What causes a ClassNotFoundException versus a NoClassDefFoundError, and how do you distinguish and diagnose each?

ClassNotFoundException is a checked exception thrown when code explicitly attempts to load a class by name at runtime (via Class.forName(), or a ClassLoader's loadClass()) and that class genuinely cannot be found anywhere on the classpath; NoClassDefFoundError is an Error thrown when a class WAS successfully available and used at compile time, but is missing from the classpath at runtime when the JVM actually attempts to load/link it (commonly caused by a dependency JAR being present at compile-time but accidentally excluded from the runtime classpath/deployment package) -- distinguishing them helps pinpoint whether the issue is a genuinely missing class entirely, or a classpath/packaging mismatch between build-time and run-time environments.

// ClassNotFoundException -- explicit runtime lookup of a class that doesn't exist anywhere
try {
    Class.forName("com.example.NonExistentClass");
} catch (ClassNotFoundException e) {
    // handle: the class genuinely isn't found by name
}

// NoClassDefFoundError -- typically NOT caught explicitly; indicates a packaging/classpath problem
// e.g., a dependency present at compile time but missing from the deployed runtime classpath