Error could not find or load main class helloworld class

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 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

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.

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

Понравилась статья? Поделить с друзьями:
  • Error could not find or load main class game
  • Error could not find or load main class forge
  • Error could not find or load main class example class
  • Error could not find or load main class cpw mods fml relauncher serverlaunchwrapper
  • Error could not find or load main class com company main