Java.Lang.Classnotfoundexception When Running in Intellij Idea

Error ClassNotFoundException in IntelliJ IDEA

Probably your project structure was src/main/java/start.java but when you added it to IntelliJ you have set src as a source folder, so IntelliJ put main.java as a package.

In Project Setting (Ctrl+Shift+Alt+S) → Modules → Sources tab set src/main/java as Source Folder. Then in your simple program change package to whatever you like (e.g. my.test). After that if IntelliJ reports any error in line with package quick fix with Alt+Enter should help

java.lang.ClassNotFoundException when running in IntelliJ IDEA

The error that you get occurs not on complilation, but when you try to run your application. It happens because Java was not able to find Table.class file inside db subdirectory of the project output directory (classpath).

It can happen for multiple reasons:

wrong main class selected in the run/debug configuration
Table.java is excluded from compilation (by accident or intentionally because it contained errors and you wanted to skip it while working on other code)
class not compiled because Build step is excluded from from Before launch steps in the Run/Debug configuration
project is misconfigured and there is no Source root defined for the directory containing db subdirectory
Table.java has incorrect package statement or is located/moved to a different package
project path contains a colon : on Mac/Linux or semicolon ; on Windows, it's used to separate the classpath and will render the classpath invalid. See this thread for details. Note that Finder on Mac may display colons in the path as slashes.
the jar may not execute if one of the dependent jars is digitally signed since the new artifact will include the partial signature of the dependency. See this answer for more details.
In project structure make sure you have the right Java version for compile.
there is a known bug that sometimes a Java project created from the Command Line template doesn't work because .idea/modules.xml file references invalid module file named untitled104.iml. Fix the module name manually or create a project from scratch and don't use a template.
on Windows "Beta: Use Unicode UTF-8 for worldwide language support" Region Setting is enabled. See IDEA-247837 for more details and workarounds.
When IntelliJ IDEA is configured to store module dependencies in Eclipse format source root configuration is lost due to a known bug. Configure the module to use IntelliJ IDEA format dependencies as a workaround.

In a properly configured project and with the correct run/debug configuration everything works just fine:

Run

intelliJ IDEA: java.lang.ClassNotFoundException

Exception in thread "main" java.lang.ClassNotFoundException: Main

In the run/debug configuration you have written wrong class name for the Main class field. The name of the class should be the class that has main method to run.

This error could also happen if you didn't create or didn't select run/debug configuration for the class. To create it use Edit Configurations -> Add new under application tree item or from the editor press Alt+hift+F10 and choose your file to run. The configuration will be added automatically. To select current run/debug configuration use dropdown from the toolbar.

Usually I prefer to choose from menu Run -> Run, but as @Bajal mentioned in the comment, you can right-click on the file from the Project Structure and choose Run from the popup menu or press Ctrl+Shift+F10.

Why get java.lang.ClassNotFoundException when run as jar, but work well with IntelliJ IDEA

There is an option called Enable launch optimization in IntelliJ IDEA run config, uncheck it and everything work as expected.

My project runs well from IntelliJ IDEA while encounters java.lang.ClassNotFoundException from CLI tool, how do I fix it?

You need to tell maven to include the dependencies. When IntelliJ runs it for you, it adds all the dependencies to the classpath, which is why it works from IntelliJ.

Try this

If you run mvn package and unzip the resulting jar with your updated pom.xml, you will see the dependencies are still not included.

The reason for this is that you need to put

                <plugin>
                    <artifactId>maven-assembly-plugin</artifactId>
                    <executions>
                        <execution>
                            <phase>package</phase>
                            <goals>
                                <goal>single</goal>
                            </goals>
                        </execution>
                    </executions>
                    <configuration>
                        <descriptorRefs>
                            <descriptorRef>jar-with-dependencies</descriptorRef>
                        </descriptorRefs>
                    </configuration>
                </plugin>

under

    <build>
        <plugins>
                <plugin>
                    <artifactId>maven-assembly-plugin</artifactId>
                    <executions>
                        <execution>
                            <phase>package</phase>
                            <goals>
                                <goal>single</goal>
                            </goals>
                        </execution>
                    </executions>
                    <configuration>
                        <descriptorRefs>
                            <descriptorRef>jar-with-dependencies</descriptorRef>
                        </descriptorRefs>
                    </configuration>
                </plugin>
        </plugins>
        <pluginManagement>
            <plugins>
                <plugin>
                    <groupId>org.apache.maven.plugins</groupId>
                    <artifactId>maven-compiler-plugin</artifactId>
                    <version>3.8.1</version>
                </plugin>
            </plugins>
        </pluginManagement>
    </build>

Not inside pluginManagement.
I tried it, it generates a jar called ...-with-dependencies and your java command then runs fine.

IntellijIDEA + Gradle + JavaFX: java.lang.ClassNotFoundException for one Run Configuration but not the other

All right.

It turns out that the widget for Build and Run is whitespace sensitive...

Which becomes obvious if one looks at the workspace.xml file

<option name="MAIN_CLASS_NAME" value="pack2.ImageViewExample " />

This is why XML should not be dismissed easily as config language.

It's Whitespace Sensitive

A telling sign, if you know what to look for:

One can see the whitespace if one is primed for it

Well, that was easy.

Error report submitted to JetBrains.