Request For Enhancement: Cell Renderers: Is the Mouse In the Cell?

Question

I would like to propose an enhancement to Cell Renderers: add some way for Cell Renderers to know if the mouse is currently over the cell being renderered. Currently, Cell Renderers are told only if a cell is SELECTED or has FOCUS, which are not the same thing.
This would be of great help especially for JTable and JTableHeader, where one could provide visual feedback as the mouse moves across cells, which could be a great help for the visually impaired.

Answer 1

Hi, Marsian -
You are absolutely right about adding methods to interfaces. My mistake.
And you are also correct that what I propose would require that cells be repainted as the mouse enters or exits that cell's bounds. If this were to prove too expensive for some applications, the JComponents that use CellRenderers could have a property to enable or disable the behavior, for example setTrackMouseMovements(boolean). But when compared to what happens when you scroll a JTable, I cannot imagine this would be anywhere nearly as expensive, and the rendering that scrolling requires--all the cells that come into view within the viewport after scrolling--seems to work just fine.
Heck, even when you select a cell, a whole lot of cells are re-rendered. Also, when a tooltip is currently visible, if you move the mouse, the CellRenderer is consulted for each and every mouse movement, even within the same cell!!!. This seems to work fine too. Would re-rendering a cell as the mouse enters or exits its bounds by anywhere nearly as expensive?
But you write something that still confuses me. Are you saying that you create a new Component each and every time the getXXXRendererComponent() method of your CellRenderer is called? I don't understand why one would want to do this. Can you please help me out here?
I would think that only one instance of each Component sub-class that the CellRenderer might return would be more than good enough. And I would think that these instances would be created in the CellRenderer's constructor. Certainly, the default CellRenderer classes do not create a new Component each time a cell needs to be renderered. In fact, most of them actually extend an extension of JComponent themselves, i.e. JLabel, and return "this". No CellRenderer I have ever written has ever created a Component each time a cell is rendered. That would be very expensive.
But still, your point about altering interfaces is well taken. Too bad.
There must be some solution to this problem. JTable could provide the cell coordinates of the cell in which the mouse currently resides, for instance, or it could have a method that indicates if the mouse is currently over a cell with given coordinates.
Thanks,
---Mark