Error: Could not find or load main class

Question

I am having trouble compiling and running my Java code, intended to allow me to interface Java with a shared object for Vensim, a simulation modeling package.

The following code compiles without error:

javac -d . -cp ./apache-log4j-1.2.16/log4j-1.2.16.jar:./vensim.jar SpatialModel.java     VensimHelper.java VensimException.java VensimContextRepository.java

However, when I try to run the following:

java -cp ./apache-log4j-1.2.16/log4j-1.2.16.jar:./vensim.jar SpatialModel vars

I get the following error: "Error: Could not find or load main class SpatialModel ". My SpatialModel.java code does contain a 'main' method (below), so I'm not sure what the problem is - can anyone please help me out? Thanks.

import java.io.File;
import java.text.NumberFormat;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

import org.apache.log4j.Logger;

public class SpatialModel {

    private VensimHelper vh;

    public static final String DLL_LIBNAME_PARAM = "vensim_lib_nam";

    public static final String MODEL_PATH_PARAM = "vensim_model_path";

    private final static int VENSIM_CONTEXT_CREATION_MAX_FAILURE_COUNT = 10;

    public SpatialModel() throws SpatialException {

        String libName = System.getProperty(DLL_LIBNAME_PARAM);
        String modelPath = System.getProperty(MODEL_PATH_PARAM);        

        if(libName == null || libName.trim().equals("")) {
            log.error("Vensim library name has to be set with -D" + DLL_LIBNAME_PARAM);
            throw new SpatialException("Vensim library name has to be set with -D" + DLL_LIBNAME_PARAM);
        }

        if(modelPath == null || modelPath.trim().equals("")) {
            log.error("Model path has to set with -D" + MODEL_PATH_PARAM);
            throw new SpatialException("Model path ahs to be set with -D" + MODEL_PATH_PARAM);
        }

        for (int i = 0; i < VENSIM_CONTEXT_CREATION_MAX_FAILURE_COUNT && vh == null; i++) {
            try {
                log.info("creating new vensim helper\n\tdll lib: " + libName + "\n\tmodel path: " + modelPath);
                vh = new VensimHelper(libName, modelPath);
            } catch (Throwable e) {
                log.error("An exception was thrown when initializing Vensim, try: " + i, e);
            }
        }
        if (vh == null) {
            throw new SpatialException("Can't initialize Vensim");
        }

    }

    public static void main(String[] args) throws VensimException {

        long before = System.currentTimeMillis();   
        String libName = System.getProperty(DLL_LIBNAME_PARAM);
        String modelPath = System.getProperty(MODEL_PATH_PARAM);

        if (libName == null) {
            libName = "libvensim";
        }
        if(modelPath == null) {
            modelPath = "~/BassModel.vmf";
        }

        System.setProperty(DLL_LIBNAME_PARAM, libName);
        System.setProperty(MODEL_PATH_PARAM, modelPath);

        if (args.length > 0 && args[0].equals("info")) {
            System.out.println(new VensimHelper(libName, modelPath).getVensimInfo());
        } else if (args.length > 0 && args[0].equals("vars")) {
            VensimHelper helper = new VensimHelper(libName, modelPath);
            String[] vars = helper.getVariables();
            for (String var : vars) {
                System.out.println(helper.getVariableInfo(var));
            }
        } else {

            File f = new File(".");
            System.out.println(f.getAbsolutePath());

            SpatialModel sm = new SpatialModel();
        }

        System.out.println("Execution time: " + (System.currentTimeMillis() - before));
    }

}

score 299 · Accepted Answer · edited Jun 03 '19 at 21:43

299

You must ensure that you add the location of your .class file to your classpath. So, if its in the current folder, add . to your classpath. Note that the Windows classpath separator is a semi-colon, i.e. a ;.

edited Jun 03 '19 at 21:43

Sae1962

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answered Sep 20 '11 at 13:16

Saket

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78

And if someone didn't notice in the question the classpath separator in Linux is a colon `:`. – Javier Mr Jun 26 '14 at 10:12
6

@JavierMr Thanks for pointing out classpath separator in Linux. I ended up using this: `javac -cp third_party_lib.jar MyClass.java` and for execute: `java -cp .:./third_party_lib.jar MyClass` (notice the dot **.** and colon **:** after -cp option) – broadband Sep 25 '14 at 08:39
1

See This one too.. http://stackoverflow.com/a/6069764/565661 – RamNat Nov 19 '14 at 22:06
2

how to do this in STS with same error? – vbNewbie Nov 23 '14 at 19:21
4

This error occurs when you write wrongly the name of the main method or you use the package statement without putting the class in the correct folder structure. – Gilberto Feb 09 '15 at 18:34
@Saket "You must ensure that you add the location of your .class file to your classpath. " How to add location of our .class to our class path in eclipse ide? – Mar 26 '15 at 16:24
I know you're supposed to avoid comments like "ugh", but that's exactly how I feel about this. I'm using a colon in my classpath and the engine isn't complaining at all about an illegal classpath! It just says "Could not find or load main class". I searched for that error when I realized I must be doing something stupid. Thank you! – Erick Robertson Jan 26 '16 at 22:07
In Windows, using a batch to run your Java may only cause this error if passing an argument of a file from a parent folder. Having the batch first cd to the same directory as the Java file will fix it. – theSparky Jul 19 '23 at 16:18

aliteralmind · Answer 2 · 2017-11-13T11:27:49.513

150

If the class is in a package

package thepackagename;

public class TheClassName {
  public static final void main(String[] cmd_lineParams)  {
     System.out.println("Hello World!");
  } 
}

Then calling:

java -classpath . TheClassName

results in Error: Could not find or load main class TheClassName. This is because it must be called with its fully-qualified name:

java -classpath . thepackagename.TheClassName

And this thepackagename directory must exist in the classpath. In this example, ., meaning the current directory, is the entirety of classpath. Therefore this particular example must be called from the directory in which thepackagename exists.

To be clear, the name of this class is not TheClassName, It's thepackagename.TheClassName. Attempting to execute TheClassName does not work, because no class having that name exists. Not on the current classpath anyway.

Finally, note that the compiled (.class) version is executed, not the source code (.java) version. Hence “CLASSPATH.”

edited Nov 13 '17 at 11:27

answered Mar 29 '15 at 16:40

aliteralmind

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And this command must be called from the directory in which the thepackagename directory exists. Easy to miss but VERY important. – Patricia Oct 08 '15 at 21:19
1

@aliteralmind your answer worked for me as my class was inside a package. Can you please explain why it needs to be done this way if the class is inside a package? – mahacoder Nov 12 '15 at 10:38
1

Thank you! Mine Stupid mistake of package name. – Mohit Jan 16 '16 at 15:24
1

This was my problem. Totally non-obvious for someone coming from a non-java background. – pixelpax Jan 26 '17 at 05:39
1

Very concise and helpful, thanks. – nrmad Apr 25 '20 at 15:55
Thanks this saved lots of my time. This is why I always feel Kafkaesque in Java – Atul Jul 26 '22 at 15:03

score 67 · Answer 3 · edited Jan 21 '12 at 17:39

67

You can try these two when you are getting the error: 'could not find or load main class'

If your class file is saved in following directory with HelloWorld program name d:\sample

java -cp d:\sample HelloWorld
java -cp . HelloWorld

edited Jan 21 '12 at 17:39

Yi Jiang

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answered Jan 21 '12 at 17:17

Sudhakar Pabbati

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9

This works great. But why please? – Sam003 May 07 '15 at 17:21
it works because it tells the java interpreter where the bytecode class files are. – rupweb Sep 17 '15 at 08:46
That's the simplest answer I think. Thanks! – Tung Oct 14 '16 at 14:19

score 39 · Answer 4 · answered Sep 20 '11 at 13:12

39

I believe you need to add the current directory to the Java classpath

java -cp .:./apache-log4j-1.2.16/log4j-1.2.16.jar:./vensim.jar SpatialModel vars

answered Sep 20 '11 at 13:12

Rafael Cordones

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Thanks for your answer. My life was saved! After previous editing of CLASSPATH. Current directory was no longer in CLASSPATH. That's why I keep getting "could not find or load main class" and "could not find symbol" when they are right in the current directory. Thank you so much!!! – Sophia Feng Dec 18 '13 at 00:53
2

For those with the same problem, here's how to add the current directory to CLASSPATH `shell$ export CLASSPATH=.:$CLASSPATH` – Sophia Feng Dec 18 '13 at 00:59
This is clearly what has to be done, but according to the Java documentation it should not be needed. Here's what the Oracle java docs http://docs.oracle.com/javase/tutorial/essential/environment/paths.html say "The default value of the class path is ".", meaning that only the current directory is searched" – Motorhead Jul 26 '17 at 02:07
@S.N. - You are misinterpreting. That is the >>default<< value for the classpath. If you supply an explicit classpath via `$CLASSPATH` or `-cp` or any other way, then the default classpath is not relevant. The OP is using `-cp`. – Stephen C Jul 26 '19 at 03:58
@StephenC: of course, I had a brain misfire when I wrote that! – Motorhead Sep 20 '19 at 18:04

score 36 · Answer 5 · answered Nov 03 '14 at 13:36

36

You have to include classpath to your javac and java commands

javac -cp . PackageName/*.java
java -cp . PackageName/ClassName_Having_main

suppose you have the following

Package Named: com.test Class Name: Hello (Having main) file is located inside "src/com/test/Hello.java"

from outside directory:

$ cd src
$ javac -cp . com/test/*.java
$ java -cp . com/test/Hello

In windows the same thing will be working too, I already tried

answered Nov 03 '14 at 13:36

Muhammad Soliman

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Clear explanation and working on Windows 10 – Jelphy Sep 14 '21 at 19:36

score 32 · Answer 6 · edited Jun 03 '19 at 18:48

32

If you work in Eclipse, just make a cleanup (project\clean.. clean all projects) of the project.

edited Jun 03 '19 at 18:48

Sae1962

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answered May 22 '13 at 14:59

cane

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score 20 · Answer 7 · edited Jun 10 '13 at 01:26

20

You have to set the classpath if you get the error:

Could not find or load main class XYZ

For example:

E:\>set path="c:\programfiles\Java\jdk1.7.0_17\bin"
E:\>set classpath=%classpath%;.;
E:\>javac XYZ.java
E:\>java XYZ

edited Jun 10 '13 at 01:26

Leigh

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answered Apr 25 '13 at 07:22

user2318595

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score 20 · Answer 8 · edited Nov 05 '18 at 01:36

20

I got this error because I was trying to run

javac HelloWorld.java && java HelloWorld.class

when I should have removed .class:

javac HelloWorld.java && java HelloWorld

edited Nov 05 '18 at 01:36

Community

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answered Aug 30 '14 at 10:12

Lri

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Could not find or load main class mainClass – Yaroslav Dukal Oct 11 '18 at 03:55
1

Glad none of my coworkers saw me trying to figure this out for half an hour – jMan Sep 23 '19 at 13:22

score 12 · Answer 9 · edited Jun 03 '19 at 16:58

12

Check your BuildPath, it could be that you are referencing a library that does not exist anymore.

edited Jun 03 '19 at 16:58

Sae1962

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answered Sep 06 '12 at 12:36

Samsky

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score 10 · Answer 10 · answered Dec 21 '15 at 23:31

10

If you're getting this error and you are using Maven to build your Jars, then there is a good chance that you simply do not have your Java classes in src/main/java/.

In my case I created my project in Eclipse which defaults to src (rather than src/main/java/.

So I ended up with something like mypackage.morepackage.myclass and a directory structure looking like src/mypackage/morepackage/myclass, which inherently has nothing wrong. But when you run mvn clean install it will look for src/main/java/mypackage/morepackage/myclass. It will not find the class but it won't error either. So it will successfully build and you when you run your outputted Jar the result is:

Error: Could not find or load main class mypackage.morepackage.myclass

Because it simply never included your class in the packaged Jar.

answered Dec 21 '15 at 23:31

Mike S

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so what is the fix for the problem. I am getting the error because of the Maven – Baradwaj Aryasomayajula Apr 03 '17 at 00:44
@BaradwajAryasomayajula the fix is to move your Java class files into the `src/main/java/` directory, because that's where Maven expects them to be. – Mike S Apr 03 '17 at 19:14
I have added class under src/main/java/ and it is also there in generated jar but I am still getting same error while running the jar :( – Joker Jan 02 '20 at 10:50

score 9 · Answer 11 · answered Apr 09 '13 at 20:53

I know this question was tagged with linux, but on windows, you might need to separate your cp args with a ; instead of a :.

java -cp ./apache-log4j-1.2.16/log4j-1.2.16.jar;./vensim.jar SpatialModel vars

http://docs.oracle.com/javase/7/docs/technotes/tools/windows/classpath.html

score 8 · Answer 12 · edited Jun 03 '19 at 16:05

8

If you try to run a java application which needs JDK 1.6 and you are trying to run on JDK 1.4, you will come across this error. In general, trying to run a Java application on old JRE may fail. Try installing new JRE/JDK.

edited Jun 03 '19 at 16:05

Sae1962

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answered Nov 11 '11 at 07:26

Shaddu

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score 6 · Answer 13 · edited Jun 03 '19 at 16:57

6

I was using Java 1.8, and this error suddenly occurred when I pressed "Build and clean" in NetBeans. I switched for a brief moment to 1.7 again, clicked OK, re-opened properties and switched back to 1.8, and everything worked perfectly.

I hope I can help someone out with this, as these errors can be quite time-consuming.

edited Jun 03 '19 at 16:57

Sae1962

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answered Mar 08 '14 at 10:32

Maxim

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score 6 · Answer 14 · answered Jul 15 '14 at 11:18

6

Project > Clean and then make sure BuildPath > Libraries has the correct Library.

answered Jul 15 '14 at 11:18

Giri

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score 6 · Answer 15 · answered Dec 04 '15 at 15:17

6

java -verbose:class HelloWorld might help you understand which classes are being loaded.

Also, as mentioned before, remember to call the full qualified name (i.e. include package).

answered Dec 04 '15 at 15:17

Jose Alban

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score 6 · Answer 16 · edited Mar 23 '12 at 19:48

6

Problem is not about your main function. Check out for

javac -d . -cp ./apache-log4j-1.2.16/log4j-1.2.16.jar:./vensim.jar SpatialModel.java     VensimHelper.java VensimException.java VensimContextRepository.java

output and run it.

edited Mar 23 '12 at 19:48

Sufian Latif

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answered Sep 20 '11 at 13:24

ymutlu

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score 5 · Answer 17 · answered Jan 14 '15 at 13:57

This problem occurred for me when I imported an existing project into eclipse. What happens is it copied all the files not in the package, but outside the package. Hence, when I tried run > run configurations, it couldn't find the main method because it was not in the package. All I did was copy the files into the package and Eclipse was then able to detect the main method. So ultimately make sure that Eclipse can find your main method, by making sure that your java files are in the right package.

score 5 · Answer 18 · answered May 12 '15 at 20:08

If so simple than many people think, me included :)

cd to Project Folder/src/package there you should see yourClass.java then run javac yourClass.java which will create yourClass.class then cd out of the src folder and into the build folder there you can run java package.youClass

I am using the Terminal on Mac or you can accomplish the same task using Command Prompt on windows

score 5 · Answer 19 · answered Jun 06 '15 at 12:05

If you are using Eclipse... I renamed my main class file and got that error. I went to "Run As" configurator and under the class path for that project, it had listed both files in the class path. I removed old class that I renamed and left the class that had the new name and it compiled and ran just fine.

score 2 · Answer 20 · answered Jun 13 '14 at 19:11

2

This solved the issue for me today:

cd /path/to/project
cd build
rm -r classes

Then clean&build it and run the individual files you need.

answered Jun 13 '14 at 19:11

Luc

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score 2 · Answer 21 · answered Jul 15 '14 at 17:23

2

I have a similar problem in Windows, it's related to the classpath. From the command line, navigate until the directory where it's located your Java file (*.java and *.class), then try again with your commands.

answered Jul 15 '14 at 17:23

edwindh

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score 2 · Answer 22 · answered Feb 20 '15 at 13:36

I use Anypoint Studio (an Eclipse based IDE). In my case everything worked well, until I found out that while running the java code, something totally different is executed. Then I have deleted the .class files. After this point I got the error message from this question's title. Cleaning the project didn't solve the problem.

After restarting the IDE everything worked well again.

Error: Could not find or load main class

22 Answers22

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