Solution,

• You have the correct class or its package.
  
• Unset the CLASSPATH environment variable, if it's set.
• Class name are case sensitive.
  
• Make sure you're spelling the class name exactly the same way as it is declared.
  
• Make sure your classes appear in the correct subdirectory.
  
• Make sure you're using the class name and not the filename. In fact, make the names the same and you won't run into this problem for this reason.