ClassNotFoundException vs NoClassDefFoundError in Java

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In Java, two commonly confused runtime issues are ClassNotFoundException and NoClassDefFoundError. Both are related to class loading issues, but they are very different in their nature, cause, and how they should be handled.

Understanding the distinction between these two is essential when debugging Java applications, especially in large projects with multiple modules, external dependencies, or dynamic class loading.

In this article, we will dive deep into:

• What ClassNotFoundException and NoClassDefFoundError are
• Why and when they occur
• Real code examples that trigger them
• Key differences between them

Overview of ClassNotFoundException

• It is a checked exception.
• Belongs to the java.lang.Exception class hierarchy.
• Occurs when a class is dynamically loaded at runtime (like using Class.forName()), but not found in the classpath.

Overview of NoClassDefFoundError

• It is an unchecked error.
• Belongs to the java.lang.Error class hierarchy.