Generic JList and ListCellRenderer?

K

Knute Johnson

I've been trying to clean up some really old code and I've hit some
snags. I've got several modified JLists and the ListCellRenderers for
them and thought it would make sense to have generic classes that could
be extended for different data types that need to be displayed. The
example below displays InetAddresses in the JList. I've got another
implementation of JList that does a lot more than what this one does but
I wanted to keep this example simple and focus on the ListCellRenderer.

MyListCellRenderer extends the getListCellRenderer method of
DefaultListCellRenderer and adds a new method, textToDisplay(). I added
a field to the constructor that specifies the class of the data element
to be displayed. That class information is the test to make the call to
the textToDisplay() method. The InetAddressListCellRenderer class
extends MyListCellRenderer to get this to display nice neat Strings for
the InetAddresses.

I think this whole thing is a little kludgie. I've been playing with it
for so long now I'm not getting anywhere anymore. I was hoping somebody
would say "gee you ought to go this way" or "no that is really brilliant
code my man!" and I'll leave it as is :).

Anyway, please take a look and if you have any great ideas, I would
appreciate the feedback.

Thanks,

import java.awt.*;
import java.io.*;
import java.net.*;
import java.util.*;
import javax.swing.*;
import javax.swing.border.*;

public class test6 extends JPanel {
public test6() throws UnknownHostException {
Vector<InetAddress> v = new Vector<>();
v.addElement(InetAddress.getByName("216.240.58.139"));
v.addElement(InetAddress.getByName("4.2.2.1"));
v.addElement(InetAddress.getByName("knutejohnson.com"));
v.addElement(InetAddress.getByName("192.168.3.6"));

MyJList<InetAddress> list =
new MyJList<>(new DefaultListModel<InetAddress>());
list.setCellRenderer(new InetAddressListCellRenderer());
list.setListData(v);
add(list);
}

public class MyJList<E> extends JList<E> {
private final DefaultListModel<E> model;

public MyJList(DefaultListModel<E> lm) {
super(lm);
this.model = lm;
}

public void setListData(E[] listData) {
model.clear();
for (E e : listData)
model.addElement(e);
}

public void setListData(Vector<? extends E> v) {
model.clear();
Iterator<? extends E> iter = v.iterator();
while (iter.hasNext())
model.addElement(iter.next());
}
}

public class MyListCellRenderer extends DefaultListCellRenderer {
private final Class clazz;

public MyListCellRenderer(Class clazz) {
super();
setOpaque(true);
setBorder(new EmptyBorder(1,1,1,1));
setName("List.cellRenderer");

this.clazz = clazz;
}

public Component getListCellRendererComponent(JList<?> list,
Object value, int index, boolean isSelected, boolean
cellHasFocus) {
setComponentOrientation(list.getComponentOrientation());

UIDefaults defaults = UIManager.getDefaults();

Color bg = null;
Color fg = null;

JList.DropLocation dropLocation = list.getDropLocation();
if (dropLocation != null &&
!dropLocation.isInsert() &&
dropLocation.getIndex() == index) {
bg = (Color)defaults.get("List.dropCellBackground");
fg = (Color)defaults.get("List.dropCellForeground");

isSelected = true;
}

if (isSelected) {
setBackground(bg == null ?
list.getSelectionBackground() : bg);
setForeground(fg == null ?
list.getSelectionForeground() : fg);
} else {
setBackground(list.getBackground());
setForeground(list.getForeground());
}

if (value instanceof Icon) {
setIcon((Icon)value);
setText("");
} else if (clazz.isInstance(value)) {
setIcon(null);
// setText(((InetAddress)value).getHostAddress());
setText(textToDisplay(value));
/*
} else {
setIcon(null);
setText((value == null) ? "" : value.toString());
*/
}

setEnabled(list.isEnabled());
setFont(list.getFont());

Border border = null;
if (cellHasFocus) {
if (isSelected)
border = (Border)defaults.get(
"List.focusSelectedCellHighlightBorder");
if (border == null)
border = (Border)defaults.get(
"List.focusCellHighlightBorder");
} else
border = new EmptyBorder(1,1,1,1);
setBorder(border);

return this;
}

public String textToDisplay(Object value) {
return value == null ? "" : value.toString();
}
}

public class InetAddressListCellRenderer extends MyListCellRenderer {
public InetAddressListCellRenderer() {
super(InetAddress.class);
}

public String textToDisplay(Object value) {
return ((InetAddress)value).getHostAddress();
}
}

public static void main(String[] args) {
EventQueue.invokeLater(new Runnable() {
public void run() {
try {
JFrame f = new JFrame();
f.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
test6 t6 = new test6();
f.add(t6);
f.pack();
f.setVisible(true);
} catch (Exception e) {
e.printStackTrace();
}
}
});
}
}
 
J

John B. Matthews

Knute Johnson said:
I've been trying to clean up some really old code and I've hit some
snags. I've got several modified JLists and the ListCellRenderers
for them and thought it would make sense to have generic classes that
could be extended for different data types that need to be displayed.
The example below displays InetAddresses in the JList. I've got
another implementation of JList that does a lot more than what this
one does but I wanted to keep this example simple and focus on the
ListCellRenderer.

Good use of a class literal as runtime type token:

MyListCellRenderer extends the getListCellRenderer method of
DefaultListCellRenderer and adds a new method, textToDisplay(). I
added a field to the constructor that specifies the class of the data
element to be displayed. That class information is the test to make
the call to the textToDisplay() method. The
InetAddressListCellRenderer class extends MyListCellRenderer to get
this to display nice neat Strings for the InetAddresses.

I think this whole thing is a little kludgie. I've been playing with
it for so long now I'm not getting anywhere anymore. I was hoping
somebody would say "gee you ought to go this way" or "no that is
really brilliant code my man!" and I'll leave it as is :).

Anyway, please take a look and if you have any great ideas, I would
appreciate the feedback.

Some suggestions:

Use the @Override annotation.

Use the for-each (Iterable) loop in setListData():

@Override
public void setListData(Vector<? extends E> v) {
model.clear();
for (E e : v) {
model.addElement(e);
}
}

Consider supporting List:

public void setListData(List<? extends E> v) {
model.clear();
for (E e : v) {
model.addElement(e);
}
}

Catch the more specific exception:

catch (UnknownHostException e)
 
K

Knute Johnson

Good use of a class literal as runtime type token:



Some suggestions:

Use the @Override annotation.

Use the for-each (Iterable) loop in setListData():

@Override
public void setListData(Vector<? extends E> v) {
model.clear();
for (E e : v) {
model.addElement(e);
}
}

Consider supporting List:

public void setListData(List<? extends E> v) {
model.clear();
for (E e : v) {
model.addElement(e);
}
}

Catch the more specific exception:

catch (UnknownHostException e)

Thanks John.
 
K

Knute Johnson

Knute said:
I've been trying to clean up some really old code and I've hit some
snags. I've got several modified JLists and the ListCellRenderers for
them and thought it would make sense to have generic classes that could
be extended for different data types that need to be displayed. The
example below displays InetAddresses in the JList. I've got another
implementation of JList that does a lot more than what this one does but
I wanted to keep this example simple and focus on the ListCellRenderer.

Without knowing what directions you intend/expect to extend the code it's a
little difficult to make sensible suggestions.

So, proceeding with due absence of sense...

I don't see what the clazz variable (and the associated instance checks) are
buying you. You could try removing them and see if that makes your code come
to life. It would certainly remove some of the "kludgy" smell.

If that doesn't help, then it may be that you're being bothered by the way that
ListCellRenderer has (so to speak) too many responsibilities -- it's
simultaneously about /how/ something is displayed and about /what/ is
displayed.

So (this is only my guess, of course) you're being pulled in two directions: on
the one hand your /how/ is pretty much fixed and you want to use nice generic
code for it whatever the types of the values, but the /what/ is not generic and
will change according to each application. You are being required to do
rampant subclassing of something that shouldn't /need/ subclassing.

If that sounds as if it might relate to your problem, you could introduce a new
kind of object who's only responsibility is to take a value, the element of the
List (of type Object -- unless you want to mess with generics), and return a
String which is to be the contents of the cell. It might also have a parallel
method which takes the (same) List element and returns an Icon (or null).

The new object would have appropriate subclasses for InetAddress etc. (which
might or might not appear formally as separate classes in your source -- you
could instead use anonymous classes to make the getTextFor() and getIconFor()
methods "pluggable").

So then you have just one subclass of ListCellRenderer which holds an instance
of [a subclass of] this new class (instead of the clazz variable), and which
always blindly uses that to "fetch" the appropriate String and/or Icon for use
for each cell.

It's difficult to think of a good name for the new kind of object (and it's
subclasses), but in this case I don't think that's the danger sign it normally
would be: the reason that it's difficult to find a name is that the obvious
names (some variant on "Renderer") are already taken. "TextSupplier" perhaps
(or ContentSupplier, or ContentTranslator) ?

It could well be that this kind of picture is over-engineered for your purposes
(I can't tell). But even if it is, then the real culprit is the Swing
architecture which is under-engineered (here), so something may be needed to
compensate.

-- chris

Thanks Chris.
 

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