ClassNotFoundException Vs NoClassDefFoundError

ClassNotFoundException :  ClassNotFoundException occurs when class loader could not find the required class in class path . So , basically you should check your class path and add the class in the classpath.

NoClassDefFoundError : This is more difficult to debug and find the reason. This is thrown when at compile time the required classes are present , but at run time the classes are changed  or removed or class's static initializes threw exceptions. It means the class which is getting loaded is present in classpath , but one of the classes which are required by this class , are either removed or failed to load by compiler .So you should see the classes which are dependent on this class .

Example :

public class Test1
{
}

public class Test 

{
   public static void main(String[] args)

   {
        Test1 = new Test1();    
    }

}

    
Now after compiling both the classes , if you delete Test1.class file , and run Test class , it will throw

Exception in thread "main" java.lang.NoClassDefFoundError: Test
        at Test1.main(Test1.java:5)
Caused by: java.lang.ClassNotFoundException: Test
        at java.net.URLClassLoader$1.run(Unknown Source)
        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)
        ... 1 more



ClassNotFoundException :-
Thrown when an application tries to load in a class through its name, but no definition for the class with the specified name could be found

NoClassDefFoundError :-
Thrown if the Java Virtual Machine tries to load in the definition of a class and no definition of the class could be found.