What are the differences between ClassNotFoundException and NoClassDefFoundError in Java?

Both ClassNotFoundException and NoClassDefFoundError are the errors when JVM or ClassLoader not able to find appropriate class while loading at run-time. ClassNotFoundException is a checked exception and NoClassDefFoundError is an Error which comes under unchecked.

There are different types of ClassLoader loads classes from difference sources, sometimes it may cause library JAR files missing or incorrect class-path which causes loader not able to load the class at run-time.

ClassNotFoundException

ClassNotFoundException comes when we try to load a class at run-time using Reflection and if those class files are missing then application or program thrown with ClassNotFoundException Exception. There is nothing to check at compile-time since it is loading the class at run-time.

Example

Live Demo

public class ClassNotFoundExceptionTest {
   public static void main(String[] args) {
      try {
         Class.forName("Test");
      } catch (ClassNotFoundException cnfe) {
         System.err.println("You are trying to search for a class is not existing. "+cnfe);
      }
   }
}

Output

You are trying to search for a class is not existing. java.lang.ClassNotFoundException: Test

NoClassDefFoundError

NoClassDefFoundError is thrown when a class has been compiled with a specific class from the classpath but if same class not available during run-time. Missing JAR files are the most basic reason to get NoClassDefFoundError. As per Java API docs "The searched-for class definition existed when the currently executing class was compiled, but the definition can no longer be found."

Example

Live Demo

class Test1 {
   public void show() {
      System.out.println("show() method called");
   }
}
public class Test2 {
   public static void main(String[] args) {
      Test1 t = new Test1();
      t.show();
   }
}

When we compile both the classes we will be getting two class files Test1.class and Test2.class, while running Test2 class just remove the Test1.class file then we will be getting NoClassDefFoundError as below

Output

Exception in thread "main" java.lang.NoClassDefFoundError: Test1
         at Test2.main(Test2.java:9)