Could Not Find Or Load Main Class – What Does It Mean?

When you receive this error message, this simply means that the system has failed during the first step itself. The command of Java that you entered could not find the class required to run the program. In most cases, the error message will also be accompanied by the class name that it was looking for, but could not find.

There may be several reasons that may cause this error message to appear on your screen. Let’s talk about them in detail:

1. You did not enter the class name correctly

One of the most common reasons is that you did not provide the correct class name, or you provided the right class name with some mistake. There can be several different ways of providing a wrong class name. Here are a few examples:

• Writing a simple class name: When you declare your class name in a package, like com.acme.example, then you need to use its full class name, including its package name in Java command. If you fail to do that, or if you do that incorrectly, then Java will not be able to identify the class, and you are likely to receive this error message. Instead of writing just a simple class name like java ListUser, you should write java com.acme.example.ListUser.
• Writing a pathname or file name instead of class name: One mistake that new Java developers commonly make is to write the pathname or file name instead of the correct class name.
• Writing the class name with incorrect casing: You need to take care of the casing as well while writing the class name.
• Making a typing or spelling mistake: Java will not identify the class name if you make a spelling or typing mistake while writing it. Make sure that the class name you are writing is completely correct and devoid of any mistakes.
• Writing a source file name: Writing the source file name in the class name will prevent Java from identifying the class name.

2. You specified incorrect class path for the application

Even if you specify the class name in the right way, Java command may not be able to find the needed class. For a deeper understanding of this concept, you need to learn some information about class path as well. If you receive the error message ‘could not find or load main class’, the first thing you need to check is your class name.

If that’s correct, the next thing you need to look into is the class path. Follow these steps to check it:

• Give attention to the command line and the class path environment variable that you used while running the Java command
• Check that the JAR file and directory names are absolutely right
• If you have mentioned some path names in the class path, make sure that they have resolved correctly while running the Java command
• Make sure that the class that is mentioned in your error message is present on the class path as well

3. There is wrong directory on the class path

When a directory is put on the class path, it should correspond to the qualified name space’s root. Class is located in the structure of the directory just below that root, and you can find it by providing a completely qualified name to your path name.

4. Subdirectory path does not match FQN

If the FQN of your class is com.acme.example.Toon, then JVM will look for Toon.class inside the directory. If the structure of your directory does not match the name of the package in a similar pattern, then the JVM will not be able to find the desired class. Renaming the class by shifting its position won’t help either in this case. You need to be careful while selecting between relative and absolute path names in the class paths. Keep in mind that you can break a relative path name if there are changes in the current directory.

5. Lack of dependencies from class path

Class path that you use should include all the non-system classes that you are depending your application on. You need to be least concerned about it, as system classes automatically find their location and you don’t have to do anything about it. In order to correctly load the main class, the JVM finds the following:

• The class
• All the interfaces and classes in the hierarchy of superclass
• All the interfaces and classes referred though variable declarations, field expressions or method calls

Please note that JVM and JLS specifications leave some space for JVM to load the classes, and it can have some effect on when it throws the class loader exception.

6. You declared the class in wrong package

There are chances that you put the source code file in a wrong folder, or you left out the declaration of the package. If you did this in some IDE, then its compiler will alert you about it immediately. If you used a Java build tool, then it will run the Javac in order to detect the problem and alert you. However, if your Java code is built by hand, then the compiler may not be able to detect it and you may receive the ‘could not find or load main class’ error message on your screen.

In order to resolve the error problem, you need to identify its cause first. Initially, you can start by checking the class name and the class path settings. Make sure that you have written the right program and its class name. In order to check that, find out the correct name of your package because you have placed your main class in that package.

If everything seems fine and there is no spelling mistake, then check that you have set the path correctly. If that’s done fine too, then you can import the related language program and compile it. Restart the program, compile it again and you will be able to run it without any error. If you are still getting the error message, then you need to check the directory on the class path, the subdirectory that should match with the FQN, and the package declaration in the class path. Once you identify the cause, you will be able to resolve it without much problem.

Java is a relatively easy language program to learn and operate. However, the ‘could not find or load main class’ error message right in the beginning of running it can be frustrating and discouraging. So, identify the issue that is causing the problem, and resolve it quickly by following the above instructions.

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