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

I'm a beginner in Java and am trying to run my code using IntelliJ that I just installed as my IDE with JDK 1.7. The following piece of code keeps does not even compile and keeps giving me the err...

I know this was asked a while ago, but I was just stumbling over this issue and thought my findings might help others. As pointed out, the error message is basically a result of the out folder. That’s because, when you’re trying to run the program, it compiles the code first, and puts the compiled result to the out location, and then it tries to load the compiled code from the out location. If the compiled code is not in the location expected, you’ll get the error.

The point I’m particularly wanting to share is that some times, the code is not compiled (built), even though your run configuration specifies «Build» in the «Before launch» section of the configuration panel.

When can this happen?
One situation that can cause this to happen is if you’re using modules and you manually delete the module out directory. For example, if I have a module named «foo», there should be a directory named foo under out/production. If you manually delete it, the build system may not know that it needs to be rebuilt.

Even worse, if you select Build | Build module ‘foo’, it still may not rebuild the module. If that’s the case, you should select a file in the module, for example ‘bar.java’ and then select Build | Recompile ‘bar.java’. Now the out directory out/production/foo should be restored.

Since IntelliJ typically knows about any changes going on, this surprised me, and took me a little time to figure out, so I thought I’d share.

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

@ivyyangyq It looks alright as it did show the run (green arrow) which means technically there is a main class or can be run (not sure if you want to try and go to AddressBook.java and click the green arrow besides the main() method line) but for some reason if you still see that error may be has something to do with the configuration (just a guess).

This might be an unpopular opinion but since you tried everything like recloning, cloning a different repository and changing settings etc.

May I suggest that I am not sure if it will work for you but it is worth a try and based on your screenshot you seem to be either using some linux distribution or macOS so if can try resetting your entire IntelliJ IDE settings to default if you want and see if it works. You can do so without reinstalling by deleting the configuration folder which can be found in this page (https://intellij-support.jetbrains.com/hc/en-us/articles/206544519-Directories-used-by-the-IDE-to-store-settings-caches-plugins-and-logs) for Windows, Linux or MacOS respectively. If you still want to keep your old settings you can try exporting it first by going File and export settings which will export as a zip store it somewhere else instead of the default directory.

Alternatively: You can try backup the configuration folder which I meant as eg. «.IdeaIC2018.3» the entire folder instead of just the config folder and then delete the entire «.IdeaIC2018.3″ (the version number may vary depending on what version you installed but it should be along the lines of .Ideal followed by whether you download community edition or the complete edition then the version number)»

After deleting the configuration for IntelliJ IDE, try relaunching IntelliJ IDE, it should show something like want to import existing configuration or do not import existing configuration. Choose do not import existing configuration to start from default settings and see maybe if it works for you.

You can always import your old settings back if it still does not work for you by going File > import settings and select the settings zip file you back up earlier.

Источник
Error:

/Library/Java/JavaVirtualMachines/zulu-11.jdk/Contents/Home/bin/java 
-javaagent:/Applications/IntelliJ IDEA CE.app/Contents/lib/idea_rt.jar=53049:
/Applications/IntelliJ IDEA CE.app/Contents/bin -Dfile.encoding=UTF-8 Scratch

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

Process finished with exit code 1

Class File:

public class HelloWorld {

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

As you can see the above class does not have any complication issues or syntax errors. There can be multiple reasons that you may get the error, below are some of the solutions that may work for you!

  • Make sure that the class name is correct.
  • Go to Menu: Build -> Re-Build Project.
  • Remove .idea directory from your project and restart IDE.
  • Go to Files: Invalidate Cache/ Restart…
  • Check your project has a out folder, if not create one, example: /Users/code2care/IdeaProjects/java-examples/out
  • Edit Build/Run Configurations and check if you have selected the correct Java class that has a main method.

Fixing Could not find or load main class error

Fixing Could not find or load main class error

Источник

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

If you are getting Error: Could not find or load main class error, it means JVM is trying to load a class with main method. Simply, JVM cannot find this class in the classpath.

main method
main() method has a special meaning in Java, it is the entry point of Java programs.

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

When you run the following command, it starts JVM, loads HelloWorld class and starts running its main method.

java HelloWorld

We will go through several possible reasons for this error. We usually use IDEs for software development, so first let’s examine how to fix this problem in our IDEs.

IntelliJ IDEA – Solution 1

If you run a main class through you IDE, it will be stored in Run/debug Configurations. You can easily run the class again with the run/debug buttons at a later time.

Run/debug Configurations

You might want to change the name of your class (Main -> Main2). If you update a class name on your IntelliJ, all references to this class will also be updated (including Run/debug Configurations). However, if you change a class name (or change its package) outside your IDE, Run/debug Configurations will become obsolete.

I renamed Main.java to Main2.java outside Intellij IDE and tried to run the code again using Run button. Now, it gives the following error message in console.

C:Java.jdksbinjava.exe "-javaagent:C:Program FilesJetBrainsIntelliJ IDEA Community Edition 211.6556.6libidea_rt.jar=14581:C:Program FilesJetBrainsIntelliJ IDEA Community Edition 211.6556.6bin" -Dfile.encoding=UTF-8 -classpath C:abcstudyguidecodesoutproductionMain com.abc.Main
 Error: Could not find or load main class com.abc.Main
 Caused by: java.lang.ClassNotFoundException: com.abc.Main
 Process finished with exit code 1

You can update Run/debug Configurations to fix the error.

IntelliJ IDEA – Solution 2

Sometimes just rebuilding your project is enough to resolve this error. To rebuild the project, select Build -> Rebuild Project.

IntelliJ IDEA – Solution 3

If there is some residue left in the cache, you can invalidate caches and restart you IDE.

Eclipse – Solution 1

Similar to IntelliJ, you can run Java application in eclipse.

Once you run your application, it is saved as a Run Configuration. If you update your application main class outside Eclipse IDE and try to run it again, you will get Error: Could not find or load main class error message.

If you have updated class name or package outside Eclipse IDE, you need to update Run Configurations accordingly.

Eclipse – Solution 2

If there is nothing wrong with your Run Configurations, you can try to clean and build your project. Note that Build Automatically option is selected.

Eclipse – Solution 3

If you copied the project from another computer, your build path might be pointing incorrect path. To check the build path of your project, select File -> Properties from menu and open Java Build Path.

Eclipse warns you with message: Build path entry is missing: C:/jar_libs/jdbc.jar. To fix the path of jar file, click on Edit button and update with correct path.

Command Line – Solution 1

You can also use command line to compile and run your Java programs. Consider the following HelloWorld class. It is a simple class with a main method. There is no package declaration in the source code.

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

You can compile your source code with javac command and run the compiled code with java command. There are a few things you need to pay attention to while running the code.

Java is case-sensitive, so you should type your class name exactly the same. If you type helloworld instead of HelloWorld, command will give error.

Also, file extensions (like HelloWorld.class) should not be used when running java command.

C:codes>javac HelloWorld.java

C:codes>java HelloWorld
Hello World!

C:codes>java helloworld
Error: Could not find or load main class helloworld

C:codes>java HelloWorld.class
Error: Could not find or load main class HelloWorld.class

Command Line – Solution 2

Following code is similar to previous one, but in this example HelloWorld class is defined under a package. In Java, packages are used to organize similar classes together.

Packages are organized as directories on your file system. For instance, HelloWorld class is created under com.abc.study.guide package, so HelloWorld.java file should be in {project-root}/com/abc/study/guide folder.

package com.abc.study.guide;

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

To run a class, we are using FQN (Fully Qualified Name) of the class – that is (package.class name). For HelloWorld class in our example FQN is com.abc.study.guide.HelloWorld. Thus, java comand to run this class is: java com.abc.study.guide.HelloWorld.

C:codes>javac comabcstudyguideHelloWorld.java

C:codes>java HelloWorld
Error: Could not find or load main class HelloWorld

C:codes>java com.abc.study.guide.HelloWorld
Hello World!

You can run HelloWorld class from any directory in file system. But if you are running java command from outside the project root directory, classpath parameter should be used. Classpath tells JVM where to look for classes – it’s path of classes. You can specify the class path is by using the -classpath (or -cp) command line switch.

C:another-directory>java com.abc.study.guide.HelloWorld
Error: Could not find or load main class com.abc.study.guide.HelloWorld

C:another-directory>java -classpath C:codes  com.abc.study.guide.HelloWorld
Hello World!

C:another-directory>java -cp C:codes  com.abc.study.guide.HelloWorld
Hello World!

C:codescomabcstudyguide>java com.abc.study.guide.HelloWorld
Error: Could not find or load main class com.abc.study.guide.HelloWorld

C:codescomabcstudyguide>java -cp ../../../.. com.abc.study.guide.HelloWorld
Hello World!

Источник

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.

Could not find or load main class in Java

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“.

Could not find or load 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

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
  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

Fix the Could Not Find or Load Main Class Error 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.

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