What does "Could not find or load main class" mean?

Asked 2023-09-21 08:09:32 View 135,542

A common problem that new Java developers experience is that their programs fail to run with the error message: Could not find or load main class ...

What does this mean, what causes it, and how should you fix it?

  • Please note that this is a "self-answer" question that is intended to be a generic reference Q&A for new Java users. I could not find an existing Q&A that covers this adequately (IMO). - anyone

Answers

The java <class-name> command syntax

First of all, you need to understand the correct way to launch a program using the java (or javaw) command.

The normal syntax1 is this:

    java [ <options> ] <class-name> [<arg> ...]

where <option> is a command line option (starting with a "-" character), <class-name> is a fully qualified Java class name, and <arg> is an arbitrary command line argument that gets passed to your application.


1 - There are some other syntaxes which are described near the end of this answer.

The fully qualified name (FQN) for the class is conventionally written as you would in Java source code; e.g.

    packagename.packagename2.packagename3.ClassName

However some versions of the java command allow you to use slashes instead of periods; e.g.

    packagename/packagename2/packagename3/ClassName

which (confusingly) looks like a file pathname, but isn't one. Note that the term fully qualified name is standard Java terminology ... not something I just made up to confuse you :-)

Here is an example of what a java command should look like:

    java -Xmx100m com.acme.example.ListUsers fred joe bert

The above is going to cause the java command to do the following:

  1. Search for the compiled version of the com.acme.example.ListUsers class.
  2. Load the class.
  3. Check that the class has a main method with signature, return type and modifiers given by public static void main(String[]). (Note, the method argument's name is NOT part of the signature.)
  4. Call that method passing it the command line arguments ("fred", "joe", "bert") as a String[].

Reasons why Java cannot find the class

When you get the message "Could not find or load main class ...", that means that the first step has failed. The java command was not able to find the class. And indeed, the "..." in the message will be the fully qualified class name that java is looking for.

So why might it be unable to find the class?

Reason #1 - you made a mistake with the classname argument

The first likely cause is that you may have provided the wrong class name. (Or ... the right class name, but in the wrong form.) Considering the example above, here are a variety of wrong ways to specify the class name:

  • Example #1 - a simple class name:

    java ListUser
    

    When the class is declared in a package such as com.acme.example, then you must use the full classname including the package name in the java command; e.g.

    java com.acme.example.ListUser
    
  • Example #2 - a filename or pathname rather than a class name:

    java ListUser.class
    java com/acme/example/ListUser.class
    
  • Example #3 - a class name with the casing incorrect:

    java com.acme.example.listuser
    
  • Example #4 - a typo

    java com.acme.example.mistuser
    
  • Example #5 - a source filename (except for Java 11 or later; see below)

    java ListUser.java
    
  • Example #6 - you forgot the class name entirely

    java lots of arguments
    

Reason #2 - the application's classpath is incorrectly specified

The second likely cause is that the class name is correct, but that the java command cannot find the class. To understand this, you need to understand the concept of the "classpath". This is explained well by the Oracle documentation:

So ... if you have specified the class name correctly, the next thing to check is that you have specified the classpath correctly:

  1. Read the three documents linked above. (Yes ... READ them! It is important that a Java programmer understands at least the basics of how the Java classpath mechanisms works.)
  2. Look at command line and / or the CLASSPATH environment variable that is in effect when you run the java command. Check that the directory names and JAR file names are correct.
  3. If there are relative pathnames in the classpath, check that they resolve correctly ... from the current directory that is in effect when you run the java command.
  4. Check that the class (mentioned in the error message) can be located on the effective classpath.
  5. Note that the classpath syntax is different for Windows versus Linux and Mac OS. (The classpath separator is ; on Windows and : on the others. If you use the wrong separator for your platform, you won't get an explicit error message. Instead, you will get a nonexistent file or directory on the path that will be silently ignored.)

Reason #2a - the wrong directory is on the classpath

When you put a directory on the classpath, it notionally corresponds to the root of the qualified name space. Classes are located in the directory structure beneath that root, by mapping the fully qualified name to a pathname. So for example, if "/usr/local/acme/classes" is on the class path, then when the JVM looks for a class called com.acme.example.Foon, it will look for a ".class" file with this pathname:

  /usr/local/acme/classes/com/acme/example/Foon.class

If you had put "/usr/local/acme/classes/com/acme/example" on the classpath, then the JVM wouldn't be able to find the class.

Reason #2b - the subdirectory path doesn't match the FQN

If your classes FQN is com.acme.example.Foon, then the JVM is going to look for "Foon.class" in the directory "com/acme/example":

  • If your directory structure doesn't match the package naming as per the pattern above, the JVM won't find your class.

  • If you attempt rename a class by moving it, that will fail as well ... but the exception stacktrace will be different. It is liable to say something like this:

    Caused by: java.lang.NoClassDefFoundError: <path> (wrong name: <name>)
    

    because the FQN in the class file doesn't match what the class loader is expecting to find.

To give a concrete example, supposing that:

  • you want to run com.acme.example.Foon class,
  • the full file path is /usr/local/acme/classes/com/acme/example/Foon.class,
  • your current working directory is /usr/local/acme/classes/com/acme/example/,

then:

# wrong, FQN is needed
java Foon

# wrong, there is no `com/acme/example` folder in the current working directory
java com.acme.example.Foon

# wrong, similar to above
java -classpath . com.acme.example.Foon

# fine; relative classpath set
java -classpath ../../.. com.acme.example.Foon

# fine; absolute classpath set
java -classpath /usr/local/acme/classes com.acme.example.Foon

Notes:

  • The -classpath option can be shortened to -cp in most Java releases. Check the respective manual entries for java, javac and so on.
  • Think carefully when choosing between absolute and relative pathnames in classpaths. Remember that a relative pathname may "break" if the current directory changes.

Reason #2c - dependencies missing from the classpath

The classpath needs to include all of the other (non-system) classes that your application depends on. (The system classes are located automatically, and you rarely need to concern yourself with this.) For the main class to load correctly, the JVM needs to find:

(Note: the JLS and JVM specifications allow some scope for a JVM to load classes "lazily", and this can affect when a classloader exception is thrown.)

Reason #3 - the class has been declared in the wrong package

It occasionally happens that someone puts a source code file into the the wrong folder in their source code tree, or they leave out the package declaration. If you do this in an IDE, the IDE's compiler will tell you about this immediately. Similarly if you use a decent Java build tool, the tool will run javac in a way that will detect the problem. However, if you build your Java code by hand, you can do it in such a way that the compiler doesn't notice the problem, and the resulting ".class" file is not in the place that you expect it to be.

Still can't find the problem?

There lots of things to check, and it is easy to miss something. Try adding the -Xdiag option to the java command line (as the first thing after java). It will output various things about class loading, and this may offer you clues as to what the real problem is.

Also, consider possible problems caused by copying and pasting invisible or non-ASCII characters from websites, documents and so on. And consider "homoglyphs", where two letters or symbols look the same ... but aren't.

You may run into this problem if you have invalid or incorrect signatures in META-INF/*.SF. You can try opening up the .jar in your favorite ZIP editor, and removing files from META-INF until all you have is your MANIFEST.MF. However this is NOT RECOMMENDED in general. (The invalid signature may be the result of someone having injected malware into the original signed JAR file. If you erase the invalid signature, you are in infecting your application with the malware!) The recommended approach is to get hold of JAR files with valid signatures, or rebuild them from the (authentic) original source code.

Finally, you can apparently run into this problem if there is a syntax error in the MANIFEST.MF file (see https://stackoverflow.com/a/67145190/139985).


Alternative syntaxes for java

There are three alternative syntaxes for the launching Java programs using the java command.

  1. The syntax used for launching an "executable" JAR file is as follows:

    java [ <options> ] -jar <jar-file-name> [<arg> ...]
    

    e.g.

    java -Xmx100m -jar /usr/local/acme-example/listuser.jar fred
    

    The name of the entry-point class (i.e. com.acme.example.ListUser) and the classpath are specified in the MANIFEST of the JAR file.

  2. The syntax for launching an application from a module (Java 9 and later) is as follows:

    java [ <options> ] --module <module>[/<mainclass>] [<arg> ...]
    

    The name of the entrypoint class is either defined by the <module> itself, or is given by the optional <mainclass>.

  3. From Java 11 onwards, you can use the java command to compile and run a single source code file using the following syntax:

    java [ <options> ] <sourcefile> [<arg> ...]
    

    where <sourcefile> is (typically) a file with the suffix ".java".

For more details, please refer to the official documentation for the java command for the Java release that you are using.


IDEs

A typical Java IDE has support for running Java applications in the IDE JVM itself or in a child JVM. These are generally immune from this particular exception, because the IDE uses its own mechanisms to construct the runtime classpath, identify the main class and create the java command line.

However it is still possible for this exception to occur, if you do things behind the back of the IDE. For example, if you have previously set up an Application Launcher for your Java app in Eclipse, and you then moved the JAR file containing the "main" class to a different place in the file system without telling Eclipse, Eclipse would unwittingly launch the JVM with an incorrect classpath.

In short, if you get this problem in an IDE, check for things like stale IDE state, broken project references or broken launcher configurations.

It is also possible for an IDE to simply get confused. IDE's are hugely complicated pieces of software comprising many interacting parts. Many of these parts adopt various caching strategies in order to make the IDE as a whole responsive. These can sometimes go wrong, and one possible symptom is problems when launching applications. If you suspect this could be happening, it is worth trying other things like restarting your IDE, rebuilding the project and so on.


Other References

Answered   2023-09-21 08:09:32

  • I had this problem when I was trying to run a Class with a 3rd party library. I invoked java like this: java -cp ../third-party-library.jar com.my.package.MyClass; this does not work, instead it is necessary to add the local folder to the class path as well (separated by :, like this: java -cp ../third-party-library.jar:. com.my.package.MyClass, then it should work - anyone
  • After years of java programming I still managed to end up on this page. For me the issue was that the classpath syntax is OS-dependent. I'm kind of new to programming on Windows and had no idea. - anyone
  • Additional notes, point 2 save me! It is sad to see that java does not say it does not find an imported class, but instead the main class you're trying to run. This is misleading, although I'm sure there's a reason for that. I had the case where java knew exactly where my class is, however it couldn't find one of the imported classes. Instead of saying that, it complained about not finding my main class. Really, annoing. - anyone
  • I had this problem twice in Eclipse. First time the signature of main() was wrong. Second time I have renamed a .jar, and even though I added the new one to the build path, Eclipse didn't find the old one, so the project didn't compile, with this error. I had to remove the .jar file from Project > Properties > Java Build Path > Libraries. - anyone
  • I've encountered it a third time. I've run the program from a Windows 10 batch file, and put the .jar name in a variable (called with "-cp %jarname%;lib*"). I've mistakenly put an extra space at the end of the jarname, and it caused the error. Hat trick :) - anyone

If your source code name is HelloWorld.java, your compiled code will be HelloWorld.class.

You will get that error if you call it using:

java HelloWorld.class

Instead, use this:

java HelloWorld

Answered   2023-09-21 08:09:32

  • The problem is that this solution only works for Java classes declared in the default package with no JAR file dependencies. (And even then, not all of the time.) Most Java programs are not that simple. - anyone
  • like Stephen said, this only works with "default package" - which means no package declaration at the top of the file. For a quick test of some code, I did: javac TestCode.java followed by java TestCode - anyone
  • This did not work for me. It still says, "Could not find or load main class HelloWorld" - anyone
  • I needed to to do java -classpath . HelloWorld - anyone
  • @ChrisPrince - Yes ... that works ... sometimes. To understand when it works, and when it doesn't work, read the top-voted answer. - anyone

If your classes are in packages then you have to cd to the root directory of your project and run using the fully qualified name of the class (packageName.MainClassName).

Example:

My classes are in here:

D:\project\com\cse\

The fully qualified name of my main class is:

com.cse.Main

So I cd back to the root project directory:

D:\project

Then issue the java command:

java com.cse.Main

This answer is for rescuing newbie Java programmers from the frustration caused by a common mistake. I recommend you read the accepted answer for more in depth knowledge about the Java classpath.

Answered   2023-09-21 08:09:32

  • This answer makes a whole load of assumptions. And there are other ways to achieve this. Instead of blindly following the above advice, I would recommend that people take the time to read the links in my Answer that explain how the Java classpath works. It is better to UNDERSTAND what you are doing ... - anyone
  • This answer makes the exact assumptions that I needed :) I was located in the directory of the .class file and java.exe wasn't working. Once I cd-ed above and ran with the package name included in the command line it worked. - anyone
  • I agree with Nick Constantine. Same here. It was an exact example that followed the steps I made, and it worked for me too. The java classpath handling has a certain logic, but I sure wouldn't have designed it that way. - anyone
  • Tnanks, your answer help me to look that i used Main with Uppercase _! - anyone
  • I'm writing this comment after trying this but it didn't work. I've a project named Helloworld which consist only 1 java file, Helloworld/src/com/firstpackage/Test.java (windows 10. intellij idea). I don't have CLASSPATH environmental variable, i want to set classpath specifically for this project. running java com.firstpackage.Test inside Helloworld directory doesn't work and neither does the command java -classpath C:\Users\matuagkeetarp\IdeaProjects\Helloworld\src\com\first‌​package Test.java set classpath variable. Can you help? - anyone

With keyword 'package'

If you have a package keyword in your source code (the main class is defined in a package), you should run it over the hierarchical directory, using the full name of the class (packageName.MainClassName).

Assume there is a source code file (Main.java):

package com.test;

public class Main {

    public static void main(String[] args) {
        System.out.println("salam 2nya\n");
    }
}

For running this code, you should place Main.Class in the package like directory:

C:\Users\workspace\testapp\com\test\Main.Java

Then change the current directory of the terminal to the root directory of the project:

cd C:\Users\workspace\testapp

And finally, run the code:

java com.test.Main

Without keyword 'package'

If you don't have any package on your source code name maybe you are wrong with the wrong command. Assume that your Java file name is Main.java, after compile:

javac Main.java

your compiled code will be Main.class

You will get that error if you call it using:

java Main.class

Instead, use this:

java Main

Answered   2023-09-21 08:09:32

  • See "Additional Notes #1" of my Answer. For a better explanation of this problem. - anyone
  • @StephenC Yes, your answer is more complete (and of course, +1), but this particular answer had the word "package" in it, which allowed me to find what I needed fast. And it worked. So +1 Razavi. StephenC, yours lacks the simple package example I needed as I am new to Java. - anyone
  • This was exactly my problem. I have been wading through tons of Java doc and this concrete example is what I needed - anyone
  • Yes, concrete example is nice, this worked perfectly. I'm sure the main answer is very thorough, but it was difficult to see the tree for the forest. Nice one @Razavi - anyone
  • I like this shorter and useful answer instead of accepted one! - anyone

When the same code works on one PC, but it shows the error in another, the best solution I have ever found is compiling like the following:

javac HelloWorld.java
java -cp . HelloWorld

Answered   2023-09-21 08:09:32

  • This is not a good recommendation. You are depending on the CLASSPATH environment variable being unset, or having a value that is consistent with ".". Yes, it works in many cases, but it won't in others. - anyone
  • Well certainly javac -classpath . HelloWorld.java would have worked! And that is a better solution in your case. - anyone
  • If you have 'package com.some.address' as a first line - this will not work. You will need to comment out 'package address'.. - anyone
  • @Joe - That hack (commenting out the package) will work (in some cases) but it is a bad idea. A better idea is to learn / understand what caused the problem and implement the correct solution. - anyone
  • unsetting the classpath variable basically worked for me. - anyone

Specifying the classpath on the command line helped me. For example:

  1. Create a new folder, C:\temp

  2. Create file Temp.java in C:\temp, with the following class in it:

     public class Temp {
         public static void main(String args[]) {
             System.out.println(args[0]);
         }
     }
    
  3. Open a command line in folder C:\temp, and write the following command to compile the Temp class:

     javac Temp.java
    
  4. Run the compiled Java class, adding the -classpath option to let JRE know where to find the class:

     java -classpath C:\temp Temp Hello!
    

Answered   2023-09-21 08:09:32

  • In Ubuntu, I also had to specify the path. Don't understand why it can't use the Current Working Directory by default. I'm convinced that Java is sponsored by Keyboard manufacturers!! - anyone
  • @gone - The reason that "." is not in $PATH by default is that it is a security trap. seas.upenn.edu/cets/answers/dot-path.html - anyone
  • Thanks a lot for this......even though not sure why java was not being able to find out the classpath even after setting it in environment variables. - anyone
  • @akash89 - The most likely reasons were: 1) java wasn't looking at $CLASSPATH (because you used -classpath or -jar) or 2) the classpath setting was not set in the environment that was not in effect in the context that java was run; e.g. because you didn't "source" the file where added the setenv commands in the right shell. - anyone
  • I still got Error: Could not find or load main class Temp could anybody help! - anyone

According to the error message ("Could not find or load main class"), there are two categories of problems:

  1. The Main class could not be found
  2. The Main class could not be loaded (this case is not fully discussed in the accepted answer)

The Main class could not be found when there is a typo or wrong syntax in the fully qualified class name or it does not exist in the provided classpath.

The Main class could not be loaded when the class cannot be initiated. Typically the main class extends another class and that class does not exist in the provided classpath.

For example:

public class YourMain extends org.apache.camel.spring.Main

If camel-spring is not included, this error will be reported.

Answered   2023-09-21 08:09:32

  • "Basically" there are lots of other categories too. And the missing superclass problem is a very unusual subcase. (So unusual that I've never seen it ... in questions asked on this site.) - anyone
  • There are TWO because the error says "Could not FIND or LOAD main class". If there are other categories, please advise me. I've seen it, so just want to share it here maybe someone else will need it. - anyone
  • I would have revised it to something like "You need to included all of the classes that is required to initiate the main class to avoid this particular error". I am not trying to convince you. It is just a way that I would like to see. I left the answer here just for people who might like reading things in this way. Let's not extend this discussion further :) I changed my statement to "not fully discussed in the accepted answer" and hope you feel better. - anyone
  • This information is crucial and deserves an explicit mention (this is the only answer that mentions extends). I've just learned the hard way that when main class fails to load because it extends another that could not be found, java does not report which actual class was not found (unlike NoClassDefFoundError). So yes it does happen, and it's a hair-pulling situation when you don't know this. - anyone
  • In this situation is there any way to tell exactly which dependency class is failing to load? - anyone

Use this command:

java -cp . [PACKAGE.]CLASSNAME

Example: If your classname is Hello.class created from Hello.java then use the below command:

java -cp . Hello

If your file Hello.java is inside package com.demo then use the below command

java -cp . com.demo.Hello

With JDK 8 many times it happens that the class file is present in the same folder, but the java command expects classpath and for this reason we add -cp . to take the current folder as reference for classpath.

Answered   2023-09-21 08:09:32

  • This only works in simple cases. More complicated cases require a more complicated classpath. - anyone
  • And for >>really<< simple cases, -cp . is unnecessary, because if $CLASSPATH is unset, then . is the default classpath. - anyone
  • No Stephen, many times in Windows default classpath does not work. I tried it out on three different machines, you can try that out as well. - anyone
  • That is probably because you have actually set the %CLASSPATH% environment variable somewhere. If you do that, then you are not using the default classpath. (What does echo %CLASSPATH% output?) And no, I can't check because I don't have a Windows PC. - anyone
  • This worked for me when I tried to run a simple program from command line - anyone

I had such an error in this case:

java -cp lib.jar com.mypackage.Main

It works with ; for Windows and : for Unix:

java -cp lib.jar; com.mypackage.Main

Answered   2023-09-21 08:09:32

  • Yes. That's most likely because your Main is not in the JAR file. -cp lib.jar; means the same thing as -cp lib.jar;. i.e. the current directory is included on the classpath. - anyone
  • Finally fixed the issue for unix .. thanks (works with :) - anyone

Try -Xdiag.

Steve C's answer covers the possible cases nicely, but sometimes to determine whether the class could not be found or loaded might not be that easy. Use java -Xdiag (since JDK 7). This prints out a nice stacktrace which provides a hint to what the message Could not find or load main class message means.

For instance, it can point you to other classes used by the main class that could not be found and prevented the main class to be loaded.

Answered   2023-09-21 08:09:32

Sometimes what might be causing the issue has nothing to do with the main class, and I had to find this out the hard way. It was a referenced library that I moved, and it gave me the:

Could not find or load main class xxx Linux

I just deleted that reference, added it again, and it worked fine again.

Answered   2023-09-21 08:09:32

  • It sounds like the problem was that you had a incorrect classpath due to a broken "reference" in your project in your IDE. I'll update my answer to cover that case. - anyone
  • @StephenC and EduardoDennis, It was that here too, there was a jar missing, that jar contained an interface that the main class depended on to be instantiated. So, the error message is too broad. I should say "could not find" if the class file is not found and "could not load (missing dependencies)" if there is something else missing but not the file itself, so the error message being too broad is missleading if you focus only on the "find" part of it :( - anyone
  • @AquariusPower - There should have been an additional "caused by" stacktrace for the "cause" exception that said wht class was missing. If you want to suggest to the Java developers that they change an error message that has been saying that for 20+ years ... feel free. (I think that the error message is correct. The problem was that >>you<< narrowed in on the wrong clause.) - anyone
  • @StephenC what I meant is, they surely have access to the information if the main class file is available or not, so why not show us a better error message saying that such file is missing. In the other hand, they could also say "The file was found but could not be loaded" at that point, we would promptly focus on dependencies instead of losing half a day researching and testing things to understand. Just that I meant :). They may do it in a limited way for 20+ years, but they can improve it and we are here to grant that will happen thru our criticism and complaints! :D - anyone
  • Please understand what >>I<< meant. Complaining about it in some obscure comment on 3 year old Q&A is not going to achieve anything. The people who might concievably act on your complaints won't notice it. If you want to do something constructive, submit a patch. (I don't rate your chances, but they will be greater than if you just whinge about it.) - anyone

I had same problem and finally found my mistake :) I used this command for compiling and it worked correctly:

javac -cp "/home/omidmohebbi/AAAATest/jars/core-1.7.jar:/home/omidmohebbi/AAAATest/jars/javase-1.7.jar:/home/omidmohebbi/AAAATest/jars/qrgen-1.2.jar" qrcode.java

But this command did not work for me (I could not find or load the main class, qrcode):

java -cp "/home/omidmohebbi/AAAATest/jars/core-1.7.jar:/home/omidmohebbi/AAAATest/jars/javase-1.7.jar:/home/omidmohebbi/AAAATest/jars/qrgen-1.2.jar" qrcode

Finally I just added the ':' character at end of the classpath and the problem was solved:

java -cp "/home/omidmohebbi/AAAATest/jars/core-1.7.jar:/home/omidmohebbi/AAAATest/jars/javase-1.7.jar:/home/omidmohebbi/AAAATest/jars/qrgen-1.2.jar:" qrcode

Answered   2023-09-21 08:09:32

In this instance you have:

Could not find or load main class ?classpath

It's because you are using "-classpath", but the dash is not the same dash used by java on the command prompt. I had this issue copying and pasting from Notepad to cmd.

Answered   2023-09-21 08:09:32

  • Wow! That is a totally bizarre cause! (But it serves you right for using Notepad instead of a real text editor :-) ) - anyone

If you use Maven to build the JAR file, please make sure to specify the main class in the pom.xml file:

<build>
    <plugins>
        <plugin>
            <artifactId>maven-jar-plugin</artifactId>
            <configuration>
                <archive>
                    <manifest>
                        <mainClass>class name us.com.test.abc.MyMainClass</mainClass>
                    </manifest>
                </archive>
            </configuration>
        </plugin>
    </plugins>
</build>

Answered   2023-09-21 08:09:32

In my case, the error appeared because I had supplied the source file name instead of the class name.

We need to supply the class name containing the main method to the interpreter.

Answered   2023-09-21 08:09:32

  • Yes. See my example #2 of the wrong ways to specify the class name!! - anyone

All answers here are directed towards Windows users it seems. For Mac, the classpath separator is :, not ;. As an error setting the classpath using ; is not thrown then this can be a difficult to discover if coming from Windows to Mac.

Here is corresponding Mac command:

java -classpath ".:./lib/*" com.test.MyClass

Where in this example the package is com.test and a lib folder is also to be included on classpath.

Answered   2023-09-21 08:09:32

  • On Linux just as on Mac. - anyone
  • Why /* is necessary? - anyone
  • It is a wildcard syntax. (It is not mandatory. You can explicitly list the JARs if you want to.) - anyone

This might help you if your case is specifically like mine: as a beginner I also ran into this problem when I tried to run a Java program.

I compiled it like this:

javac HelloWorld.java

And I tried to run also with the same extension:

java Helloworld.java

When I removed the .java and rewrote the command like java HelloWorld, the program ran perfectly. :)

Answered   2023-09-21 08:09:32

  • This is because you are executing the compiled version of your .java. It is actually executing the .class file - anyone
  • For the record, this is the same as Reason #1, Example #5 in my Answer ... - anyone

I thought that I was somehow setting my classpath incorrectly, but the problem was that I typed:

java -cp C:/java/MyClasses C:/java/MyClasses/utilities/myapp/Cool  

instead of:

java -cp C:/java/MyClasses utilities/myapp/Cool   

I thought the meaning of fully qualified meant to include the full path name instead of the full package name.

Answered   2023-09-21 08:09:32

  • I've updated my answer to try to address that confusion. - anyone
  • Neither of these is correct. The class must be given as utilities.myapp.Cool or whatever its package name is, if any. - anyone

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Class file location: C:\test\com\company

File Name: Main.class

Fully qualified class name: com.company.Main

Command line command:

java  -classpath "C:\test" com.company.Main

Note here that class path does not include \com\company.

Answered   2023-09-21 08:09:32

When running the java with the -cp option as advertised in Windows PowerShell you may get an error that looks something like:

The term `ClassName` is not recognized as the name of a cmdlet, function, script ...

In order to for PowerShell to accept the command, the arguments of the -cp option must be contained in quotes as in:

java -cp 'someDependency.jar;.' ClassName

Forming the command this way should allow Java process the classpath arguments correctly.

Answered   2023-09-21 08:09:32

On Windows put .; at the CLASSPATH value in the beginning.

The . (dot) means "look in the current directory". This is a permanent solution.

Also you can set it "one time" with set CLASSPATH=%CLASSPATH%;.. This will last as long as your cmd window is open.

Answered   2023-09-21 08:09:32

  • This advice may or may not help. It will help if the class tree containing the classes in the current directory. It won't if they are not. I actually wouldn't do this. Instead I would create a one-liner wrapper script that worked whether or not the user is in the "right" directory. - anyone

This is a specific case:

Windows (tested with Windows 7) doesn't accept special characters (like á) in class and package names. Linux does, though.

I found this out when I built a .jar in NetBeans and tried to run it in command line. It ran in NetBeans, but not on the command line.

Answered   2023-09-21 08:09:32

What fixed the problem in my case was:

Right click on the project/class you want to run, and then Run AsRun Configurations. Then you should either fix your existing configuration or add a new one in the following way:

Open the Classpath tab, click on the Advanced... button, and then add bin folder of your project.

Answered   2023-09-21 08:09:32

First set the path using this command;

set path="paste the set path address"

Then you need to load the program. Type "cd (folder name)" in the stored drive and compile it. For Example, if my program stored on the D drive, type "D:" press enter and type " cd (folder name)".

Answered   2023-09-21 08:09:32

  • This does not help. This Question is about Java programs, not ordinary executables. Java does not use PATH to locate anything, and if "cd" helps then it by luck rather than by judgement. - anyone
  • if "cd" helps then it by luck rather than by judgement. This is wrong (I believe), as java uses the current directory . as part of the classpath by default. - anyone
  • @GKFX - That's what I mean. Unless you know that you are using the default classpath (or a classpath with "." on it), "cd" will have no effect. This solution works more by luck (i.e. guessing / hoping that "." is on the classpath) than by judgement (i.e. checking that "." is on the classpath). Furthermore you are incorrect about the default. Java uses "." as the classpath by default, not as part of the classpath by default. - anyone
  • On Windows, presumably? - anyone

In Java, when you sometimes run the JVM from the command line using the Java interpreter executable and are trying to start a program from a class file with public static void main (PSVM), you might run into the below error even though the classpath parameter to the JVM is accurate and the class file is present on the classpath:

Error: main class not found or loaded

This happens if the class file with PSVM could not be loaded. One possible reason for that is that the class may be implementing an interface or extending another class that is not on the classpath. Normally if a class is not on the classpath, the error thrown indicates as such. But, if the class in use is extended or implemented, Java is unable to load the class itself.

Reference: https://www.computingnotes.net/java/error-main-class-not-found-or-loaded/

Answered   2023-09-21 08:09:32

  • Did you read the accepted answer? Does your answer add anything new? - anyone
  • @StephenC I've tried to find a reason in your list by looking at "Reason" categories and their points. I couldn't find the matching point in "Reason #1" and "Reason #2" title didn't look close to my case (because I was sure there is no problem with classpath itself). I've found the reason by performing experiments and I was surprised that in my case "main class not found" error was shown because implementing interface was not on the class path. Sure you can say "you should read everything described in the post" but it seems to me your reason list can be improved. - anyone
  • The link is broken: "Hmm. We’re having trouble finding that site. We can’t connect to the server at www.computingnotes.net." - anyone

You really need to do this from the src folder. There you type the following command line:

[name of the package].[Class Name] [arguments]

Let's say your class is called CommandLine.class, and the code looks like this:

package com.tutorialspoint.java;

    /**
     * Created by mda21185 on 15-6-2016.
     */

    public class CommandLine {
        public static void main(String args[]){
            for(int i=0; i<args.length; i++){
                System.out.println("args[" + i + "]: " + args[i]);
            }
        }
    }

Then you should cd to the src folder and the command you need to run would look like this:

java com.tutorialspoint.java.CommandLine this is a command line 200 -100

And the output on the command line would be:

args[0]: this
args[1]: is
args[2]: a
args[3]: command
args[4]: line
args[5]: 200
args[6]: -100

Answered   2023-09-21 08:09:32

  • A class cannot be called "CommandLine.class". That would be a Java syntax error. (What you mean is that the file containing the compiled class is called "CommandLine.class" ... ). The other problem is that your instruction to "cd to the source directory" only works if you compiled the code >>into<< the source directory tree. Finally, if your compile used a "-cp" argument, then you need an equivalent on when you run. - anyone
  • In my project I have the src folder and bin folder at the root. I had to cd into src and then run the command java ../bin com.blah.blah.MyClass which worked for me. So thanks for the tip! - anyone

I also faced similar errors while testing a Java MongoDB JDBC connection. I think it's good to summarize my final solution in short so that in the future anybody can directly look into the two commands and are good to proceed further.

Assume you are in the directory where your Java file and external dependencies (JAR files) exist.

Compile:

javac -cp mongo-java-driver-3.4.1.jar JavaMongoDBConnection.java
  • -cp - classpath argument; pass all the dependent JAR files one by one
  • *.java - This is the Java class file which has main method. sdsd

Run:

java -cp mongo-java-driver-3.4.1.jar: JavaMongoDBConnection
  • Please do observe the colon (Unix) / comma (Windows) after all the dependency JAR files end
  • At the end, observe the main class name without any extension (no .class or .java)

Answered   2023-09-21 08:09:32

  • This all assumes that 1) JavaMongoDBConnection has no package, and 2) you don't change directory. It is, to say the least, fragile. And by not explaining the issues, it will lead newbies to try this approach in situations where it won't work. In short, it encourages "voodoo programming techniques": en.wikipedia.org/wiki/Voodoo_programming - anyone

All right, there are many answers already, but no one mentioned the case where file permissions can be the culprit.

When running, a user may not have access to the JAR file or one of the directories of the path. For example, consider:

Jar file in /dir1/dir2/dir3/myjar.jar

User1 who owns the JAR file may do:

# Running as User1
cd /dir1/dir2/dir3/
chmod +r myjar.jar

But it still doesn't work:

# Running as User2
java -cp "/dir1/dir2/dir3:/dir1/dir2/javalibs" MyProgram
Error: Could not find or load main class MyProgram

This is because the running user (User2) does not have access to dir1, dir2, or javalibs or dir3. It may drive someone nuts when User1 can see the files, and can access to them, but the error still happens for User2.

Answered   2023-09-21 08:09:32

I was unable to solve this problem with the solutions stated here (although the answer stated has, no doubt, cleared my concepts). I faced this problem two times and each time I have tried different solutions (in the Eclipse IDE).

  • Firstly, I have come across with multiple main methods in different classes of my project. So, I had deleted the main method from subsequent classes.
  • Secondly, I tried following solution:
    1. Right click on my main project directory.
    2. Head to source then clean up and stick with the default settings and on Finish. After some background tasks you will be directed to your main project directory.
    3. After that I close my project, reopen it, and boom, I finally solved my problem.

Answered   2023-09-21 08:09:32

  • Deleting main methods won't fix the problem. There is nothing technically wrong with an application that has multiple entry points. - anyone

Sometimes, in some online compilers that you might have tried you will get this error if you don't write public class [Classname] but just class [Classname].

Answered   2023-09-21 08:09:32

  • Examples (of offending compilers) please. While it is conventional / normal to make an "entry point" class public, the Java specs don't require this, and neither does the standard Oracle / OpenJDK java command. - anyone