Error could not find or load main class main caused by java lang noclassdeffounderror

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 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.

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)

In some situations, while executing a Java program from the command prompt, we may face the error “Could not find or load main class”. This occurs mainly when the JVM fails to find the main class or .class file. Whenever we compile a Java code, the compiler will automatically create a .class file with the same name as the class name. This will be present in the same directory where we have the .java file.

Let us see a simple Java code and try to compile and execute it from the command prompt.

public class HelloJava {

  public static void main(String[] args) {
    System.out.println("Java programming language");

  }

}

First, we need to compile the code using the javac command.

D:Sample>javac HelloJava.java

This will produce the right result as below since we have compiled it correctly.

D:Sample>java HelloJava
Java programming language

Now let us see the different reasons or causes to generate the error “Could not load or find the main class in Java“.

Whenever we compile the java code, the compiler creates a .class file in the same name as the class name. After compilation, if we try to run the code by providing an incorrect file name in the java command, it will result in an error “Could not load or find the main class in Java“.

D:Sample>java hellojava
Error: Could not find or load main class hellojava
Caused by: java.lang.NoClassDefFoundError: HelloJava (wrong name: hellojava)

Since the class names “HelloJava” and “hellojava” are different, it could not find the right file to execute. The Java file names are case-sensitive during execution.

Could not find or load main class in Java: Incorrect file extension

During compilation, we provide the file extension as.java. In this case, it compiles and successfully generates the .class file. But for executing a java code, we do not need to provide the .class extension. In case we provide the .class as file extension, it will generate the below error “Could not find or load main class in Java”.

D:Sample>java HelloJava.class
Error: Could not find or load main class HelloJava.class
Caused by: java.lang.ClassNotFoundException: HelloJava.class

If we try to execute without the file extension, it will execute successfully.

D:Sample>java HelloJava
Java programming language

Package names

When we have multiple classes that we want to group together, we can group them under a single package. When we place a Java file inside a package, we need to specify the fully qualified name. This means we need to execute the file as packagename.filename.

Now, let us see how to execute the java code when we place the HelloJava.java file inside the package “javaDemo“. In the below example, we are already within the directory “javaDemo“. Hence the compiler tries to find the class file javaDemo.HelloJava and fails to find it. This will result in an error “Could not find or load main class in Java”.

D:SamplejavaDemo>java javaDemo.HelloJava
Error: Could not find or load main class javaDemo.HelloJava
Caused by: java.lang.ClassNotFoundException: javaDemo.HelloJava

Suppose we move one directory up. Now, it runs successfully. Whenever we execute a java file with a fully qualified name, we need to move one directory up.

D:Sample> java javaDemo.HelloJava
Java programming language

Could not find or load main class in Java: Invalid classpath

A classpath is a path that instructs the JVM where the .class files are present. We can also execute a Java file by specifying the classpath using –cp or -classpath option in the java command.

We can specify the classpath after the -cp command as below. Here HelloJava is present inside the directory javaDemo.

C:Usersnandieclipse-workspaceFirstJavaProjectsrc>java -cp ./javaDemo;. HelloJava
Java programming language

If we try to specify the classpath when we are already within the target directory, we will get the below error “Could not find or load main class in Java”. This is because it searching for the package javaDemo which is not present since we are already within the same directory.

C:Usersnandieclipse-workspaceFirstJavaProjectsrcjavaDemo>java -cp ./javaDemo;. HelloJava
Error: Could not find or load main class HelloJava
Caused by: java.lang.ClassNotFoundException: HelloJava

Reference

1. Could Not Find Error Due to Passing the Wrong Name in Java
2. Could Not Find Error Due to Wrong Package Name in Java
3. Could Not Find Error Due to Wrong CLASSPATH in Java

This tutorial introduces the could not find or load main class errors in Java.

Suppose we have written a code and compiled it. Till now, everything is working fine, but when we finally ran it, an error showed up.

could not find or load main class

This tutorial will discuss why this error occurs and how to resolve it. Let us first recap how we run a java program using the command prompt.

First, we compile the code using the javac command like below:

After executing the above command, A file with the .class extension gets created into the current folder.

The .class file will have the same class as the .java program. We then run the .class file using the following command to execute the Java code:

We may get the could not find or load main class error. This error is a runtime error and occurs when the Java Virtual machine cannot locate the main class (class containing the main method) we are trying to run.

This error most commonly occurs when running our Java programs using the command prompt. Before discussing the causes of this error, let us first understand CLASSPATH.

CLASSPATH in Java

This is the executable.class and other resource files.

The JVM uses it to locate the files. The default CLASSPATH is the current directory unless we explicitly set the CLASSPATH in the system variables.

To run a program, we need to pass the class name. We take the following example to illustrate the point:

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

Let’s first compile it using the javac command:

C:UsersUserDocumentsDelftStackjava>javac DelftStack.java
C:UsersUSerDocumentsDelftStackjava>

After the above command execution, a DelftStack.class file gets created in our current directory. Let’s run that file by using the java command.

C:UsersUserDocumentsDelftStackjava>java DelftStack.class
Error: Could not find or load main class DelftStack.class
Caused by: java.lang.ClassNotFoundException: DelftStack.class

Here, we are getting an error because we are trying to run the .class file. Instead, we just need to pass the class name.

Look below:

C:UsersUserDocumentsDelftStackjava>java DelftStack
Hello from DelftStack

Could Not Find Error Due to Passing the Wrong Name in Java

The could not find or load the main class can also occur when we pass the wrong class name. By continuing the previous example, if we try to run the program with the wrong name as follows:

C:UsersUserDocumentsDelftStackjava>java DelftStac
Error: Could not find or load main class DelftStac
Caused by: java.lang.ClassNotFoundException: DelftStac

We get the error above because we have misspelled the class name. Here, the JVM is trying to run a class named DelftStac, which doesn’t exist.

We can resolve this issue by correctly spelling out the class name as follows:

C:UsersUserDocumentsDelftStackjava>java DelftStack
Hello from DelftStack

We should also note here that the class name is case-sensitive. If we run the class Delftstack, we will get an error.

Look below:

C:UsersUserDocumentsDelftStackjava>java Delftstack
Error: Could not find or load main class Delftstack
Caused by: java.lang.NoClassDefFoundError: Delftstack (wrong name: Delftstack)

We should use the correct spelling and the correct cases to run a file successfully.

Could Not Find Error Due to Wrong Package Name in Java

Let’s move our DelftStack class into the com.DelftStack package. A package is used to keep similar classes together.

Look at the following code:

package com.DelftStack;

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

To compile a package in Java, we use the following command:

javac -d . <.java file name>

The -d flag switch is used to tell where to keep the generated class file. The . means the current directory.

We compile the above code as follows:

C:UsersUserDocumentsDelftStackjava>javac -d . DelftStack.java

After executing the above command, the following folder structure gets created in our current directory.

comDelftStackDelftStack.class

As we can see, our class file is two folders deep from our current directory. So if we try to run our class file like we were doing in previous cases, we get an error.

C:UsersUserDocumentsDelftStackjava>java DelftStack
Error: Could not find or load main class DelftStack
Caused by: java.lang.ClassNotFoundException: DelftStack

The reason for this error is that no DelftStack class exists in our current folder. To run the class present in a package, we need to pass its fully qualified name (com.DelftStack.DelftStack in this case).

C:UsersUserDocumentsDelftStackjava>java com.DelftStack.DelftStack
Hello from DelftStack

This tells Java to look for the class inside the comDelftStack folder.

Could Not Find Error Due to Wrong CLASSPATH in Java

The CLASSPATH tells the JVM where the .class files are present.

Suppose we are currently in a different folder, and we want to run a Java program whose class file exists in a different folder. In this case, we can pass the location of the class file using the -classpath option.

For example:

java -classpath XYZ/ABC <class name>

The above command tells Java to look for the .class file inside the ZYX/ABC folder.

In the previous case, we created a package.

Suppose we want to run the file inside the com/DelftStack folder. Using the following command, we can do so:

>java -classpath ../../ com.DelftStack.DelftStack
Hello from DelftStack

The ../ means the parent directory. So ../../ means to lookup two directory levels.

Let us take another example, suppose we are at the desktop (folder) location, and we want to run a class file somewhere else on the computer. We can do so by below.

>java -cp C:UsersUserDocumentsDelftStackjava com.DelftStack.DelftStack
Hello from DelftStack

The -cp flag is the shorthand for -classpath. Here, we passed the full location of the folder where the .class file is present.

Error: Could not find or load main class HelloWorld comes when you are trying to run your Java program using java command with the main class as HelloWorld but Java is not able to find the class. In order to solve this error, you must know how Java find and loads the classes, that’s a little bit complex topic for beginners, but we will touch the same base here. For the curious reader, I would suggest reading my post How Classpath works in Java, a must read for a beginner. For now, you just remember that there is an environment variable called CLASSPATH which includes directories where Java looks for all class files and if it doesn’t find your main class there then it throws «Error: Could not find or load main class XXX», where XXX is the name of your main class.

Since many Java programmer is now started programming using Eclipse they face this issue when they first try to run their Java program from command line. In Eclipse, it’s easy to compile and run the program because Eclipse takes care of all Classpath setup, but when you run your Java program from command line, CLASSPATH environment variable comes in picture.

Personally, I don’t like this environment variable and doesn’t define in my environment variable, because its confusing and source of so many classpath related issue. Instead, I use -cp or -classpath option with java command to run my program. This way you always know which JARs are included in your classpath.

For beginners, another important thing to understand is the difference between PATH and CLASSPATH, you must know that PATH is used locate system executable, commands or .exe, .dll files (in Windows) and .so files (in Linux). It is also used to locate native libraries used by your Java program. While, CLASSPATH is used to locate the class file or JAR files. It’s Java class loader who looked into CLASSPATH for loading classes.

Coming back to the problem in hand, if you are a beginner in Java, who are able to run the program from Eclipse but getting «Error: Could not find or load main class HelloWorld» when trying to run the same program from the command line then follow the steps given here to solve it.

Solving Error: Could not find or load main class HelloWorld 

Unfortunately beginner’s book like Head First Java, which many developers used to learn Java, doesn’t teach you how to deal with this kind of errors. You need to build this skill by doing active development.  In order to understand the problem little better, let’s reproduce it. This is one of the most important troubleshooting skill which will help you a long way in your career. Half of the problem is solved when you are able to reproduce it.

For our purpose we will use following HelloWorld program for our testing, interestingly I have named it HelloHP and it resides in a package called «dto». I have purposefully chosen a class with a package instead of HelloWorld in the default package because many programmers get «Could not find or load main class» error when they try to run a class which is inside a package.

package dto;
/**
 * Simple Java program to demonstrate following error
 * Error :Could not find or load main class
 * 
 * @author Javin Paul
 */
public class HelloHP {

    public static void main(String args[]) {
        System.out.println("My first program in Java, HelloWorld !!");
       
    }

}

When you run this from Eclipse, by Right click on the source file and Choosing «Run as Java Program», it will run fine and print following line:

My first program in Java, HelloWorld !!

Everything as expected, Now we will try to run same Java program from command line. Since I am using Maven with Eclipse, its build process creates class files in project_directorytargetclasses directory. If you are not using Maven with Eclipse, then you can see the class file created by Eclipse’s Java compiler in project_directorybin. It doesn’t matter how those class files are created, but, what is important is the location of the class file.

If your class is inside a non-default package e.g. «dto» in our case then compiler the will put the HelloHP.class file, which contains Java bytecode in a directory named «dto». In our case the full name of class dto.HelloHP and it is present in C:UsersWINDOWS 8workspaceDemotargetclassesdto. So in the first try, I go there and execute java command to launch my program, as seen below:

C:UsersWINDOWS 8workspaceDemotargetclassesdto>java HelloHP
Error: Could not find or load main class HelloHP

Do you see the error? It’s coming because the full name of the class should be dto.HelloHP and not HelloHP. So let’s correct this error and try to run the same command from the same location but this time with fully qualified name:

C:UsersWINDOWS 8workspaceDemotargetclassesdto>java dto.HelloHP
Error: Could not find or load main class dto.HelloHP

Still same error, right. Why? because I don’t have any CLASSPATH environment variable, neither I am using -classpath or -cp option to suggest the path, So by default Java is only searching in the current directory. It is looking for dto/HelloHP.class but since we are already inside dto, it is  not able to find the class. So, what should we do now? let’s go to the parent directory «C:UsersWINDOWS 8workspaceDemotargetclasses» and execute the same command, this time, it should work:

C:UsersWINDOWS 8workspaceDemotargetclassesdto>cd ..

C:UsersWINDOWS 8workspaceDemotargetclasses>java dto.HelloHP
My first program in Java, HelloWorld !!

Bingo!!, our program ran successfully because, without any hint about where to find class files, Java is by default looking into the current directory, denoted by . (dot) and able to locate ./dto/HelloHP.class.

Now, what if you want to run this program from any other directory? Well, for that purpose whether we need to define CLASSPATH or just use -classpath or -cp option. I like the second option because it’s easier to control and change. Also, remember, it overrides any CLASSPATH environment variable. If you like to set CLASSPATH environment variable in Windows, see that tutorial.

Now let’s run the program target directory first without using -classpath option:

C:UsersWINDOWS 8workspaceDemotargetclasses>cd ..

C:UsersWINDOWS 8workspaceDemotarget>java dto.HelloHP
Error: Could not find or load main class dto.HelloHP

You can see we are again started getting the same error, Why? because Java is still looking into the current directory and there is no .targetdtoHelloHP.class there, as it’s one level down e.g. .targetclassesdtoHelloHP.class

Now let’s run the same command using -classpath option from target directory itself:

C:UsersWINDOWS 8workspaceDemotarget>java -cp ./classes;. dto.HelloHP
My first program in Java, HelloWorld !!

Bingo!!, our program ran successfully again because now Java is also looking at ./classes directory and there it is able to find dtoHelloHP.class file.

There are many ways Error: Could not find or load main class HelloWorld manifests itself, but if you know the basics of Java Classpath, you can easily sort out the problem. Most of the time you just need to either correct your CLASSPATH environment variable or run your program with java -cp or -classpath option. By the way, there are more to it e.g. Main class defined in the manifest.mf file and that’s why I suggest reading about How Classpath works in Java (see the link in the first paragraph).

Summary

If you are getting «Error: Could not find or load main class XXX», where XXX is the name of your main class while running Java program then do this to solve that error:

1) If you are running Java program right from the directory where .class file is and you have CLASSPATH environment variable defined then make sure it include current directory. (dot). You can include it as set CLASSPATH=%CLASSPATH%;. in Windows and export CLASSPATH = ${CLASSPATH}:. (see the separator, in Windows it’s;(semicolon) while in Linux it is (colon), also note we have included current directory in existing classpath. If you still face the issue of setting classpath, see this step by step guide to set the classpath. Same thing applies if you are running your program using -cp or -classpath option.

2) If you are running Java program from the directory, your .class file is and you don’t have any CLASSPATH or -cp option then check whether your class is the in the package or not. If it’s the in the package then go outside of the package directory and run java command with fully qualified name e.g. if your program is com.abc package then runs following command from the parent directory of «com»

java com.abc.HelloWorld

without any classpath hints, Java will look into the current directory and search for comabcHelloWorld.class in Windows, so if com directory exists in your current directory, your program will run otherwise you will get «Error: Could not find or load main class dto.HelloHP».

3) You can run your Java program from anywhere with the help of proper CLASSPATH or java -cp option as shown below:

java -cp C:test;. com.abc.HelloWorld

If you still facing any issue just check whether you have accidentally using CLASSPATH environment variable, you can check this in Windows by running echo %CLASSPATH% command and in Linux by running echo $CLASSPATH. If CLASSPATH is nonempty then it will print its value otherwise just echo the same command.

4) If you are running in Java version 1.6 or 1.5, then instead of receiving «Error: Could not find or load main class», you will get Exception in thread «main» java.lang.NoClassDefFoundError: HelloWorld. It’s only from JDK 1.7 onward we are started receiving this new error. The solution is exactly same, every bit of discussion applies to that case as well. So if you are not able to solve that problem by following steps here, do let me know and I will try to work with you to troubleshoot the problem.

Here is the screenshot of how I tried to reproduce and solve the error as discussed in the previous paragraph:

That’s all about how to solve «Error: Could not find or load main class HelloWorld» in Java. Classpath is little confusing topic to master, but you will understand it once you started writing and running some Java program. If you are still not able to fix your problem then post a comment there with what you have tried and we will try to troubleshoot together.

My goal is not just to give you solution but also make you able to explain why the solution is working and CLASSPATH basics are very important for a Java developer. I have seen many programmers getting frustrated, losing interest in Java due to various PATH and CLASSPATH issues e.g. NoClassDefFoundError and ClassNotFoundException and this is my humble effort to bring them back and empower with practical knowledge. Hope you understand.

I have already shared the common problem java beginners face i.e illegal start of expression error. You can not find these kind of errors in java books. The only way to
familiarize yourself about the most common errors by using active java
development. Today we will look into another common problem for java beginners,i.e  how to fix «error: could not find or load main class» error in java. As the name suggest,this error occurs when java is not able to find the class you are trying to execute. To better understand the error you should be familiar with CLASSPATH. If you are not familiar with CLASSPATH then  please check this out  what is CLASSPATH and how it differs from PATH.

Read Also:  Difference between PATH and CLASSPATH

«Could not find or load main class : XXX»  where the class name is XXX occurs if the CLASSPATH environment variable where java looks for all class files, does not found the class.

[Fixed] Error : Could not find or load main class

1. Calling  .class file from java command

public class HelloWorld {

    public static void main(String args[]) {
        System.out.println(" You have just run HelloWorld !");       
    }
}

Suppose I have a simple HelloWorld java program. If I will compile it using command

javac HelloWorld.java

then  HelloWorld.class file is created.

You will get the Error : Could not find or load main class if  you try to run .class file using java command as shown below:

java HelloWorld.class

instead you should try

java HelloWorld

Yayy!! your problem is resolved.

You have just run HelloWorld !

2. If your casing incorrect 

After compiling the code , you run the following command

java helloworld

then also «Could not find or load main class helloworld» error appears. Make sure casing is correct. In our program it should be HelloWorld ( where ‘H’ of hello and ‘W’ of world is in uppercase).

java HelloWorld

Yayy ! It will work fine and print

You have just run HelloWorld !

3. Class in a package 

In the below example I have a HelloWorld class inside the com.javahungry package.

package com.javahungry;
/**
 * Java program to demonstrate 
 * Error :Could not find or load main class
 * 
 * @author Subham Mittal
 */
public class HelloWorld {

    public static void main(String args[]) {
        System.out.println(" You have just run HelloWorld !");       
    }
}

If you try to call

java HelloWorld

It will result in Error : Could not find or load main class HelloWorld. The error occurs because it must be called with its fully qualified name.  To
be clear, the name of this class is not HelloWorld, It’s
com.javahungry.HelloWorld . Attempting to execute HelloWorld does not work,
because no class having that name exists. Not on the current classpath
anyway.

java  com.javahungry.HelloWorld

Above command will also result in Error : Could not find or load main class HelloWorld because CLASSPATH environment variable is not set. I do not use -classpath or -cp command to suggest the path. By default java is searching the class file in the default directory. 

java -cp . com.javahungry.HelloWorld

If your classpath is correct then above command will run the HelloWorld program. Otherwise, It will also result in Error : Could not find or load main class HelloWorld because -cp . command make sure the JVM looks for the class file in  the current directory.

Follow below steps if error persist.

You must ensure that the location of the .class file is added in the
CLASSPATH. Suppose if «/Users/SubhamMittal/Desktop/» is on the classpath
and JVM looks for the class called «com.javahungry.HelloWorld», it will
look for the .class file in the following path:

«/Users/SubhamMittal/Desktop/com/javahungry/HelloWorld»

java -cp  /Users/SubhamMittal/Desktop/  com.javahungry.HelloWorld

 Yayy ! It will print

 You have just run HelloWorld !

The above example is performed on Mac OS.

4. Class in a Package for Windows OS 

Suppose My java class i.e HelloWorld (given above)  after compilation is in the below path

D:projectcomjavahungry

 
The full name of the HelloWorld class is

com.javahungry.HelloWorld

so use cd .. back command to reach to the main directory 

D:project

 Then issue the java command

java com.javahungry.HelloWorld

 Yayy ! The program ran successfully without setting any classpath. It is because java is looking for  the current directory denoted by .(dot) and able to locate the .comjavahungryHelloWorld.class .

It will print

 You have just run HelloWorld !

5. Setting  CLASSPATH to current directory

if .class file in the current folder then set  .   to your CLASSPATH (note in Windows the CLASSPATH separator is semi colon ; while in Linux the separator is colon :).
You can set the CLASSPATH  in Windows :

set CLASSPATH = %CLASSPATH%;.

 (note . added at last for current directory)

You can set the CLASSPATH in Linux :

export CLASSPATH = ${CLASSPATH}:.

 (note . added at last for current directory)

If you are using java 1.5 or 1.6 , instead of getting error : could not find or load main class you will get Exception in thread «main» java.lang.NoClassDefFoundError : HelloWorld. Since jdk 1.7 we started receiving this new error. The good news, Solution is exactly same.

If you are not able to solve the problem by using above steps then please mention in the comments.