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

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?

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:

• The java command documentation
• Setting the Classpath.
• The Java Tutorial - PATH and CLASSPATH

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:

• the class itself.
• all classes and interfaces in the superclass hierarchy (e.g. see https://stackoverflow.com/questions/42880748)
• all classes and interfaces that are referred to by means of variable or variable declarations, or method call or field access expressions.

(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

• From the Oracle Java Tutorials - Common Problems (and Their Solutions)

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

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.}

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

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

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!
   

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.

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.

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.

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

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.

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

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.

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.

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>

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. :)

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.

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.

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.

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.

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.

What fixed the problem in my case was:

Right click on the project/class you want to run, and then Run As → Run 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.

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.

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)".

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/

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

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)

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.

I got this error after doing mvn eclipse:eclipse. This messed up my .classpath file a little bit.

I had to change the lines in .classpath from

<classpathentry kind="src" path="src/main/java" including="**/*.java"/>
<classpathentry kind="src" path="src/main/resources" excluding="**/*.java"/>

to

<classpathentry kind="src" path="src/main/java" output="target/classes" />
<classpathentry kind="src" path="src/main/resources" excluding="**"  output="target/classes" />

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.

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].

In my case, I got the error because I had mixed UPPER- and lower-case package names on a Windows 7 system. Changing the package names to all lower case resolved the issue. Note also that in this scenario, I got no error compiling the .java file into a .class file; it just wouldn't run from the same (sub-sub-sub-) directory.

I had a weird one:

> Error: Could not find or load main class mypackage.App

It turned out I had a reference to POM (parent) coded up in my project's pom.xml file (my project's pom.xml was pointing to a parent pom.xml) and the relativePath was off/wrong.

Below is a partial of my project's pom.xml file:

<parent>
    <groupId>myGroupId</groupId>
    <artifactId>pom-parent</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <relativePath>../badPathHere/pom.xml</relativePath>
</parent>

Once I resolved the POM relativePath, the error went away.

Go figure.

Here's another issue that took me a bit of time: The command line class path param doesn't behave as you'd expect. I'm on MacOS calling the CLI directly, and I'm including two jars in the call.

For example, both of these were confusing the tool about the name of the main class:

This one because the asterisk was causing it to parse the args incorrectly:

java -cp path/to/jars/* com.mypackage.Main

And this one because -- I'm not sure why:

java -cp "*.jar" com.mypackage.Main

This worked:

java -cp "path/to/jars/*" com.mypackage.Main

Listing the two jars explicitly also worked:

java -cp path/to/jars/jar1.jar:path/to/jars/jar2.jar com.mypackage.Main

After searching for 2 days I found this solution and this works. It is pretty weird but it works for me.

package javaapplication3;
public class JavaApplication3 {

/**
 * @param args the command line arguments
 */
public static void main(String[] args) {
    // TODO code application logic here
    System.out.println("Hello");
}

}

this is my program i want to run that locates at C:\Java Projects\JavaApplication3\src\javaapplication3

Now open cmd on this location and compile program using this command

javac JavaApplication3.java

After compiling navigate one directory down i.e. C:\Java Projects\JavaApplication3\src

now run following command to execute program

java javaapplication3.JavaApplication3

Scenario: using command prompt(CMD in Windows) for compile and run a simple 'java' program which have only 'Main.java' file, with specified 'package main'.

Source file path :

> some-project-name-folder\src\main\Main.java

Destination folder :

> some-project-name-folder\dest

Destination file path (folder '\main' and file '\Main.class' will be produced by 'javac') :

> some-project-name-folder\dest\main\Main.class

Main.java is as follow :

package main;

public class Main {
    public static void main(String[] args) {
        System.out.println("Hello world");
    }
}

Compilation :

// 'javac' compiler will produce 'Main.class' in the 'dest\main' folder.
// 'main' folder is created because in the source file(in our case: 'Main.java') is
// specified 'package main'.

javac -d ./dest ./src/main/Main.java

Run compiled file (in our case: 'Main.class') :

// '-cp'(is the same as '-classpath')
// './dest'(means destination folder, where resides compiled 'Main.class').
// 'main.Main'(means 'package' 'main', which contains class 'Main'('Main.class'))
// WARNING: when run 'java' class, MUST NOT type extension '.class'
//          after 'class name
//          (in our case: 'main.Main'(<package>.<class-name>) WITHOUT extension 
//           '.class').

java -cp ./dest main.Main

// Hello world

I came one level up. So, now the HelloWorld.class file is in hello\HelloWorld.class and I ran the below command. Where cp is classpath and . means check in current directory only.

java -cp . hello.HelloWorld

Output

Hello world!

In the context of IDE development (Eclipse, NetBeans or whatever) you have to configure your project properties to have a main class, so that your IDE knows where the main class is located to be executed when you hit "Play".

1. Right click your project, then Properties
2. Go to the Run category and select your Main Class
3. Hit the Run button.

If this issue is Eclipse-related:

Try adding the project to your class path.

See the below image:

This method worked for me.

This happened to me too. In my case, it only happened when a HttpServlet class was present in source code (IntelliJ IDEA didn't give a compile time error; the servlet package got imported just fine, however at run time there was this main class error).

I managed to solve it. I went to menu File → Project Structure...:

Then to Modules:

There was a Provided scope near the servlet module. I changed it to Compile:

And it worked!

If you use IntelliJ and get the error while running the main method from the IDE, just make sure your class is located in java package, not in kotlin

[Java Version: 11]

If you are using Java 11 then you don't need to compile and run your java file.

Just run like

Java ClassName.java

Example:

class abc{ 
	public static void main(String[] args){
		System.out.println("hello Jarvis ");	
	}
}

Now Run the command

java abc.java

By default, Java uses ., the current working directory, as the default CLASSPATH. What this means is that when you type a command at the prompt e.g. java MyClass, the command is interpreted as if you had type java -cp . MyClass. Did you see that dot between -cp and MyClass? (cp is short for the longer classpath option)

This is sufficient for most cases and things seems to work just fine until at some time you try to add a directory to your CLASSPATH. In most cases when programmers need to do this, they just run a command like set CLASSPATH=path\to\some\dir. This command creates a new environment variable called CLASSPATH having the value path\to\some\dir or replaces its value with path\to\some\dir if CLASSPATH was already set before.

When this is done, you now have a CLASSPATH environment variable and Java no longer uses its default classpath (.) but the one you've set. So the next day you open your editor, write some java program, cd to the directory where you saved it, compile it, and try to run it with the command java MyClass, and you are greeted with a nice output: Could not find or load main class ... (If your commands were working well before and you are now getting this output, then this might be the case for you).

What happens is that when you run the command java MyClass, Java searches for the class file named MyClass in the directory or directories that you have set in your CLASSPATH and not your current working directory so it doesn't find your class file there and hence complains.

What you need to do is add . to your class path again which can be done with the command set CLASSPATH=%CLASSPATH%;. (notice the dot after the semicolon). In plain english this command says "Pick what was initially the value of CLASSPATH (%CLASSPATH%), add . to it (;.) and assign the result back to CLASSPATH".

And voila, you are once again able to use your command java MyClass as usual.

Solving "Could not Load main class error"

After reading all the answers, I noticed most didn't work for me. So I did some research and here is what I got. Only try this if step 1 doesn't work.

1. Try to install JRE 32 or 64. If it doesn't work,
2. Open go to C:\Program Files (x86)\Java or C:\Program Files\Java
   • i. open the jdk folder and then the bin folder.

• ii. Copy the path and add it to environment variables. Make sure you separate variables with a semi-colon, ;. For example, "C:\Yargato\bin;C:\java\bin;". If you don't, it will cause more errors.

• iii. Go to the jre folder and open its bin folder.

• iv. Here search for rt.jar file. Mine is:

  > C:\Program Files (x86)\Java\jre1.8.0_73\lib\rt.jar Copy and under environment variable and search for the classpath variable and paste it there.

• v. Now restart cmd and try running again. The error will disappear.

• vi. I will post a link to my YouTube video tutorial.

I got this issue for my demo program created in IntelliJ.

There are two key points to solve it:

1. the package name of program
2. the current working dir of the terminal/cmd prompt

my demo program:

package io.rlx.tij.c2;

public class Ex10 {
    public static void main(String[] args) {
        // do something
    }
}

the path of source code:

../projectRoot/src/main/java/io/rlx/tij/c2/Ex10.java

1. go to java dir: cd ../projectRoot/src/main/java
2. compile to class: javac ./io/rlx/tij/c2/Ex10.java
3. run program: java io.rlx.tij.c2.Ex10

if I run program in ../projectRoot/src/main/java/io/rlx/tij/c2 or I run it without package name, I will get this error: Error: Could not find or load main class.

• In IntelliJ IDEA, check your global libraries* and local libraries
• Check in the libraries version file pom.xml, Maybe it is an old library
• They are so many possibilities mentioned in the previous answers that need to be tried too

The simplest way you can fix it is by redownloading the Maven Apache here: https://maven.apache.org/download.cgi

Than you set your path again:

System variable

Put the path there where you set your Apache Maven folder like: C:\Program Files\apache-maven-3.8.4\bin

Restart the terminal or IDE, and it should work.

It always works for me.

Seems like when I had this problem, it was unique.

Once I removed the package declaration at the top of the file, it worked perfectly.

Aside from doing that, there didn't seem to be any way to run a simple HelloWorld.java on my machine, regardless of the folder the compilation happened in, the CLASSPATH or PATH, parameters or folder called from.

Right click the project.

1. Select "Open module settings"
2. Mark the src folder as "sources"
3. Go to edit configurations, and then select your main class
4. Click OK or the Apply button

This worked for me.

One more scenario that got me scratch my head, and I found no reference to it herein, is:

package com.me
Public class Awesome extends AwesomeLibObject {
    ....
    public static void main(String[] argv) {
	     System.out.println("YESS0");
    }
}

Where AwesomeLibObject is a class defined in an external lib. I got the same confusing error message for it:

Error: Could not find or load main class com.Awesome

The resolution is simple: the external lib must be in classpath as well!

Reason #2 - the application's classpath is incorrectly specified. Read the three documents linked previously. (Yes ... read them! It is important that a Java programmer understands at least the basics of how the Java classpath mechanisms works.) I want to add this documentation to this very good post from above.

JDK Tools and Utilities General General Information (file structure, classpath, how classes are found, changes) Enhancements (enhancements in JDK 7) Standard JDK Tools and Utilities

https://docs.oracle.com/javase/7/docs/technotes/tools/index.html

https://docs.oracle.com/javase/7/docs/technotes/tools/findingclasses.html

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

How the Java Launcher Finds Classes Understanding the class path and package names

https://docs.oracle.com/javase/7/docs/technotes/tools/solaris/javac.html

ClassLoader in Java The Java ClassLoader is a part of the Java Runtime Environment that dynamically loads Java classes into the Java Virtual Machine. The Java run time system does not need to know about files and file systems because of classloaders.

Java classes aren’t loaded into memory all at once, but when required by an application. At this point, the Java ClassLoader is called by the JRE and these ClassLoaders load classes into memory dynamically.

https://en.wikipedia.org/wiki/Java_Classloader

https://www.geeksforgeeks.org/classloader-in-java/

https://en.wikipedia.org/wiki/Java_virtual_machine

It basically means that there isn't a static main(String[] args) method. It should resolve it automatically. If it doesn't, something was screwed when the program was either made (it doesn't have a main method) or when the program was packaged (incorrect manifest information.)

This works

public class Hello {
    public static void main(String[] args) {
        System.out.println("Hello");
    }
}

This doesn't work

public class Hello {
    public Hello() {
        System.out.println("Hello");
    }
}

For a database connection I was getting this one. I just added the following in the class path:

export CLASSPATH=<path>/db2jcc4.jar:**./**

Here I appended ./ in last so that my class also get identified while loading.

Now just run:

java ConnectionExample <args>

It worked perfectly fine.

Answering with respect to an external library -

Compile:

javac -cp ./<external lib jar>: <program-name>.java

Execute:

java -cp ./<external lib jar>: <program-name>

The above scheme works well in OS X and Linux systems. Notice the : in the classpath.

If it's a Maven project:

1. Go to the POM file.
2. Remove all the dependencies.
3. Save the POM file.
4. Again import only the necessary dependencies.
5. Save the POM file.

The issue should go away.

This is how I solved my issue.

I noticed if you are including jar files with your compilation, adding the current directory (./) to the classpath helps.

javac -cp "abc.jar;efg.jar" MyClass.java
java -cp "abc.jar;efg.jar" MyClass

vs.

javac -cp "**./**;abc.jar;efg.jar" MyClass.java<br>
java -cp "**./**;abc.jar;efg.jar" MyClass

If your package name is javatpoint and the package name is com.javatpoint and the class name is AnotherOne, then compile your class like this type:

cd C:\Users\JAY GURUDEV\eclipse-workspace\javatpoint\src\com\javatpoint
javac AnotherOne.java

And run this class using

cd C:\Users\JAY GURUDEV\eclipse-workspace\javatpoint\src
java com.javatpoint.AnotherOne

(Here the package name should be excluded.)

In my case I just change the JRE in Eclipse.

Please find the attached screen shot:

Excluding the following files solved the problem.

META-INF/*.SF

META-INF/*.DSA

META-INF/*.RSA

Added the following code in build.gradle

jar {
    from {
        configurations.compile.collect { it.isDirectory() ? it : zipTree(it) }
    }
    {
        exclude "META-INF/*.SF"
        exclude "META-INF/*.DSA"
        exclude "META-INF/*.RSA"
    }
    manifest {
        attributes(
                'Main-Class': 'mainclass'
        )
    }
}