Build id: 20100218-1602)