import org.apache.commons.io.FilenameUtils;
public class Test {
    public static void main(String[] args) throws InterruptedException {
        System.out.println(FilenameUtils.getExtension("a.zip"));
    }
}
as shown above, the simple code relies on the commons-io.jar package
 compile command: 
  javac-cp commons-io-2.4.jar-d. Test.java  
 compiled successfully! 
 execute the command: 
  java-cp commons-io-2.4.jar Test  
 error: 
  error: the main class Test cannot be found or cannot be loaded  
 change to the specified directory: 
  java-cp. Test  
 error: 
Exception in thread "main" java.lang.NoClassDefFoundError: org/apache/commons/io/FilenameUtils
        at Test.main(Test.java:6)
Caused by: java.lang.ClassNotFoundException: org.apache.commons.io.FilenameUtils
        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 moreexcuse me, what is the cause?
