If you see an error like ‘could not find or load main class,’ this mostly means that you must have made any mistake in declaring the particular class while writing a java program. The context is launching a java program by running a java command into the command prompt. There are varieties of mistakes that usually occur from the user’s end.
This article aims to tell you all such mistakes and scenarios and the methods for fixing them.
Elaborating the syntax ‘java <class-name>
Before we start the process of troubleshooting the java function’s issue, it is very important to know why the terminal keeps giving the error message when tried upon for execution. This will also be better facilitated with a basic understanding of the Java language of programming and how to go about it.
You will be able to better understand by knowing the syntax of the particular command. It is here that the main problem lies. If you use the wrong syntax, then it will inevitably lead to the error.
This is the correct syntax of the java command that we are talking about:
java [ <option> … ] <class-name> [<argument> … ]
In the address above, the <option> denoted the option of the command line. Also, the <class-name> is the name of a fully authentic java class. Lastly, the <argument> denotes the argument of the command line that gets transferred to the application during the entire package is in the process of getting complied.
Let us also give you an example of one valid command to help you understand better:
Java –Xmx100m com.acme.example.ListAppuals kevin arrows bart
When you enter the command given above, then these operations will be executed as a result of it:
- The command will start running a search to find a fully compiled version associated with the class named ‘com.acme.example.ListAppuals‘.
- Once the search finishes, it will start loading the class.
- At this stage, once the class finishes loading, the command will give way for investigating the class to find the main method that has a modifier, return type, and a valid signature. Let us show you an example of the main class that would look something like:
Public static void main(String)
Lastly, the operation will call the method by entering arguments like Bart, arrows, and Kevin in the form of the string.
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The reasons behind the main class not getting found error
When you get the error message like ‘could not find or load main class,’ then this means that while you were trying to run the Java operation, the first step failed. The Java engine could not find the class to execute it. The java command could not find the class inside the directory.
In certain cases, you will have to enable the java terminal to find the correct location by adding the right file path. The computer might not be able to find the class or location because you are using the command line terminal to execute the command. This problem doesn’t occur in the targeted IDE’s because the IDE uses a pointer to point at the directory that is currently working.
Solutions to deal with the main class error
Just like there are many reasons that cause the ‘could not find or load main class’ error, there are many solutions to deal with that. It is clear that the error we are considering is totally unrelated to any fault in the system. It is mostly user-generated as we saw the scenarios that could emerge. In this part, we will give you the best methods that will help you fix the error with a few easy steps.
1. Checking the argument of the class name
One of the very common mistakes that the users make is while entering the class name into the argument box. Either they enter the incorrect class name or enter the wrong form even while entering the right class name. In both cases, the error is bound to occur. As we are setting the parameters into the command line, we will be considering the case that is the user entering the argument of the class name in the incorrect form.
Now we will be listing all the similar scenarios and mistakes that the users can make.
- The first instance of mistake that can happen is while entering the simple class name. When you are declaring the name of the class into a package like ‘cm.acme.example,’ then you should be careful about entering the full class name into the Java command along with the package.
Enter – java com.acme.examples.ListAppuals
In place of
- Take care not to declare the pathname or filename and declare the class name instead, which is the better option. Some of the wrong entries that can be made will look like:
- You must always be considerate about the casing in the commands. The Java commands of programs are highly case sensitive. The whole purpose will go wrong if you enter even one single letter in the wrong case. The result will be the error ‘could not find or load main class.’ Any incorrect command with case mistake would look like:
- It is important that you should avoid declaring the source filename. As we have mentioned before, you only have to be careful about declaring the class in the right format with the full class name. The mistake can look like:
- The last reason for the error related to class name can be when you do any type or may just skip writing the class name fully.
If you may have any such sloppy mistake, be it any from the above, check them up and remove them. After that, you can again try to run the program and see if the ‘could not find or load main class’ error happens again.
2. Check the classpath to fix ‘Could Not Find Or Load Main Class’
Now, if you looked and rectified everything that we mentioned in the method above and still face the same error, then it was probably another reason troubling you. Maybe the java command failed to find the verified class name in the path. This lets the java runtime search for class files and resources in it. Setting a classpath is easy, and you can do the same in two different ways by entering separate commands.
These commands are:
C:> sdkTool –classpath classpath1;classpath2…
C:> set CLASSPATH=classpath1;classpath2…
3. Check the directory to fix ”Could Not Find Or Load Main Class”
It’s important to remember that when we declare any directory for the classpath, the namespace and its root will always be corresponded. Let’s take an example; if you enter “/usr/local/acme/classes” into the classpath, the java command will look for the class “com.acme.example.Appuals“.
The java command will do a search for the following class and pathname:
On the other hand, if you will enter another address like the one below into the classpath, then Java will fail to find the correct class:
Another thing to be taken care of is the subdirectory. You must check if it is similar to the FQN. If the FN of your class is “com.acme.example.Appuals”, the java command will try searching for the “Appuals.class” in the “com/acme/example” directory.
We will be giving you certain examples by taking up some scenarios:
- The class that you are trying to launch is: com.acme.example.Appuals
- The full file path name is: /usr/local/acme/classes/com/acme/example/Appuals.class
- The current working directory name is: /usr/local/acme/classes/com/acme/example/
These scenarios would look like:
# wrong, FQN is needed
# wrong, there is no ‘com/acme/example’ folder in the current working directory
# wrong, similar to above scenario
java –classpath . com.acme.example.Appuals
# OK : a relative classpath is set
java –classpath ../../.. come.acme.example.Appuals
# OK: an absolute classpath is set
java –classpath /usr/local/acme/classes com.acme.example.Appuals
A thing to note here is that the while entering the classpath, you also have to enter every other class (non-system) that the application requires to.
4. Check the class package to fix ”Could Not Find Or Load Main Class”
If the solutions that we gave above proved helpful in the situation you are facing, you must take care of some other things. You must check if you have placed the source code in the appropriate folder. Another thing to check is that the package declared by you is correct. If you try running the code along with an IDA, then you will probably get informed about the issue and all the relevant information.
However, in the case that we are considering, we are using the command prompt. It gives way to chances that you will not notice the mistake, and the ‘could not find or load main class’ error will be the result.
Conclusion on How to Fix Could Not Find Or Load Main Class
These were all the mistakes that you can avoid or remove to get rid of the error ‘could not find or load main class.’ Go through the programs you wrote thoroughly, identify the mistakes, and apply the right solution.