JAVA : ClassNotFoundException vs. NoClassDefFoundError

ClassNotFoundException and NoClassDefFoundError occur when a particular class is not found at runtime. However, they occur at different scenarios.

ClassNotFoundException is an exception that occurs when you try to load a class at run time using Class.forName() or loadClass() methods and mentioned classes are not found in the classpath.

NoClassDefFoundError is an error that occurs when a particular class is present at compile time, but was missing at run time.

ClassNotFoundException

For example, the below program will throw ClassNotFoundException if the mentioned class “oracle.jdbc.driver.OracleDriver” is not found in the classpath.

public class MainClass

{

    public static void main(String[] args)

    {

        try

        {

            Class.forName("oracle.jdbc.driver.OracleDriver");

        }catch (ClassNotFoundException e)

        {

            e.printStackTrace();

        }

    }

}

java.lang.ClassNotFoundException: oracle.jdbc.driver.OracleDriver

NoClassDefFoundError







class A








{







  // some code






}






public class B






{






    public static void main(String[] args)






    {






        A a = new A();






    }






}

When you compile the above program, two .class files will be generated. One is A.class and another one is B.class. If you remove the A.class file and run the B.class file, Java Runtime System will throw NoClassDefFoundError like below:

Exception in thread "main" java.lang.NoClassDefFoundError: A

OR 

java TestTestTest

-> if TestTestTest.class is not there then also we get.