I have encountered a strange problem

While making package in java ....code as follows

Code:
____________________________________________________________________________
_____
package packageOne;

import java.io.*;

public class mainClass
{
public static void main(String args[])
{
System.out.println("Hello to all");

}
}
____________________________________________________________________________
_____

Above code compiles fines with no error, but when execute it saying "java
mainClass" it gives error.

Exception in thread "main" java.lang.NoClassDefFoundError: mainClass (wrong
name
: packageOne/mainClass)
at java.lang.ClassLoader.defineClass0(Native Method)
at java.lang.ClassLoader.defineClass(ClassLoader.java:537)
at
java.security.SecureClassLoader.defineClass(SecureClassLoader.java:12
3)
at java.net.URLClassLoader.defineClass(URLClassLoader.java:251)
at java.net.URLClassLoader.access$100(URLClassLoader.java:55)
at java.net.URLClassLoader$1.run(URLClassLoader.java:194)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:187)
at java.lang.ClassLoader.loadClass(ClassLoader.java:289)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:274)
at java.lang.ClassLoader.loadClass(ClassLoader.java:235)
at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:302)
Press any key to continue . . .