类声明错误（由ClassNotFoundException引起的NoClassDefFoundError）

Question

Please advise me what's wrong with this class declaration:

ExchEngine.java

package engine;

public class ExchEngine {

    public ExchEngine() {
    }

    public static void main(String[] args) {

        ExchEngine engine = new ExchEngine();

    }
}

When I compile this file, I always get exception:

java.lang.NoClassDefFoundError: test_engine/ExchEngine
Caused by: java.lang.ClassNotFoundException: test_engine.ExchEngine
 at java.net.URLClassLoader$1.run(URLClassLoader.java:202)
 at java.security.AccessController.doPrivileged(Native Method)
 at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
 at java.lang.ClassLoader.loadClass(ClassLoader.java:307)
 at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301)
 at java.lang.ClassLoader.loadClass(ClassLoader.java:248)
Exception in thread "main"

This seems very weird that ExchEngine.java is inside a package and it cannot run itself. Thanks for any help.

Answer 1

You should put your source file in a directory called "engine", since that's the package name of gave it.

Compile the file with javac engine/ExchEngine.java and run it with java engine.ExchEngine .

Answer 2

Your class is engine.ExchEngine . Something somewhere is looking for test_engine.ExchEngine , and is causing this error when it fails to do so. You presumably need to change the two so that they match.

Answer 3

This isn't a compile time problem, it is a run time error.

Need more information. For instance, how are you trying to run it? In Eclipse?

If in Eclipse: You may need to clean the project. Or you might be trying an old run configuration from before you changed the package.

Answer 4

Specify the classpath when running your application

Assuming after compiling, you have the following directory structure:

./test_engine/ExchEngine.class

then run it like so:

java -cp . test_engine.ExchEngine

Answer 5

If your using an IDE, reconfigure the project, mainly where it looks for the Main method. Clean, rebuild, restart, do what ever you need to do.