Swing Hacks

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Chapter 2, Lists and Combos

#19 Turn Methods into List Renderers

HACK

The previous code replaces the label.setText( ) method in the original ren-

derer. It retrieves the specified method from the current list value and stores

it in a

Method

object. This object represents the abstract method itself. If you

did

new String("text").getClass( ).getMethod("toString",null)

, then you

would get an object that represents the

toString( )

method on any string.

Once you have this method, you can call it on the actual object at hand.

meth.invoke( ) will invoke the method on the real list item, returning a value

into

retval. Both getMethod( ) and invoke( ) take an additional argument,

which is

null in the previous code. This argument is actually an array repre-

senting the arguments to the method being called. For this hack to work,

you must assume that the method has no arguments, so

null is used.

Once you have the return value of the method in hand, you can call

setText( ). I set it to ""+retval because that will automatically handle nulls

and call

toString( ) on the value itself. The reflection code can throw an

exception, so it’s all wrapped up on a

try-catch block. If the reflection fails,

then it will set the text using

toString( ) on the list item as a backup, which

is what the standard renderer would do.

Putting It All Together

To use this new renderer, you need to create a JList and set its CellRenderer

property. The following code creates a frame with one JList in it, creates a

custom renderer for the

toString( ) method, then packs and shows the

frame on screen. The list contains an array of strings, representing common

subatomic particles:

public static void main(String[] args) {

String[] data = { "Proton", "Neutron", "Electron" };

JList list = new JList(data);

GenericListCellRenderer renderer =

new GenericListCellRenderer("toString");

list.setCellRenderer(renderer);

JFrame frame = new JFrame("Cell Renderer Hack");

frame.getContentPane( ).add(list);

frame.pack( );

frame.setVisible(true);

}

The generic renderer can call any method on the list item objects as long as

the method has no arguments and doesn’t return void. It will even work

with primitives. You could call

hashCode( ), which returns an int, like this:

GenericListCellRenderer renderer =

new GenericListCellRenderer("hashCode");