Question

Is it normal that “java Hello3” (with Hello3.class a java class) gives me:

Exception in thread "main" java.lang.NoClassDefFoundError: Hello3 (wrong name: pp/Hello3)

Hello3.class is the compiled binary of Hello3.java. This java file is declared to be under some package “pp”, but it is not put in the “pp” directory.

package pp;
public class Hello3 {
    public static void main(String[] args) {
        System.out.println("Hello World");
    }
}

My command java Hello3.java is done at the same directory as Hello3.java. What’s more, even the command “java -cp . Hello3” doesnot work! Maybe a problem from mine??

Since I got your confirmation from the following answers, I may draw a naive conclusion here: it’s too easy to make a .class that never works. Simply declare your .java under package XXX but do not put it in the directory XXX. IMHO, I think this can be used as an attack!

Answer 1

You indeed need to have Hello3.class in a directory named pp. The directory containing pp should be in the classpath. Suppose this directory is called classes:

...
  |_ classes
         |_ pp
            |_ Hello3.class

The command to launch your Java program should thus be

java -cp /the/path/to/classes pp.Hello3

or, on Windows

java -cp c:\the\path\to\classes pp.Hello3