Question: What is a ClassNotFoundException?

Answer: A ClassNotFoundException can happen when you use Java's Reflection capability to dynamically create a class. Here's some example code where I intentionally throw a ClassNotFoundException by trying to create and use a class ("FooClass") that I know doesn't exist:

package com.devdaily.javasamples;

import java.lang.reflect.Method;

public class JavaReflectionExample1
{

  public JavaReflectionExample1()
  {
    Class c;
    try
    {
      c = Class.forName("FooClass");
      Method m[] = c.getDeclaredMethods();
      System.out.println(m[0].toString());
    }
    catch (ClassNotFoundException e)
    {
      // deal with the exception here ...
      e.printStackTrace();
    }
  }

  public static void main(String[] args)
  {
    new JavaReflectionExample1();
  }

}

Because that class doesn't exist, when I try to run this program I get the following Java error message (stack trace):

java.lang.ClassNotFoundException: FooClass
   at java.net.URLClassLoader$1.run(URLClassLoader.java:200)
   at java.security.AccessController.doPrivileged(Native Method)
   at java.net.URLClassLoader.findClass(URLClassLoader.java:188)
   at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
   at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:268)
   at java.lang.ClassLoader.loadClass(ClassLoader.java:251)
   at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:319)
   at java.lang.Class.forName0(Native Method)
   at java.lang.Class.forName(Class.java:164)
   at com.devdaily.javasamples.JavaReflectionExample1.(JavaReflectionExample1.java:13)
   at com.devdaily.javasamples.JavaReflectionExample1.main(JavaReflectionExample1.java:26)