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Troubleshooting "Could Not Find or Load Main Class" When Running JAR Files: A Comprehensive Guide

Encountering the "Could not find or load main class" error when trying to run a JAR file is a common headache for Java developers, especially beginners. This error indicates that the Java Virtual Machine (JVM) can't locate the designated entry point – the `main` method – within your compiled code. This tutorial will delve into the common causes, provide solutions with code examples, and offer best practices to avoid this issue altogether.

*Understanding the Error*

The `java.lang.NoClassDefFoundError: Could not initialize class ...` is a related error often associated with this, which signifies the main class was found but failed to initialize due to a dependency issue. Fixing the root cause of the `Could not find or load main class` often resolves that as well.

*Common Causes and Solutions*

Let's break down the most frequent reasons why you might encounter this error, along with detailed solutions:

*1. Missing or Incorrect `Main-Class` Attribute in the JAR Manifest*

*Cause:* Every executable JAR file must have a manifest file (`META-INF/MANIFEST.MF`) that specifies the entry point for the application. This is done using the `Main-Class` attribute. If this attribute is missing, incorrect, or contains a typo, the JVM won't know where to start.

*Solution:*

*Verify the Manifest:* Open your JAR file using a ZIP utility (like 7-Zip, WinRAR, or even the `jar` command itself). Navigate to the `META-INF` directory and examine the `MANIFEST.MF` file.

*Check the `Main-Class` attribute:* Ensure the manifest contains a line that precisely identifies your main class, including the complete package name. For example:



*Case Sensitivity Matters:* The `Main-Class` attribute is case-sensitive. Ensure that `com.example.MyApplication` exactly matches the fully qualified name of your `main` class.

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