Develop Reference

When I try to run the application jar file, I get the error: Could not find or load main class [duplicate]

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:
Search for the compiled version of the com.acme.example.ListUsers class.
Load the class.
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.)
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:
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.)
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.
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.
Check that the class (mentioned in the error message) can be located on the effective classpath.
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 Java class is present in classpath but startup fails with Error: Could not find or load main class)
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.
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.
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>.
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:
Create a new folder, C:\temp
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]);
}
}
Open a command line in folder C:\temp, and write the following command to compile the Temp class:
javac Temp.java
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:
The Main class could not be found
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.

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

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.

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.

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>

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.

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.

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.

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.

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

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 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)

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:
Right click on my main project directory.
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.
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].

Could not find or load main class swing.ComboDemo Caused by: java.lang.ClassNotFoundException: swing.ComboDemo on this code [duplicate]

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:
Search for the compiled version of the com.acme.example.ListUsers class.
Load the class.
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.)
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:
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.)
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.
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.
Check that the class (mentioned in the error message) can be located on the effective classpath.
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 Java class is present in classpath but startup fails with Error: Could not find or load main class)
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.
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.
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>.
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:
Create a new folder, C:\temp
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]);
}
}
Open a command line in folder C:\temp, and write the following command to compile the Temp class:
javac Temp.java
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:
The Main class could not be found
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.

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

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.

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.

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>

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.

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.

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.

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.

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

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 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)

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:
Right click on my main project directory.
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.
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].

Error executing Java jar file : Could not find or load main class A [duplicate]

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:
Search for the compiled version of the com.acme.example.ListUsers class.
Load the class.
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.)
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:
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.)
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.
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.
Check that the class (mentioned in the error message) can be located on the effective classpath.
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 Java class is present in classpath but startup fails with Error: Could not find or load main class)
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.
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.
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>.
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:
Create a new folder, C:\temp
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]);
}
}
Open a command line in folder C:\temp, and write the following command to compile the Temp class:
javac Temp.java
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:
The Main class could not be found
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.

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

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.

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.

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>

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.

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.

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.

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.

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

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 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)

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:
Right click on my main project directory.
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.
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].

Java package issue

I am trying to import a package found at /home/jirwin/ptplot5.8/ptolemy/plot/plot.jar. I am using import ptolemy.plot.* and compiling with javac -cp /home/jirwin/ptplot5.8/ptolemy/plot/plot.jar The Class.java. When I run (using java -cp ...same... TheClass) I get Error:Could not find or create main class TheClass.
When I take away the -cp from the java call the Could not find or create error goes away...
I know this must be something simple but I can't figure this out!

If you specify that the classpath is a single jar files -- as you seem to be doing -- then Java won't find any classes outside of that jar file. You need your classpath to include both the jar file and the location of your compiled classes. You can use "." to mean the current directory; i.e.,
java -cp .:/home/jirwin/ptplot5.8/ptolemy/plot/plot.jar TheClass
Note the "dot colon" prepended to the beginning of the classpath.

You need to put your full package name in front of your .class when you run it with java. Otherwise it looks in the wrong place, or something. (I don't understand java well enough to give you the "why" but it's the idea.)
java -cp /home/jirwin/ptplot5.8/ptolemy/plot/plot.jar {package}.TheClass

Can you run a .class file from terminal that is outputted by an IDE

I am trying to run a file from command line. The file is a .class file and is apart of a larger project that I compiled in Netbeans. I navigated to the .class file and ran
java MyFile
And I got:
Exception in thread "main" java.lang.NoClassDefFoundError: PersonTest/class
Caused by: java.lang.ClassNotFoundException: PersonTest.class
at java.net.URLClassLoader$1.run(URLClassLoader.java:217)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:205)
at java.lang.ClassLoader.loadClass(ClassLoader.java:321)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:294)
at java.lang.ClassLoader.loadClass(ClassLoader.java:266)
Could not find the main class: PersonTest.class. Program will exit
Whats up with that? (I should mention that i'm running ubuntu)

You need to check this useful link java - the Java application launcher:
By default, the first non-option
argument is the name of the class to
be invoked. A fully-qualified class
name should be used
So, you have to write the full qualified name of the class (this includes the package name).
So, the right way to execute your command is this (from the root dir where your class files are stored):
> java my.package.MyFile
Also, make sure to include all the needed dependencies at the classpath (-cp) argument (check the referenced link).
UPDATE: to include a classpath setting example:
java -classpath C:\MyProject\classes;C:\MyProject\lib\utility.jar my.package.MyFile
With this, the java runtime will search for the classes at the C:\MyProject\classes directory, and at the C:\MyProject\lib\utility.jar JAR file. You'll need not only your class direct dependencies, but the dependencies needed by the referenced files (the whole tree).

The answer appears to be in this line:
Exception in thread "main" java.lang.NoClassDefFoundError: PersonTest/class
It means you didn't type:
java MyFile
as you said in your original post, you typed
java PersonTest.class
you should have typed
java PersonTest

Yes you can, they are compiled by a java compiler. If you have the right version of the jvm (often other versions work aswell) than it can be run. The information about your error is not enough to tell what went wrong.
Your probably in the wrong folder, mistyped the classname, used a class in your code that couldn't be found, etc.

Unless your class is entirely standalone (i.e. only references java.lang classes like String), you'll need to add other classes/JARs to the classpath when you invoke Java.
The NoClassDefFoundError (which usually states the name of the class by the way, and always includes a stacktrace) indicates that an external class that was available when your class was compiled, is not available on the classpath at runtime.
EDIT based on update:
You're invoking your process incorrectly. You don't need to append the .class suffix of the file - doing so makes Java look for a file class class in a subpackage.
(P.S. you said you ran java MyFile. That's a lie, you actually ran java PersonTest.class. If you'd noted that to start with, it would have made it much easier for people to answer the question!)

Just consider this example
say I already have a folder src and I wrote in my notepad
package test.oye;
class testclass {
static public void main (String [] args)
{
int a=3;
System.out.println(a);
}
}
then what go to src folder and you ,yourself create a folder named test and inside it oye . Then put your .java file in it . Then cd src/test/oye only(in Command prompt or terminal).From there itself
javac testclass.java
cd src
java test.oye.testclass
This will work for sure.
If you don’t want to put .java file there … then just compile your .java file and get the .class file .
Now create the test folder and then oye inside it ….and put .class file inside it ….
Now go back to src …and then type
java test.oye.testclass;

according to terminal ide, android requires classes in DEX format when running them.
Try:
dx --dex --output=practice.jar practice.class
Then run using this:
java -jar practice.jar practice