Related
NetBeans 12.0
Jdk 15
I cannot launch my program, clicking the green run button doesn't do anything, and right click + run file on MainFrame gives the error "Class does not have a main method". But as you can see in the GIF below, it does have a main(String args[]).
Does anyone have any idea why this happens?
Initial suggestions:
Make sure all files are saved first. If you try to run a program where the class containing the main method has not been saved, this can happen.
Right-click on the project name (in the Projects explorer), then select Properties > Run - and then make sure the main class is selected there. Again, if you have not saved the files NetBeans may not be able to find the main method (yet).
It may also be worth checking whether you have any other classes in your project which also have a public static void main(String args[]) method.
If all else fails:
Clearing the NetBeans cache may help.
Go to Help > About, and note the location of the cache directory. Exit from NetBeans. Delete the Cache directory. Restart NetBeans.
WARNING
Please note what #skomisa points out in the comments:
NetBeans 12.0 does not support Java 15, and while it may work for a trivial project, there are still issues (as shown by the comments here). It would be reckless to use that combination for production code, and the IDE (as opposed to the compiler) may not handle new features in Java 15 appropriately.
Here is what worked for me on Netbeans 12.3 JDK 11:
Right Click Project -> Properties
In the Sources Tab, click Add Folder under Source Package Folders. Choose src.
Press Ok.
It appears Netbeans re-scans the project folder, and the main class is found.
I just downloaded eclipse for Java Yesterday but when I was trying to get make my first program, I kept on getting this error:
must declare a named package eclipse because this compilation unit is associated to the named module x.
How do I fix this?
Just delete module-info.java at your Project Explorer tab.
The "delete module-info.java at your Project Explorer tab" answer is the easiest and most straightforward answer, but
for those who would want a little more understanding or control of what's happening, the following alternate methods may be desirable;
make an ever so slightly more realistic application; com.YourCompany.etc
or just com.HelloWorld (Project name: com.HelloWorld and class name: HelloWorld)
or
when creating the java project; when in the Create Java Project dialog, don't choose Finish but Next, and deselect Create module-info.java file
Reason of the error: Package name left blank while creating a class. This make use of default package. Thus causes this error.
Quick fix:
Create a package eg. helloWorld inside the src folder.
Move helloWorld.java file in that package. Just drag and drop on
the package. Error should disappear.
Explanation:
My Eclipse version: 2020-09 (4.17.0)
My Java version: Java 15, 2020-09-15
Latest version of Eclipse required java11 or above. The module feature is introduced in java9 and onward. It was proposed in 2005 for Java7 but later suspended. Java is object oriented based. And module is the moduler approach which can be seen in language like C. It was harder to implement it, due to which it took long time for the release. Source: Understanding Java 9 Modules
When you create a new project in Eclipse then by default module feature is selected. And in Eclipse-2020-09-R, a pop-up appears which ask for creation of module-info.java file. If you select don't create then module-info.java will not create and your project will free from this issue.
Best practice is while crating project, after giving project name. Click on next button instead of finish. On next page at the bottom it ask for creation of module-info.java file. Select or deselect as per need.
If selected: (by default) click on finish button and give name for module. Now while creating a class don't forget to give package name. Whenever you create a class just give package name. Any name, just don't left it blank.
If deselect: No issue
When I create a new class inside a package for some reason it creates two class files: one inside of the package and one out of it. I wanted to know why this is happening? Also, at times, Eclipse are kind of "hiding" some of my classes, so that they do not appear in the project explorer, although when you create a new class file with the same name of the class that disappeared, it says the class already exists...
When I refresh (F5) the package, these problems are sort of solved. But they happen again when I create a new class or when I restart Eclipse. So I wanted a different solution from keep pressing F5 everytime...
So I had this same problem and it may be for the same reason. The problem can be because you are creating the .java files in the Project Explorer window. Close this window. Then, go to Window -> Show View -> Package Explorer. Then create your .java files there. The issue is that the Project Explorer window looks EXACTLY the same way as the Package Explorer.
#Rammohan's answer helped me but I can't vote it up or apparently add a comment because of low reputation. Being new to Eclipse I didn't get what he meant at first, so wanted to add a little detail for other people.
From the top menu it's Window -> Perspective -> Open Perspective -> Java.
I had unwittingly switched to the Debug perspective and had the double-creation problem happening in src/test/java (but not in src/main/java, still don't get why). Switching to the Java perspective cleared it right up.
I also had same problem with eclipse. This issue is resolved after changing Perspective mode from "Debug" to "Java". Two same class files appear when Eclipse Perspective is in "Debug" mode. In "Java" perspective, this issue is resolved.
Hope this helps if someone facing same issue.
So I'm making a java application in Neatbeans 7.4, been working at it for a while, everything was fine, running the project worked fine, but now when I hit run project, I get the error
Error: Could not find or load main class phleveledit.MainWindow
Java Result: 1
BUILD SUCCESSFUL (total time: 0 seconds)
I can't think of what I did right before this started happening, so I don't know what I should change.. The code doesn't appear to have any errors. Here's a screenshot of the IDE+project folder
Image (http://puu.sh/5ldYB) :
Any ideas?
Edit: Unfortunately I happened to fix this problem by removing some code, which probably had some errors Netbeans couldn't detect, but I don't know what was exactly wrong so can't vote on a correct answer.
Right click on your Project in the project explorer
Click on properties
Click on Run
Make sure your Main Class is the one you want to be the entry point. (Make sure to use the fully qualified name i.e. mypackage.MyClass)
Click OK.
Clean an build your project
Run Project :)
If you just want to run the file, right click on the class from the package explorer, and click Run File, or (Alt + R, F), or (Shift + F6)
Just close the Netbeans. Go to C:\Users\YOUR_PC_NAME\AppData\Local\Netbeans and delete the Cache folder. The open the Netbeans again and run the project.
It works like magic for me.
(AppData folder might be hidden probably, if so, you need to make it appear in Folder Options).
You can :
RightClick on project node and go to Set configuration
Select the main class for your application.
Then clean and build.
Even if the above steps don't work for you then then delete the Netbeans cache by deleting the (index) folder
User\.netbeans\SOME_NUMBER_reflecting_your_version\var\cache\index\
Sometimes due to out of memory space error, NetBeans does not load or find main class.
If you have tried setting the properties and still it is not working then try
Select the project from the project explorer
Click on Run in the Menu Bar
Click on Compile
It worked for me.
This condition happens to me every 6-months or so. I think it happens when closing NetBeans under very low memory conditions. I discovered that it could be easily corrected by (1) Rename your project, including its folder name using right-click on project explorer's project name---I put a simple suffix on the original name ("_damaged"). (2) Try BUILD. If that is successful, which it is for me, give three cheers. (3) Repeat step (1) to restore the original project name. BUILD and RUN should start without trouble.
I guess that the 'rename the project and folder' process causes a special rediscovery of the applications main location.
Try to rename the package name and the class/jframe names... The clean and build the application.
Right Click on the package name
Go to Refactor
Select Rename
Give it a meaningful name, preferably all in small letters
Click on Refactor
Do the same for the class/jframe names.
Last Select Run from Menu
7.Select Clean and build main project
That should do it!!! All best
I had the same issue once. The problem was not in the code. The cause was... renaming the project folder to some other non supporting name. My project name was "MobStick" and I renamed it to "MobStick - May 26, 2014 04:00PM". Renaming it back to normal solved my problem.
I have run into this error a couple of times as well and for me the above solutions did not work. What does seem to work is going to the Project Properties, and under Compiling toggling Compile on Save.
Using NetBeans 8.1, I got the dread
Error: Could not find or load main class
from carelessly leaving an empty line in the Project Properties > Run > VM Options field. Until you click in the field, you may not see the caret flashing out of place. Remove the empty line to restore equanimity.
I just ran into this problem. I was running my source from the command line and kept getting the same error. It turns out that I needed to remove the package name from my source code and then the command line compiler was happy.
The solutions above didn't work for me so maybe this will work for someone else with a similar problem.
I had the same problem, I had the package and class named the same. I renamed the class, then clean and build. Then I set the main class in the "run" under the properties of the project. I works now.
I found the following steps useful:
Right-click on the project in the left toolbar.
Hover over the 'Set Configuration' item.
Click on 'Customize...'
Click on 'Browse...' by the 'Main Class:' item.
Select the correct class.
Click 'Select Main Class'.
Click 'OK'.
My problem was that, apparently, my package name was being listed twice. Selecting the class using the dialog changed 'aclass.MainClass' to just 'MainClass'.
Hope this helps,
-HewwoCraziness
Edit: This is expanding on Mary Martinez's answer.
You can solve it in these steps
Right-click on the project in the left toolbar.
Click on properties.
Click on Run
Click the browse button on the right side.(select your main class)
Click ok
Possible Fixes:
Fix 1
Go to project properties (right click on the folder of your project in netbeans)
On left tab where it shows the categories, click on the "Run" selection
Then click on Browse to find the Main class you use on your project
Fix 2
Go to C:\Users\name\AppData\Local\Netbeans
delete the Cache folder.
Rebuild and Run
Fix 3
Download most recent version of Netbeans
Fix 4
Download most recent version of JDK and configure Netbeans to use that
I had the same problem for 3,4 days. On my PC my Jar file snapshot would give me this error while on my laptop it would work fine, I tried all the tricks shown above and on other forums like deleting cache, selecting main project file, etc, but somehow I was sure the reason it cannot find the main class when I would execute the JAR file was may be due to classpath issue in maven configuration, and I was right and I fixed it using following steps:
Right-click on the project, and go to the properties
Inside the properties go to Actions
On the right side in Actions select "Run Project" and you will see properties below
Inside "Set Properties" make sure exec.args=classpath %classpath "package_name"
In my case, the package name was accompanied by the main class. So my main class was Login while the package name was com.mycompany.islamic_center_app1, When I checked the entry was
com.mycompany.islamic_center_app1.Login
All I did was remove ".Login" from com.mycompany.islamic_center_app1 and it was fixed, no more errors.
close netbeans.
open netbeans again.
choose new project>>java application.
click next.
deselect create main class.
now make the application
clean build run
For more reference watch this video
try this it work out for me perfectly
go to project and right click on your java file at the right corner,
go to properties,
go to run,
go to browse, and then
select Main class.
now you can run your program again.
I had the same problem and I moved the project to a location where the path had no none-english letter and that fixed the problem
if you are on window os, then try to start NetBeans via administrative mode. right click on NetBeans icon and "Run as Administrative".
If none of the above works (Setting Main class, Clean and Build, deleting the cache) and you have a Maven project, try:
mvn clean install
on the command line.
Had the same problem here. Usually Clean and Build solves much of the problem. It happened to be caused by a wrongly installed plugin.
I faced the similar issue with Netbeans 10 and JDK 1.8.
I was not able to choose the right class to launch the project
When I compile or run the project, it shows me the Class name as "initializing view, please wait ...", I could not select the class name.
The issue was resolved with the NetBeans11.3, I am able to choose the correct Class file without any other changes, and the project is launched without any issues.
I had the same issue but none of this thread's solutions worked for me. Finally, it was OneDrive that caused the issue (for once more). So, I simply moved the NetBeansProjects folder from Documents which is synced with OneDrive, to C:\Users\yourName\AppData\Local\NetBeans (selected this path as there is already a NetBeans folder) and that was it, case closed.
If you also have NetBeansProjects to a OneDrive syncing folder it is worth trying this solution, just be sure that the path you will select is not synced with OneDrive. Also, remember to close Netbeans before making the folder change and after you move the folder to the new path you need just to open Netbeans again, go to file menu/open project and select your project from the new path.
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