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.
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.
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!
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.javaHere, 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 helloworldAnd 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.javaBut 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.classAgain, 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 HelloWorldBut 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.javaFirst, 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.HelloWorldStill, 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 HelloWorldHere, 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.
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: MyClassCaused by: java.lang.ClassNotFoundException: MyClassThis 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 myclassThis 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.javaor
java MyClass.classThe correct way is to run without any extension:
java MyClass4. 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/pathWhile 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 MyClass3. 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.MyClass4. 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?
@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.
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 1Class 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