Question

I could not understand this error, the scenario is:

This is the directory structure:

classes/
    root/
        pkgmain/Cmain.class
        pkg1/C1.class
        pkg2/C2.class

The problem is when I try to run Cmain file (main() is in this file) from the following path, I get the error:

C:\Ravi\MakingJarFile\classes\root\pkgmain>java Cmain
Exception in thread "main" java.lang.NoClassDefFoundError: Cmain (wrong name: root/pkgmain/Cmain)
        at java.lang.ClassLoader.defineClass1(Native Method)
        at java.lang.ClassLoader.defineClassCond(Unknown Source)
        at java.lang.ClassLoader.defineClass(Unknown Source)
        at java.security.SecureClassLoader.defineClass(Unknown Source)
        at java.net.URLClassLoader.defineClass(Unknown Source)
        at java.net.URLClassLoader.access$000(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)
Could not find the main class: Cmain.  Program will exit.**

What could be the solution for this problem, Thanks.

Answer 1

You need to provide the fully qualified class name, and do it from the root directory (or put that directory in the classpath):

C:\Ravi\MakingJarFile\classes> java root.pkgmain.Cmain

or

C:\Anywhere> java -cp C:\Ravi\MakingJarFile\classes root.pkgmain.Cmain

You’re telling the java command the full name of a class, and that’s what it’s looking for. When it finds a relevant class file it will check that the class within it really does have that fully-qualified name