Question

I am using Eclipse + Window Builder + few third party libraries to build a gui application,

Entry point for my application resides in MainWindow.java file, which also contains the gui of the application, other than that I have few helper classes.

My application works fine, but when I checked into bin folder I find four more class files there namely:
1. MainWindow$1.class
2. MainWindow$2.class
3. MainWindow$3.class
4. MainWindow$4.class.

Now I don’t understand why these files are here, When I deleted these files, Eclipse throws an exception

Exception in thread "main" java.lang.NoClassDefFoundError: gui/MainWindow$1
at gui.MainWindow.main(MainWindow.java:71)
    Caused by: java.lang.ClassNotFoundException: gui.MainWindow$1
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)

the code that throws the exception is:

public static void main(String[] args) {
    EventQueue.invokeLater(new Runnable() {
        public void run() {
            try {
                MainWindow window = new MainWindow();
                window.frmMailExtractor.setVisible(true);
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    });

I am not able to find out why these files are there and who put them? any ideas?

OK so these are the anonymous classes, but why the Exception then?…and why can’t I see them in my eclipse..becasue when I export it as a runnable jar, the jar throws the same exception

Answer 1

These are class files generated for anonymous inner classes inside MainWindow. It probably contains a bunch of event listeners implemented as such.

Anonymous inner classes get compiled into separate class files with the name <OuterClass>$<nnn>.class, where <nnn> is a compiler-generated number.