Multi-level inheritance and foreach Java loop

Question

I had a bunch of table views that had code duplicated in them. They used to directly inherit the AbstractView class. So then I made them inherit AbstractListView(Which would in it's turn inherit from AbstractView) which would be the new class holding all the common properties for these tables.

Since I made that change this loop is behaving oddly:

for (AbstractViewPanel view : registeredViews) {
        view.modelPropertyChange(evt);
}

Here is the arrayList: registeredViews:

private ArrayList<AbstractViewPanel> registeredViews;

I've run the debugger and the registeredViews arrayList has all of the views including the table ones. For some reason it stalls at the last directly inherited view in the collection and completely skips them.

All my views used to directly inherit:

public class someView extends AbstractViewPanel

Now since my change some of the views are like this:

public class someOtherView extends AbstractListView

Here's AbstractListView:

public abstract class AbstractListView extends AbstractViewPanel

Here's AbstractViewPanel

public abstract class AbstractViewPanel extends JPanel {


public abstract void modelPropertyChange(PropertyChangeEvent evt);

}

The loop is simply not going over anything that inherits AbstractListView, yet I would assume that all of the views in the end are of type AbstractView.

UPDATE I've managed to find a workaround it simply involves changing the loop:

for (int i = 0; i < registeredViews.size(); i++) 
{
        registeredViews.get(i).modelPropertyChange(evt);
}

I would like to know though why the foreach style loop was giving me headaches.

Answer 1

For reference, I've created a skeleton sscce of your design. List<E> implements Iterable<E>, so there's no a priori reason your foreach should fail. Here are a few thing to check:

• Verify that GUI objects are constructed and manipulated only on the event dispatch thread.

• Verify that you are not overlooking a ClassCastException when an instance of SomeOtherView is retrieved from the List.

• As the List is heterogenous, see Bloch, Effective Java 2nd ed., Item 29: Consider typesafe heterogeneous containers." The pattern is mentioned here.

• Addendum: Would you not see a concurrent modification exception in that case?

  No, ConcurrentModificationException "does not always indicate that an object has been concurrently modified by a different thread." The exception is thrown if the "list has been structurally modified" outside of the iterator, on a best-effort basis. More likely, the failing iterator was obtained after adding some views but before adding others. This could happen unexpectedly if the list were constructed on the initial thread and iterated on the EDT.

SSCCE:

import java.awt.EventQueue;
import java.awt.GridLayout;
import java.util.ArrayList;
import java.util.List;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;

public class Test {

    private List<AbstractViewPanel> registeredViews;

    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {

            @Override
            public void run() {
                new Test().init();
            }
        });
    }

    private void init() {
        JFrame f = new JFrame("Test");
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.setLayout(new GridLayout(0, 1));

        registeredViews = new ArrayList<AbstractViewPanel>();
        for (int i = 0; i < 2; i++) {
            registeredViews.add(new SomeView());
            registeredViews.add(new SomeOtherView());
        }
        for (AbstractViewPanel view : registeredViews) {
            f.add(view);
        }

        f.pack();
        f.setLocationRelativeTo(null);
        f.setVisible(true);
    }

    abstract class AbstractViewPanel extends JPanel {

        public AbstractViewPanel() {
            add(new JLabel(getClass().toString()));
        }
    }

    abstract class AbstractListView extends AbstractViewPanel {
    }

    class SomeView extends AbstractViewPanel {
    }

    class SomeOtherView extends AbstractListView {
    }
}

Answer 2

I believe that your modelPropertyChange changes the registeredViews list.

So when you run the loop using the counter i the loop is limited to the size of the list at the beginning of the loop.

But when you run with the for each loop, if the modelPropertyChange change the list maybe your iterator give you some problem.