Error could not find or load main class program class

A common problem that new Java developers experience is that their programs fail to run with the error message: Could not find or load main class ... What does this mean, what causes it, and how ...

The java <class-name> command syntax

First of all, you need to understand the correct way to launch a program using the java (or javaw) command.

The normal syntax1 is this:

    java [ <options> ] <class-name> [<arg> ...]

where <option> is a command line option (starting with a «-» character), <class-name> is a fully qualified Java class name, and <arg> is an arbitrary command line argument that gets passed to your application.

1 — There are some other syntaxes which are described near the end of this answer.

The fully qualified name (FQN) for the class is conventionally written as you would in Java source code; e.g.

    packagename.packagename2.packagename3.ClassName

However some versions of the java command allow you to use slashes instead of periods; e.g.

    packagename/packagename2/packagename3/ClassName

which (confusingly) looks like a file pathname, but isn’t one. Note that the term fully qualified name is standard Java terminology … not something I just made up to confuse you :-)

Here is an example of what a java command should look like:

    java -Xmx100m com.acme.example.ListUsers fred joe bert

The above is going to cause the java command to do the following:

  1. Search for the compiled version of the com.acme.example.ListUsers class.
  2. Load the class.
  3. Check that the class has a main method with signature, return type and modifiers given by public static void main(String[]). (Note, the method argument’s name is NOT part of the signature.)
  4. Call that method passing it the command line arguments («fred», «joe», «bert») as a String[].

Reasons why Java cannot find the class

When you get the message «Could not find or load main class …», that means that the first step has failed. The java command was not able to find the class. And indeed, the «…» in the message will be the fully qualified class name that java is looking for.

So why might it be unable to find the class?

Reason #1 — you made a mistake with the classname argument

The first likely cause is that you may have provided the wrong class name. (Or … the right class name, but in the wrong form.) Considering the example above, here are a variety of wrong ways to specify the class name:

  • Example #1 — a simple class name:

    java ListUser

    When the class is declared in a package such as com.acme.example, then you must use the full classname including the package name in the java command; e.g.

    java com.acme.example.ListUser

  • Example #2 — a filename or pathname rather than a class name:

    java ListUser.class
java com/acme/example/ListUser.class

  • Example #3 — a class name with the casing incorrect:

    java com.acme.example.listuser

  • Example #4 — a typo

    java com.acme.example.mistuser

  • Example #5 — a source filename (except for Java 11 or later; see below)

    java ListUser.java

  • Example #6 — you forgot the class name entirely

    java lots of arguments

Reason #2 — the application’s classpath is incorrectly specified

The second likely cause is that the class name is correct, but that the java command cannot find the class. To understand this, you need to understand the concept of the «classpath». This is explained well by the Oracle documentation:

  • The java command documentation
  • Setting the Classpath.
  • The Java Tutorial — PATH and CLASSPATH

So … if you have specified the class name correctly, the next thing to check is that you have specified the classpath correctly:

  1. Read the three documents linked above. (Yes … READ them! It is important that a Java programmer understands at least the basics of how the Java classpath mechanisms works.)
  2. Look at command line and / or the CLASSPATH environment variable that is in effect when you run the java command. Check that the directory names and JAR file names are correct.
  3. If there are relative pathnames in the classpath, check that they resolve correctly … from the current directory that is in effect when you run the java command.
  4. Check that the class (mentioned in the error message) can be located on the effective classpath.
  5. Note that the classpath syntax is different for Windows versus Linux and Mac OS. (The classpath separator is ; on Windows and : on the others. If you use the wrong separator for your platform, you won’t get an explicit error message. Instead, you will get a nonexistent file or directory on the path that will be silently ignored.)

Reason #2a — the wrong directory is on the classpath

When you put a directory on the classpath, it notionally corresponds to the root of the qualified name space. Classes are located in the directory structure beneath that root, by mapping the fully qualified name to a pathname. So for example, if «/usr/local/acme/classes» is on the class path, then when the JVM looks for a class called com.acme.example.Foon, it will look for a «.class» file with this pathname:

  /usr/local/acme/classes/com/acme/example/Foon.class

If you had put «/usr/local/acme/classes/com/acme/example» on the classpath, then the JVM wouldn’t be able to find the class.

Reason #2b — the subdirectory path doesn’t match the FQN

If your classes FQN is com.acme.example.Foon, then the JVM is going to look for «Foon.class» in the directory «com/acme/example»:

  • If your directory structure doesn’t match the package naming as per the pattern above, the JVM won’t find your class.

  • If you attempt rename a class by moving it, that will fail as well … but the exception stacktrace will be different. It is liable to say something like this:

    Caused by: java.lang.NoClassDefFoundError: <path> (wrong name: <name>)

    because the FQN in the class file doesn’t match what the class loader is expecting to find.

To give a concrete example, supposing that:

  • you want to run com.acme.example.Foon class,
  • the full file path is /usr/local/acme/classes/com/acme/example/Foon.class,
  • your current working directory is /usr/local/acme/classes/com/acme/example/,

then:

# wrong, FQN is needed
java Foon

# wrong, there is no `com/acme/example` folder in the current working directory
java com.acme.example.Foon

# wrong, similar to above
java -classpath . com.acme.example.Foon

# fine; relative classpath set
java -classpath ../../.. com.acme.example.Foon

# fine; absolute classpath set
java -classpath /usr/local/acme/classes com.acme.example.Foon

Notes:

  • The -classpath option can be shortened to -cp in most Java releases. Check the respective manual entries for java, javac and so on.
  • Think carefully when choosing between absolute and relative pathnames in classpaths. Remember that a relative pathname may «break» if the current directory changes.

Reason #2c — dependencies missing from the classpath

The classpath needs to include all of the other (non-system) classes that your application depends on. (The system classes are located automatically, and you rarely need to concern yourself with this.) For the main class to load correctly, the JVM needs to find:

  • the class itself.
  • all classes and interfaces in the superclass hierarchy (e.g. see Java class is present in classpath but startup fails with Error: Could not find or load main class)
  • all classes and interfaces that are referred to by means of variable or variable declarations, or method call or field access expressions.

(Note: the JLS and JVM specifications allow some scope for a JVM to load classes «lazily», and this can affect when a classloader exception is thrown.)

Reason #3 — the class has been declared in the wrong package

It occasionally happens that someone puts a source code file into the
the wrong folder in their source code tree, or they leave out the package declaration. If you do this in an IDE, the IDE’s compiler will tell you about this immediately. Similarly if you use a decent Java build tool, the tool will run javac in a way that will detect the problem. However, if you build your Java code by hand, you can do it in such a way that the compiler doesn’t notice the problem, and the resulting «.class» file is not in the place that you expect it to be.

Still can’t find the problem?

There lots of things to check, and it is easy to miss something. Try adding the -Xdiag option to the java command line (as the first thing after java). It will output various things about class loading, and this may offer you clues as to what the real problem is.

Also, consider possible problems caused by copying and pasting invisible or non-ASCII characters from websites, documents and so on. And consider «homoglyphs», where two letters or symbols look the same … but aren’t.

You may run into this problem if you have invalid or incorrect signatures in META-INF/*.SF. You can try opening up the .jar in your favorite ZIP editor, and removing files from META-INF until all you have is your MANIFEST.MF. However this is NOT RECOMMENDED in general. (The invalid signature may be the result of someone having injected malware into the original signed JAR file. If you erase the invalid signature, you are in infecting your application with the malware!) The recommended approach is to get hold of JAR files with valid signatures, or rebuild them from the (authentic) original source code.

Finally, you can apparently run into this problem if there is a syntax error in the MANIFEST.MF file (see https://stackoverflow.com/a/67145190/139985).

Alternative syntaxes for java

There are three alternative syntaxes for the launching Java programs using the java command.

  1. The syntax used for launching an «executable» JAR file is as follows:

    java [ <options> ] -jar <jar-file-name> [<arg> ...]

    e.g.

    java -Xmx100m -jar /usr/local/acme-example/listuser.jar fred

    The name of the entry-point class (i.e. com.acme.example.ListUser) and the classpath are specified in the MANIFEST of the JAR file.

  2. The syntax for launching an application from a module (Java 9 and later) is as follows:

    java [ <options> ] --module <module>[/<mainclass>] [<arg> ...]

    The name of the entrypoint class is either defined by the <module> itself, or is given by the optional <mainclass>.

  3. From Java 11 onwards, you can use the java command to compile and run a single source code file using the following syntax:

    java [ <options> ] <sourcefile> [<arg> ...]

    where <sourcefile> is (typically) a file with the suffix «.java».

For more details, please refer to the official documentation for the java command for the Java release that you are using.

IDEs

A typical Java IDE has support for running Java applications in the IDE JVM itself or in a child JVM. These are generally immune from this particular exception, because the IDE uses its own mechanisms to construct the runtime classpath, identify the main class and create the java command line.

However it is still possible for this exception to occur, if you do things behind the back of the IDE. For example, if you have previously set up an Application Launcher for your Java app in Eclipse, and you then moved the JAR file containing the «main» class to a different place in the file system without telling Eclipse, Eclipse would unwittingly launch the JVM with an incorrect classpath.

In short, if you get this problem in an IDE, check for things like stale IDE state, broken project references or broken launcher configurations.

It is also possible for an IDE to simply get confused. IDE’s are hugely complicated pieces of software comprising many interacting parts. Many of these parts adopt various caching strategies in order to make the IDE as a whole responsive. These can sometimes go wrong, and one possible symptom is problems when launching applications. If you suspect this could be happening, it is worth trying other things like restarting your IDE, rebuilding the project and so on.

Other References

  • From the Oracle Java Tutorials — Common Problems (and Their Solutions)
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

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

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

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

Добрый день. Специально зарегестрировался, чтобы описать решение этой проблемы.
Сам недели две назад тоже мучился и думал что я м.д.к, потому ничего не работает.

Это не совсем так, поэтому читайте ниже.

Суть проблемы:
При повторение видео урока, а точнее действий в нем, желаемый результат не выводился. Конкретнее была написана программа вывода сообщения Hellow World в консоли WIndows
А именно, компилированный ява файл, вызывался командой java ИМЯ.class, и выходила соответствующая ошибка: «could not find or load main class ИМЯ_класса»
т
Как решил:
Вызывать надо уже ОТКОМПИЛИРОВАННЫЙ файл командой[b][i] java ИМЯ [/i][/b]И все!!! Тогда консоль нормально воспроизводит программу, если, разумеется, в ней нет ошибок.
Возможно для гуру программирования, это очевидный ответ, и кажется что это норма, но я как 3-ью неделю изучающий яву, на второй день реально не понимал в чем суть проблемы. И так как несмотря на то что обучение идет трудно, мне было обидно и непонятно почему это не работает.

Ниже распишу полный алгоритм работы с ява машиной на базовом уровне, может быть кому-то это поможет.

1) Скачать JDK c оф сайта (ссылку давать не буду, это точно найти сможете)
2) Установить скачанный пакет
3) Подключить установленную ява машину к нашей системе ( у меня это Windows 7)
а) зайти в папку с установленным пакетом и найти файл java.exe (у меня на Windows это было: C:Program FilesJavajdk1.8.0_05binjava.exe)
_______обращаю внимание, что находить именно файл java.exe не обязательно, просто я сделал так=)
б) Нажать на java.exe правой кнопкой мыши, выбрать «Свойства». В выскочевшем окошке, во вкладке «Общие», ищите строку «Расположение». Выделяйте и копируйте путь (у меня это C:Program FilesJavajdk1.8.0_05bin)
в) Открываем пуск, наводим мышку на «Мой компьютер», жмем правую кнопку мыши, жмем «Свойства».
г) В открывшемся окне, слева, жмем кнопку «Дополнительный параметры системы»
д) В отрывшемся окне «Свойства системы», во вкладке «Дополнительно» (она по умолчанию выделена/открыта), внизу ищем кнопку «Переменные среды». Жмем на нее
е) В открывшемся окне «Переменные среды», в верхней его части, а именно «Переменная среды пользователя…» нажимаем кнопку создать
ж) В открывшемся окне вводим имя переменной, у меня это Path (можно ли другую, я не знаю), а в поле «Значение» вводим наш скопированный путь (C:Program FilesJavajdk1.8.0_05bin)
д) Жмем «ОК», сохраняем все.
4) Проверяем нашу ява машину
а) Запускаем консоль (либо WIN + r => затем cmd и Enter либо Пуск => вводим в поиск над пуском cmd и жмем Enter)
б) В открывшейся консоли, вводим java
в) После этого должно появиться куча команд ява, служебная информция и прочее, если все нормально подключилось, если нет, то выдаст ошибку, вроде «Нет такой команды», или что-то похожее. Если что-то не так, делай действия выше по новой.
5)Если все ок, идем дальше. Создаем нашу простую программу на java.
6)Открываем блокнот, и пишем там код ниже:
[code=java]public class Hellow
{
public static void main(String[] args)
{
System.out.println(«Hellow World!»);

}
}
[/code]
Обращаю внимание, что тут частые ошибки, это не написан метод «main», и «println» ошибочно написано. Внимательно проверьте что напечатали сами. Если вы абсолютный новичек, то прежде чем будете понимать что происходит, должно пройти время, поэтому не умничайте, и проверяйте вплоть до каждой запятой.
7) Сохраняем наш файл. Вот тут обратите внимание на деталь: «В строке public class Hellow, слово Hellow, это имя класса, оно по сути, может быть почти любым, но очень важно, чтобы это имя совпадало с именем вашего файла, что вы сохраняете, причем если у вас он с большой буквы, значит и имя файла должно быть с большой. Сегодня я выяснил что в java вообще принято давать имена классов с большой буквы, так что не ленитесь, пишите с большой, но с маленькой, тоже не будет ошибкой, главное сохранить правило, какое имя класса, такое и имя файла.
Сохранять надо с расширением .java
В итоге, в нашем примере нужно сохранить файл в таком виде Hellow.java (вроде бы есть исключения, типо можно обозвать файл как хочу, но я не уверен, увы, но пока этого не знаю, так что делайте как выше написано, потом если узанете правду, напишите мне, благодарен буду)
8) На данном этапе, мы просто создали файл, который может откомпилировать программа, вторая ошибка, это желать сразу открыть этот файл и исполнить его. Деталей я не знаю, но суть в том, что современные Операционки и процессоры, не умеют это делать, не знаю почему, вроде что то там с процессом производства компьютеров, они вроде 4-битные, или наоброт не 4 битные, короче этот код просто так не запустить, его нужно откомпилировать, перевести в машинный язык, который можно запускать, для этого идем к след шагу
9)Запускаем консоль, в ней переходим в папку с нашим файлом Hellow.java
Обращаю внимание, так как я полный новичек, то как переходить в консоли, тоже понятия не имел, на всякий случай распишу минимум, который нужно знать
Чтобы зайти в каталог, нужно ввести команду cd ИМЯ_каталога. Например, нам нужно зайти в каталог/папку Desktop , вводим cd desktop. Разумеется, зайти мы туда сможем, только если эта папка находимся в том месте, где мы есть. Текущее местоположение отображается слева от вводимой команды в виде C:Program Files
Чтобы вернуться в корневую папку диска C нужно ввести cd
Как-то можно подыматься на каталог выше, и прочие действия, но тут уже гуглите, у меня задача другая.
Команда dir показывает список всех папок в данном каталоге/папке
Если вы знаете точный путь к каталогу, можно из любого месте прописать полный путь в виде C:Program FilesJava… и вы туда попадете. В конце обязательно ставте , а в начале диск C например. Иначе не сможете зайти.
Команад help выводит вроде базовый список возможных команд

Итого, алгоритм действий для попадания в нашу папку, где лежит файл Hellow.java
ввести cd
потом dir
прочитать что там есть, зайти в нужную папку командой cd …
снова dir
и так далее
Разумеется нужно понимать самому, где находиться твой файл, например, путь для файла, сохраненного на рабочем столе будет таким c:usersИМЯПОЛЬЗОВАТЕЛЯdesktop
10) Теперь нам нужно откомпилировать файл Hellow.java. Для этого в консоли пишем javac Hellow.java
Если все ок, через пару секунд снова загорится поле ввода команды.
Если выскочила ошибка, как правило это какой либо косяк на этапе компиляции, читайте что там написано, там всегда пишут в чем была ошибка. Чаще всего это косяк в коде. Заходите в ваш файл и проверяйте внимательно строки. Чем чаще будете читать ошибки, тем быстрее научитесь их распозновать, Знание английского в 100500 раз облегчит работу.
11) Когда файл откомпилировался, он сохраняется в той же папке, что и основной (Hellow.java), принимая имя, вроде бы класса, это детали, я их увы, не знаю, но расширение станет уже .class В нашем случае это будет файл Hellow.class
12) Вот только теперь можно запускать этот файл, для проверки, что он у нас есть в папке, где мы находимся, в консоли вводим команду dir
видим наш файл, и вводим команду java Hellow
Причем обращаю внимание, не java Hellow.class , а именно java Hellow. Так как первый вариант выдаст ошибку, уж почему, извините, я не знаю.

13) в консоли, на новой строке, выскочит сообщение Hellow World!
Все.

Вот собственно алгоритм запуска ява программ из консоил виндоус.
Я не претендую на полноту изложения, убер крутой и новый материал, просто я две недели сам мучился с этой проблемой, и писал код в среде разработчика IDEA. И мучался тем, что имея уже какое-то представление о Java, не мог запустить ее через консоль. Меня это мучило, поэтому когда я смотря очередную лекцию, увидел в чем была моя ошибка, меня осенило, и я решил свою проблемы, сняв камень с души, и как следствие, успокоившись. На радостях, я решил, что если в мире есть хотя бы еще один человек, которому эта информация может помочь, то пусть будет так.

Спасибо за внимание и заранее извините за ошибки. Если когда-нибудь надо будет, может быть я исправлю все недочеты своей короткой статьи, а пока держите, как есть.
Удачи в программировании!

Источник

Introduction

Java developers often face the ‘could not find or load main class’ error out of the blue during compilation. If you keep getting this error without any specific reason, you are not alone. Whether you’re just starting out as a programmer or have some experience under the belt, we’ve all seen this error at least once. And we know it has nothing to do with our code.

Why does the “JVM could not find or load the main class” error occur?

As the name suggests, the ‘could not find or load main class’ error means that the JVM (Java Virtual Machine) could not locate the main class in your code and throws this runtime error. The question is can we not find it?

It is one of the most unpredicted and spontaneous errors in Java, which occurs due to the tendency of JVM to stick with a default classpath, the “main class not found issue” is something that haunts amateurs and professionals alike. As serious as it seems, it is not that difficult to fix. We will be exploring in this article how you can easily fix this annoying Java compilation error.

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What Is Classpath?

Before we dive into the how and why of it, we need to understand what Classpath is and its role in Java.

The classpath is the file path where the JRE (Java runtime environment) searches for the classes and other resource files to run the code. As the name suggests, it is simply a file path where the .class format files can be found in a JDK package or directory.

Classpath can be set using two ways:

  1. Using the -classpath option at the time of executing the code,
  2. By setting the file path to the system CLASSPATH environment variable.

When the JVM is unable to locate the main class, it is usually because you would have entered the wrong .class name to run the classpath or the corresponding .class files have been altered.

See this example of generating a class file of a simple code:

1. public class example01 {
2. public static void main(String[] args) {
3. out.println("This is a simple code");
4. }
5. }

To run a .class file, you can use the following command:

java <.class filename>

Now if we run this line of code to run the class we made.

$ java eg01

Output:

Error: Could not find or load main class

It will fail with the error “Could not find or load main class eg01.”

As mentioned earlier, the .class file will have the same name given to the Java class of the program. So, in this case, the main class will have the name example01, not eg01.

Let’s try this one more with the correct name:

$ java example01

Output:

This is a simple code

Now it ran successfully.

While it’s not as simple in a project, the easiest way to rectify it is by either manually specifying the classpath or using packages.

Using Packages

Packages are used in Java to group similar classes or to provide a unique namespace to a group of classes. We will be now creating a class called example02 and place it in a package called example02Package.

1. package example02Package;
2. public class example02 {
3. public static void main(String args[]) {
4. out.println("File is found successfully!");
5. }
6. }

We will then use this package to visualize how the classpath works in Java. In your files directory, a package is represented as an independent folder by its name that you can easily observe in a file manager application.

After ensuring that the working directory is the same as the one that contains the package folder, you can also change the working directory on the terminal (command prompt) by using the cd command on almost every popular operating system.

Example02.java can be compiled by running the following command:

  1. package example02Package;
  2. javac example02Package/example02.java

This will now save the compiled .class file in the example02Package directory.

To run the compiled class, you need to type in the fully qualified class name in the command line. The fully qualified name of a Java class is written by prefixing it with the package name.

For this example, this is the fully qualified name:

java example02Package.example02

Using packages also allows Java developers to call executables from different packages from the same working directory. It can be easily done just by modifying the fully qualified class name without getting the ‘could not find or load main class error.

Manually Specifying Classpath

The other way to prevent could not find or load main class error is by manually specifying the classpath. It is recommended to manage your java files by creating separate directories for all source files and classes.

Just like the .class files are labeled as classes. the directory with source files is labeled as src. It also helps in significantly reducing the chances of JVM not being able to find the main class.

If you use this method for organization, this is how the directory structure of your projects will look like before compilation:

|___Project01
|     |___src
|           |___example02Package
|                       |___example02.java
|
|     |___classes

The indentations in the above example show the one level of the file hierarchy that your project should be following.

When compiling this code, you must make sure that your working directory is Project01. The following command is used to execute it:

javac -d classes src/testPackage/Test.java

The .class executable file must be saved in Project01/classes/example02Package. Now, the file directory structure will look like this:

|___Project01
|     |___src
|           |___example02Package
|                       |___example02.java
|
|         classes
|           |___example02Package
|                       |___example02.class

To run the .class file, you have to run the Java command with the fully qualified class name and the specification of the local classpath. Every path is declared relative to the working directory, which in this example is Project01.

java -classpath classes example02Package

Running this command will now provide you with the desired output without any chances of getting the error. The question is, why do you need to reorganize the files to solve such a small runtime error?

Why Organizing Files is important in Java?

The primary reason behind why the ‘could not find or load main class ’ is encountered is because JVM is unable to locate where your .class files were being saved.

The easiest way to resolve this error and prevent it from ever happening is to organize where the .class files are saved. Developers have to explicitly indicate the JVM to look for the .class file in the assigned location. This can only be done by organizing the source files and executables separately and using the working directory to manage either manually or using packages.

Your project code will likely keep expanding over time as the project work keeps going. By adding more constructs such as inheritance, inner classes, and more to your project, the file system keeps getting more complex. In such projects, this simple practice of organizing files can save you several hours of precious time that it would take in debugging if something goes wrong and you end up getting the could not find or load main class error.

See Also: What Are Annotations in Java – How do They Work

Conclusion

“Could not find or load main class” error is very common in Java however, we have discussed some effective ways to prevent this error in the article above. The file organization methods we discussed not only prevent this error but also make your code and directories manageable. These fixes go a long way for Java developers as they save a lot of time and trouble when debugging, especially for complex codes!

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Источник

Java Guide to Fix the Error «Could not find or load main class».

1. Overview

In this tutorial, We’ll learn What does «Could not find or load main class» mean?

A common problem that new Java developers experience is that their programs fail to run with the error message: Could not find or load main class.

What does this mean, what causes it, and how should you fix it?

The problem is with this java run command

We have gathered all these solutions from StackOverflow page and arranged in a simple manner to understand easily.

[Fixed] Error: Could not find or load main class

Please read section 2 and 5 to get a clear understanding. Section 3 tells about all areas where we do mistake usually. But, recommend reading the complete article.

2. Error Definition

According to the error message («Could not find or load main class»), there are two categories of problems:

2.1 Main class could not be found

The main class could not be found when there is a typo or wrong syntax in the fully qualified class name or it does not exist in the provided classpath.

2.2 Main class could not be loaded

The main class could not be loaded when the class cannot be initiated, typically the main class extends another class and that class does not exist in the provided classpath.

For example:

public class YourMain extends org.apache.camel.spring.Main

If camel-spring is not included, this error will be reported.

Note: To solve «Could not find or load main class», we. should go to the starting of the class package folder and run java command with the fully classified class name. Or missing the class in classpath. 

3. Java run command syntax

First of all, we need to understand the correct way to launch a program using the java (or javaw) command.

The normal syntax1 is below:

java [ <option> ... ] <class-name> [<argument> ...]

<option>: is a command line option — starting with a «-» character
<class-name> is a fully qualified Java class name
<argument> is an arbitrary command line argument that gets passed to your application.

The fully qualified name (FQN) for the class is conventionally written as in Java source code.

e.g.

packagename.packagename2.packagename3.ClassName

However, some versions of the java command allow you to use slashes instead of periods.

e.g.

packagename/packagename2/packagename3/ClassName

which (confusingly) looks like a file pathname but isn’t one. Note that the term fully qualified name is standard Java terminology … not something I just made up to confuse you :-)

Here is an example of what a java command should look like:

java -Xmx100m com.java.w3schools.ListUsers jhon peter

where -Xmx indicates the maximum allocated memory for this run.

The above command performs the following steps when we run command.

A) Search for the compiled version of the com.java.w3schools.ListUsers class.
B) Load the class.
C) Check that the class has a main method with signature, return type and modifiers given by public static void main(String[]). (Note, the method argument’s name is NOT part of the signature.)
D) Call that method passing it the command line arguments («Jhon», «Peter») as a String[] and passed to the main method.

4. Reasons why Java cannot find the class

When we get the message «Could not find or load main class …», that means that the first step A has failed. The java command was not able to find the class.

4.1 You made a mistake with the class name argument

Example #1 — a simple class name

java ListUser

When the class is declared in a package such as com.acme.example, then you must use the full class name including the package name in the java command; e.g.

java com.java.w3schools.ListUser

Example #2 — a filename or pathname rather than a class name

java ListUser.class
java com/java/example/ListUser.class

Example #3 — a class name with the case type incorrect

java com.java.w3schools.listuser

Example #4 — a typo

java com.java.w3schools.mistuser

Example #5 — a source filename

java ListUser.java

Example #6 — you forgot the class name entirely

java "jhon" "peter" -> lots of arguments

4.2 The application’s classpath is incorrectly specified

The second likely cause is that the class name is correct, but that the java command cannot find the class. To understand this, you need to understand the concept of the «classpath». This is explained well by the Oracle documentation:

The java command documentation
Setting the Classpath.
The Java Tutorial — PATH and CLASSPATH

To give a concrete example, supposing that:

you want to run com.java.w3schools.Foon class,
the full file path is /usr/local/java/classes/com/java/w3schools/Foon.class
your current working directory is /usr/local/java/classes/com/java/w3schools/

# wrong, FQN is needed
java Foon

# wrong, there is no com/java/w3schools` folder in the current working directory
java com.java.w3schools.Foon

# wrong, similar to above
java -classpath . com.java.w3schools.Foon

# fine; relative classpath set
java -classpath ../../.. com.java.w3schools.Foon

# fine; absolute classpath set
java -classpath /usr/local/java/classes com.java.w3schools.Foon

4.3 Dependencies Missing from Classpath

The classpath needs to include all of the other (non-system) classes that your application depends on. (The system classes are located automatically, and you rarely need to concern yourself with this.) For the main class to load correctly, the JVM needs to find:

A) The class itself.
B) All classes and interfaces in the superclass hierarchy (e.g. see Java class is present in classpath but startup fails with Error: Could not find or load main class)
C) All classes and interfaces that are referred to by means of variable or variable declarations, or method call or field access expressions.

4.4 The class has been declared in the wrong package

It occasionally happens that someone puts a source code file into the wrong folder in their source code tree, or they leave out the package declaration. If you do this in an IDE, the IDE’s compiler will tell you about this immediately. Similarly, if you use a decent Java build tool, the tool will run javac in a way that will detect the problem. However, if you build your Java code by hand, you can do it in such a way that the compiler doesn’t notice the problem, and the resulting «.class» file is not in the place that you expect it to be.

5. Another Example to Java Newbie

If the compiled classes are in packages then we have to cd to the root directory of the project and run using the fully qualified name of the class (packageName.MainClassName).

Example:

My classes are in here:

The fully qualified name of my main class is:

com.cse.Main

So I cd back to the root project directory:

D:project

Then issue the java command:

java com.cse.Main

This answer is for rescuing newbie java programmers from the frustration caused by a common mistake, I recommend you read the accepted answer for more in-depth knowledge about the java classpath.

6. Conclusion

In this post, We have discussed a problem «Error: Could not find or load main class». Every Java programmer must have seen this when they are new to the java.

And seen the common error-prone areas for this problem and provided solutions.

Original Article

Источник

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