Error could not find or load main class misc tweaker stubmain

Explore the reasons for the error "Could not find or load main class" and learn how to avoid them.

1. Overview

Occasionally when we run a Java program, we might see “Could not find or load main class.” It’s easy to guess the reason: The JVM failed to find the main class and gave this error. But why couldn’t it?

In this tutorial, we’ll discuss the probable reasons for failure to find the main class. We’ll also see how to fix them.

2. Sample Program

We’ll start with a HelloWorld program:

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

Now let’s compile it:

$ javac HelloWorld.java

Here, the compiler will generate a .class file for our program. This .class file will be generated in the same directory. The .class file will have the same name as the class name given in the Java program. This .class file is executable.

In the following sections, we’ll run this .class file and try to understand the probable reasons for error “Could not find or load main class.”

3. Wrong Class Name

To run a .class file generated by Java compiler, we can use this command:

java <.class filename>

Now let’s run our program:

$ java helloworld
Error: Could not find or load main class helloworld

And it failed with the error “Could not find or load main class helloworld.”

As discussed earlier, the compiler will generate the .class file with the exact same name given to the Java class in the program. So in our case, the main class will have the name HelloWorld, not helloworld.

Let’s give it one more try with correct casing:

$ java HelloWorld
Hello world..!!!

This time it ran successfully.

3.1. File Extension

To compile a Java program, we must provide the file name with its extension (.java):

$ javac HelloWorld.java

But to run a .class file, we need to provide the class name, not the file name. So there is no need to provide the .class extension:

$ java HelloWorld.class
Error: Could not find or load main class HelloWorld.class

Again, let’s run our program using the correct class name:

$ java HelloWorld 
Hello world..!!!

4. Java Package Names

In Java, we keep similar classes together in what we call a package.

Let’s move HelloWorld class into the com.baeldung package:

package com.baeldung;

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

Now let’s compile and run the updated HelloWorld program like before:

$ java HelloWorld
Error: Could not find or load main class HelloWorld

But again, we get the error “Could not find or load main class HelloWorld.”

Let’s try to understand what we missed here.

To run a Java class that is in a package, we must provide its fully qualified name. So in our case, HelloWorld‘s fully qualified name is com.baeldung.HelloWorld.

Now, when we created com.baeldung package, we actually created this folder structure:

com/baeldung/HelloWorld.java

First, let’s try to run our program from the com/baeldung directory:

$ java com.baeldung.HelloWorld
Error: Could not find or load main class com.baeldung.HelloWorld

Still, we are not able to run our program.

Here, when we specified the fully qualified class name com.baeldung.HelloWorld, Java tried to find the HelloWorld.class file in com/baeldung, under the directory from where we were running the program.

As we were already inside com/baeldung, Java failed to find and run the HelloWorld program.

Now let’s move back to the parent folder and run it:

$ java com.baeldung.HelloWorld
Hello world..!!!

And we are again able to say “Hello” to the world.

5. Invalid Classpath

Before going ahead, let’s first understand what the classpath is. It’s the set of classes available to our currently running JVM.

We use the classpath variable to tell the JVM where to find the .class files on the file system.

While running a program, we can provide the classpath using -classpath option:

java -classpath /my_programs/compiled_classes HelloWorld

Here, Java will look for the HelloWorld.class file in /my_programs/compiled_classes folder, a folder whose name we just made up. By default, the classpath variable is set to “.”, meaning the current directory.

In the above section, we changed our directory to run our program. But what if we want to run it from some other folder? That’s when the classpath variable helps us.

To run our program from the directory com/baeldung, we can simply state that our classpath is two directories up — one for each package part:

$ java -claspath ../../ com.baeldung.HelloWorld
Hello world..!!!

Here, “..” represents the parent directory. In our case “../../” represents the top of our package hierarchy.

6. Conclusion

In this article, we learned the probable reasons for the error “Could not find or load main class.”

Then, of course, we also learned how to solve this error.

Источник

The Java “Could not find or load main class” error is thrown when the JVM fails to find or load the main class while executing a program. It usually occurs when executing a Java program from the command line.

Install the Java SDK to identify and fix these errors

What Causes Error: Could not find or load main class

The «Could not find or load main class» error occurs when the JVM fails to load the main class. This can happen due to various reasons, such as:

  • The class being declared in the incorrect package.
  • The file path of the class not matching the fully qualified name.
  • Incorrectly specified classpath of the application.
  • Missing dependencies from the classpath.
  • Incorrect directory path on the classpath.
  • A typo in the class name.

Error: Could not find or load main class Example

Here’s an example of the Java «Could not find or load main class» error thrown when an incorrect class name is specified during execution:

Here’s an example Java class MyClass.java:

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

Now the above class is compiled using the command line:

$ javac MyClass.java

The compiler generates an executable .class file for MyClass:

$ ls
MyClass.class   MyClass.java

Now if the java command is used to execute the .class file with an incorrect name, the «Could not find or load main class» error is thrown:

$ java Myclass
Error: Could not find or load main class Myclass

The generated .class file has the exact same name as the Java class, which in this case is MyClass.class. Specifying the correct name will execute the program successfully:

$ java MyClass
Hello World

How to Fix Error: Could not find or load main class

There are several ways the «Could not find or load main class» error can occur while executing Java programs. Most of the time, it occurs because of specifying an incorrect class name, class file extension, file path or classpath.

The following tips can be useful to resolve the «Could not find or load main class» error:

  • Using correct class name — The spelling and casing of the class name should be checked when executing the program.
  • Using the class name without the .class extension — The java command expects the class name for executing the program, without the .class extension. Therefore, the following syntax should be used to execute Java classes: java <classname>
  • Using the correct file path — The path to the .class file should be checked and corrected if the error occurs. Remember to use the fully qualified name of the class that is in a package if executing it from outside the directory structure of the package.
  • Correct classpath definition — The classpath should be checked and defined correctly if the error comes up. It can also be specified using the java -cp or -classpath command line arguments.

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Источник

Java Could Not Find or Load Main Class

When starting your Java application, you may encounter this error:

Error: Could not find or load main class MyClass
Caused by: java.lang.ClassNotFoundException: MyClass
Caused by: java.lang.ClassNotFoundException: MyClass

This error is very common when creating new Java based projects. Whether you’re using Gradle or Maven, Spring Boot or Kafka, chances are you’ve encountered this error before.

Sometimes the error will occur unexpectedly. Sometimes the error is specific to your IDE.

Regardless, fixing the error is easy and it starts with understanding the cause:

What Causes the «Could Not Find or Load Main Class» Error?

This error is thrown whenever Java can’t find or load the main class of your application.

Let’s say you define a class like this: 

public class MyClass {
  public static void main(String[] args) {
    System.out.println("My class is working!");
  }
}
  public static void main(String[] args) {
    System.out.println("My class is working!");
  }
}

When running this simple class, you could get the «could not find or load main class» error for several reasons…

1. IDE Configuration Issue

Most IDEs let you configure the starting point for your application. For example, in IntelliJ you can edit configuration to select a main class for running the project.

If you’re running your application through an IDE, make sure that it is configured properly to look for the main class in the right place.

2. Wrong Class Name

Remember that class names must be unique in Java. Furthermore, they are case sensitive…

Let’s say you are running your program from the CLI using the java tool..

java myclass

This will result in the «Could not find or load main class» error because class names are case sensitive.

3. Wrong Extension

When running from the command line, many developers accidentally append an extension like:

java MyClass.java

or

java MyClass.class

The correct way is to run without any extension:

java MyClass

4. Wrong Location

Let’s say your class is part of a package like this:

package com.myproject;
public class  MyClass {
  public static void main(String[] args) {
    System.out.println("My class is working!");
  }
}
public class  MyClass {
  public static void main(String[] args) {
    System.out.println("My class is working!");
  }
}

If you don’t run your class with the fully qualified name AND from the right directory, you will get the «Could not find or load main class» error…

5. Wrong Class Path

The class path is where the JVM looks for classes to load into your program. Sometimes developers provide a specified path like this:

java MyClass -cp /usr/local/path

While the optional -cp argument allows you to specify your own class path, you can easily get the «Could not find or load main class» error if this is incorrect…

How to fix the «Could Not Find or Load Main Class» Error

1. Make sure your IDE is configured properly

Make sure that your IDE has the correct configuration for finding the main class/entry point of your application.

2. Make sure your class name is correct

If you are running your program from the CLI, make sure that you are specifying the right class name without extensions…

java MyClass

3. Make sure you are running your application from the right directory

Make sure you are running your application from the right folder. If your class is part of a package then you must run it from the parent directory….

java com.myproject.MyClass

4. Make sure your class path is correct

Make sure your class path is correct. By default, the class path is the current working directory «.». If you override this with the -cp argument then make sure it’s accurate!

Understanding the Java Error «Could Not Find or Load Main Class»

While this error is self explanatory and easy to fix, it’s worth understanding how Class Loaders work behind the scenes. This gives you a better understanding of why the «Could Not Find or Load Main Class» error happens…

When are Classes Loaded in Java?

Classes are loaded dynamically. This means classes are loaded into memory only when they are needed.

Unlike C++, Java is a dynamically compiled language. This means the language is compiled to machine code while the program is running.

Of course, some classes must be loaded initially when your program starts. The JRE utilizes a native class loader to load the main entry point of your application. From here, class loaders are used to dynamically load (lazy load) classes as they are needed by the application.

The Class Loading Mechanism in Java

Java utilizes a delegation mechanism for loading classes at runtime. There are 3 built-in class loaders used by the JRE at runtime:

1. Bootstrap class loader: This loads the standard runtime classes found in rt.jar

2. Extensions: This loads any extension classes used by the JRE

3. System: This loads classes defined by the application and found on the class path

Each class loader first checks a cache to see if the requested class has already been loaded into memory. If nothing is found in the cache, it delegates the finding of the class to the parent class loader.

This process happens recursively…

If the system class loader can’t find the class, it delegates to the extension class loader.

If the extension class loader can’t find the class, it delegates to the bootstrap class loader.

If the bootstrap class loader can’t find the class, it tells the extension class loader to find it

If the extension class loader can’t find the class, it tells the system class loader to find it

If the system class loader can’t find it, it throws an ClassNotFound exception

This mechanism works to ensure uniqueness, visibility and delegation are applied to the class loading mechanism in Java.

Uniqueness explains the reason why no two classes can have the same name. By keeping class names unique, class loaders can easily find the single representation of a defined class.

Visibility explains the child-parent relationship between class loaders. While children can view parent classes, parents can’t view child classes. This ensures an isolation level needed to create the hierarchy between class loaders.

Delegation explains how the class loaders work together to recursively retrieve a unique class. By delegating to parent classes, class loaders ensure only one representation of a defined class exists.

Java Class Loading Order

1) Class loader searches cache for loaded classes

2) If cache has the class, it is returned. Otherwise, the class loader delegates to parent class to retrieve the class

3) Parent class loaders ultimately delegate to the bootstrap class loader. If the class isn’t found, the bootstrap loader returns responsibility to child loader.

4) Either the system loader finds and loads the class, or a ClassNotFound exception is thrown.

Custom Class Loaders

You can create your own class loaders by extending the ClassLoader class:

public class CustomClassLoader extends ClassLoader { ...

Most developers don’t need to worry about creating custom class loaders. There are times where it makes sense however. Sometimes custom class loaders are used to implementing class versioning. Other custom class loaders allow you to create classes dynamically or switch implementations etc.

Conclusion

The «Could not find or load main class» error is common and easy to fix. Its cause usually has to do with specifying the wrong class name, extension, or class path.

This error can be easily fixed by checking IDE configurations, class path variables, class names, and making sure you’re running the application from the right directory.

The JRE utilizes a class loading mechanism to dynamically load classes into memory. This mechanism relies on a recursive process where class loaders delegate retrieval to parent loaders if they can’t find the class already loaded in memory.

You can create your own custom class loaders for dynamic class creation and versioning.

Your thoughts?

Источник

The main() method is required to run/execute programs developed in the Java programming language since it is where the program execution begins. When starting a Java program, you could encounter the warning “error: Could not find or load main class.” You’re having this problem because you’re using the java command to run main() from within the class.

Note: You can also learn Errors and Exception in Python.

How to fix could not find or load the main class?

There are many ways to solve this issue depending on the reason of occurring this error. We will discuss each reason one by one and try to fix this problem.

What are the possible causes or reasons of this error?

There are several reasons for this problem, which are listed below.

  • File Extension
  • Wrong Package
  • Classpath is not valid
  • The class name is incorrect

File Extension

We need to save the Java source code file with the extension .java to compile it. To compile a Java program, Java Compiler is being used as (javac command). After compilation, the .java file will be converted to a .class file.

As a result, your source code file will end in.java, while the produced file will end in .class. For compiling source code, we were using filename, but for running a compiled file, we cannot use the file name but the class name. Else it will throw an error like in the below example.

Example: HelloWorld.java

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

Compiling HelloWorld Program

Compilation

Running/Executing HelloWorld with the filename.

Running/Executing HelloWorld with the class name.

Wrong Package

Packages used in Java for group-related classes to write better maintainable code. To avoid name conflicts in group classes, we can use packages in Java programming. To launch a Java class in a package, we must use packageName with a fully qualified className. We cannot run it directly using the class name as in the previous example, and if we do so, we will get the error.

Example: HelloWorld.java

Package com.baeldung;
public class HelloWorld {
    public static void main(String[] args) {
        System.out.println("Hello, World!");
    }
}

Compiling HelloWorld Program

Running/Executing HelloWorld without package name.

without package name

Running/Executing HelloWorld with the class name.

with package name

It is still showing an error because it could not find the HelloWorld file inside com/baeldung. We need to move back to the parent directory and rerun it.

Classpath is not valid.

The Java Virtual Machine searches the classpath for user-defined classes, packages, and resources in Java programs. If you correctly stated the class name but still received the same error, the Java command likely could not locate the supplied class name at the location. As a result, you must first confirm that the location of your .class file is included in your classpath.

Example: HelloWorld.java

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

Compiling HelloWorld Program

Running/Executing HelloWorld at the default location.

default location

Running/Executing HelloWorld at the location where the file exists.

file location

The class name is incorrect.

This problem can occur if the name of your Java file (.java) and the primary class name is different. For the example, we have done class name HelloWorld, and the file name is HelloWorld.java. Let change the class name and execute the program.

Example: HelloWorld.java

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

Compiling HelloWorld Program

Conclusion

In this article, we tried to resolve a Java error that could not find or load the main class differently. We discussed some reasons for this error and, depending on those reasons, applied the methods to fix the error. Also, we performed coding examples with executions with error-occurring conditions and solved the error accordingly.

Источник

Introduction

Java developers often face the ‘could not find or load main class’ error out of the blue during compilation. If you keep getting this error without any specific reason, you are not alone. Whether you’re just starting out as a programmer or have some experience under the belt, we’ve all seen this error at least once. And we know it has nothing to do with our code.

Why does the “JVM could not find or load the main class” error occur?

As the name suggests, the ‘could not find or load main class’ error means that the JVM (Java Virtual Machine) could not locate the main class in your code and throws this runtime error. The question is can we not find it?

It is one of the most unpredicted and spontaneous errors in Java, which occurs due to the tendency of JVM to stick with a default classpath, the “main class not found issue” is something that haunts amateurs and professionals alike. As serious as it seems, it is not that difficult to fix. We will be exploring in this article how you can easily fix this annoying Java compilation error.

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What Is Classpath?

Before we dive into the how and why of it, we need to understand what Classpath is and its role in Java.

The classpath is the file path where the JRE (Java runtime environment) searches for the classes and other resource files to run the code. As the name suggests, it is simply a file path where the .class format files can be found in a JDK package or directory.

Classpath can be set using two ways:

  1. Using the -classpath option at the time of executing the code,
  2. By setting the file path to the system CLASSPATH environment variable.

When the JVM is unable to locate the main class, it is usually because you would have entered the wrong .class name to run the classpath or the corresponding .class files have been altered.

See this example of generating a class file of a simple code:

1. public class example01 {
2. public static void main(String[] args) {
3. out.println("This is a simple code");
4. }
5. }

To run a .class file, you can use the following command:

java <.class filename>

Now if we run this line of code to run the class we made.

$ java eg01

Output:

Error: Could not find or load main class

It will fail with the error “Could not find or load main class eg01.”

As mentioned earlier, the .class file will have the same name given to the Java class of the program. So, in this case, the main class will have the name example01, not eg01.

Let’s try this one more with the correct name:

$ java example01

Output:

This is a simple code

Now it ran successfully.

While it’s not as simple in a project, the easiest way to rectify it is by either manually specifying the classpath or using packages.

Using Packages

Packages are used in Java to group similar classes or to provide a unique namespace to a group of classes. We will be now creating a class called example02 and place it in a package called example02Package.

1. package example02Package;
2. public class example02 {
3. public static void main(String args[]) {
4. out.println("File is found successfully!");
5. }
6. }

We will then use this package to visualize how the classpath works in Java. In your files directory, a package is represented as an independent folder by its name that you can easily observe in a file manager application.

After ensuring that the working directory is the same as the one that contains the package folder, you can also change the working directory on the terminal (command prompt) by using the cd command on almost every popular operating system.

Example02.java can be compiled by running the following command:

  1. package example02Package;
  2. javac example02Package/example02.java

This will now save the compiled .class file in the example02Package directory.

To run the compiled class, you need to type in the fully qualified class name in the command line. The fully qualified name of a Java class is written by prefixing it with the package name.

For this example, this is the fully qualified name:

java example02Package.example02

Using packages also allows Java developers to call executables from different packages from the same working directory. It can be easily done just by modifying the fully qualified class name without getting the ‘could not find or load main class error.

Manually Specifying Classpath

The other way to prevent could not find or load main class error is by manually specifying the classpath. It is recommended to manage your java files by creating separate directories for all source files and classes.

Just like the .class files are labeled as classes. the directory with source files is labeled as src. It also helps in significantly reducing the chances of JVM not being able to find the main class.

If you use this method for organization, this is how the directory structure of your projects will look like before compilation:

|___Project01
|     |___src
|           |___example02Package
|                       |___example02.java
|
|     |___classes

The indentations in the above example show the one level of the file hierarchy that your project should be following.

When compiling this code, you must make sure that your working directory is Project01. The following command is used to execute it:

javac -d classes src/testPackage/Test.java

The .class executable file must be saved in Project01/classes/example02Package. Now, the file directory structure will look like this:

|___Project01
|     |___src
|           |___example02Package
|                       |___example02.java
|
|         classes
|           |___example02Package
|                       |___example02.class

To run the .class file, you have to run the Java command with the fully qualified class name and the specification of the local classpath. Every path is declared relative to the working directory, which in this example is Project01.

java -classpath classes example02Package

Running this command will now provide you with the desired output without any chances of getting the error. The question is, why do you need to reorganize the files to solve such a small runtime error?

Why Organizing Files is important in Java?

The primary reason behind why the ‘could not find or load main class ’ is encountered is because JVM is unable to locate where your .class files were being saved.

The easiest way to resolve this error and prevent it from ever happening is to organize where the .class files are saved. Developers have to explicitly indicate the JVM to look for the .class file in the assigned location. This can only be done by organizing the source files and executables separately and using the working directory to manage either manually or using packages.

Your project code will likely keep expanding over time as the project work keeps going. By adding more constructs such as inheritance, inner classes, and more to your project, the file system keeps getting more complex. In such projects, this simple practice of organizing files can save you several hours of precious time that it would take in debugging if something goes wrong and you end up getting the could not find or load main class error.

See Also: What Are Annotations in Java – How do They Work

Conclusion

“Could not find or load main class” error is very common in Java however, we have discussed some effective ways to prevent this error in the article above. The file organization methods we discussed not only prevent this error but also make your code and directories manageable. These fixes go a long way for Java developers as they save a lot of time and trouble when debugging, especially for complex codes!

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Источник

Популярное сообщение об ошибке для новых пользователей Java. 


Error: Could not find or load main class ClassName.class
Caused by: java.lang.ClassNotFoundException: ClassName.class

1. Нет пакета

1.1 Обзоры простой Java Hello World, без пакета.

C: /projects/HelloWorld.java 


public class HelloWorld {

public static void main(String[] args) {

System.out.println("Hello World");

}

}

Терминал 


# Compile Java source code
C:projects> javac HelloWorld.java

C:projects> dir
15/04/2019  01:20 PM               425 HelloWorld.class
15/04/2019  01:42 PM               138 HelloWorld.java

# Run Java
C:projects>java HelloWorld.class
Error: Could not find or load main class HelloWorld.class
Caused by: java.lang.ClassNotFoundException: HelloWorld.class

Чтобы исправить это, запустите java ClassName без .class расширение.

Терминал 


C:projects>java HelloWorld
Hello World

2. С пакетом

2.1 Простой Hello World и package это так же как com.mkyong

C: /projects/HelloWorld.java 


package com.mkyong;

public class HelloWorld {

public static void main(String[] args) {

System.out.println("Hello World");

}

}

Терминал 


# Compile Java source code
C:projects> javac HelloWorld.java

C:projects> dir
15/04/2019  01:20 PM               425 HelloWorld.class
15/04/2019  01:42 PM               138 HelloWorld.java

C:projects>java HelloWorld
Error: Could not find or load main class HelloWorld
Caused by: java.lang.NoClassDefFoundError: com/mkyong/HelloWorld (wrong name: HelloWorld)

2.2 Если package определяется, классы Java .class должен разместить в правильном месте папки.

Например :

C: /projects/HelloWorld.java 


package com.mkyong;

Выше сгенерированный файл класса должен поместить в C:projectscommkyongHelloWorld.class

2.3 Чтобы исправить это , мы можем использовать -d указать, где разместить сгенерированные файлы классов.

Терминал 


# Compile Java source code
C:projects> javac -d . HelloWorld.java

C:projects> dir

15/04/2019  02:01 PM              com
15/04/2019  01:51 PM               161 HelloWorld.java

C:projectscommkyongHelloWorld.class
C:projectsHelloWorld.java

2.4 Теперь мы можем бежать так java {package}.HelloWorld

Терминал 


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

C:projects>java com.csharpcoderr.HelloWorld
Hello World

Рекомендации

  • Википедия — пакет Java

распространенная ошибка Java

Java — не удалось найти или загрузить основной класс

0.00 (0%) votes

Источник

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.

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