ClassNotFoundException vs. NoClassDefFoundError

Many a times we will come across these exceptions, though they look similar there is a clear difference between these two classes.

package com.test;

/**
 * @author Vijayan Srinivasan
 * @since Sep 22, 2010 9:16:46 PM
 */
public class A {

 public void print() {
  System.out.println("I am A");
 }

}

package com.test;

/**
 * @author Vijayan Srinivasan
 * @since Sep 22, 2010 9:16:46 PM
 */
public class B {

 public static void main(String[] args) {
  try {
   Class clazz = Class.forName("com.test.A");
   A a = (A) clazz.newInstance();
   a.print();
  } catch (ClassNotFoundException e) {
   e.printStackTrace();
  } catch (InstantiationException e) {
   e.printStackTrace();
  } catch (IllegalAccessException e) {
   e.printStackTrace();
  }
 }

}

package com.test;

/**
 * @author Vijayan Srinivasan
 * @since Sep 22, 2010 9:16:46 PM
 */
public class C {

 private A a = new A();

 public static void main(String[] args) {
  new C().a.print();
 }

}

First compile all these classes. Once compilation is complete, then run the class B and C
You will get the following output.

    C:\Test\bin>java com.test.C
    I am A

    C:\Test\bin>java com.test.B
    I am A

Now delete class A and re-run the classes B & C

C:\Test\bin>java com.test.C
Exception in thread "main" java.lang.NoClassDefFoundError: com/test/A
    at com.test.C.(C.java:12)
    at com.test.C.main(C.java:15)
    Caused by: java.lang.ClassNotFoundException: com.test.A
    at java.net.URLClassLoader$1.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.net.URLClassLoader.findClass(Unknown Source)
    at java.lang.ClassLoader.loadClass(Unknown Source)
    at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
    at java.lang.ClassLoader.loadClass(Unknown Source)
    at java.lang.ClassLoader.loadClassInternal(Unknown Source)
    ... 2 more

    C:\Test\bin>java com.test.B
    java.lang.ClassNotFoundException: com.test.A
    at java.net.URLClassLoader$1.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.net.URLClassLoader.findClass(Unknown Source)
    at java.lang.ClassLoader.loadClass(Unknown Source)
    at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
    at java.lang.ClassLoader.loadClass(Unknown Source)
    at java.lang.ClassLoader.loadClassInternal(Unknown Source)
    at java.lang.Class.forName0(Native Method)
    at java.lang.Class.forName(Unknown Source)
    at com.test.B.main(B.java:15)