Java Reference
In-Depth Information
return an object which has a
toString()
, we're fine. Both
User
and
SAMoney
do have such a method, so they fit fine here.
The more complex answer has to do with why
JTable
calls the
toString()
method at all. The
JTable
uses, behind the scenes, a complex
table cell display mechanism, called a
table cell renderer
. A renderer is an object
that displays data in a certain way. Each table cell renderer returns a GUI
component, and if you don't want to use the default renderer, you can define
your own table cell renderer for your table. This allows you to display almost
anything you can imagine inside a table's cell. The renderer acts as a template
for those cells and will be called upon with the result of the
getValueAt()
,
along with a few more parameters, so that it can build and display the
resulting cell.
Let's revisit our simple explanation above, in light of the concept of a
renderer. The default cell renderer for a
JTable
uses just a
JLabel
. When
called upon, the default cell renderer is given the object returned by
getValueAt()
and the renderer fills its
JLabel
by calling its
setText()
method, passing in the result of
toString()
on the given object. That's how
toString()
got called on all our results. You can explicitly set a different
renderer using the
setDefaultRenderer()
method on
JTable
.
In the Javadoc for Swing table objects we find this interface:
public Component
getTableCellRendererComponent(JTable table,
Object value,
boolean isSelected,
boolean hasFocus,
int row,
int column)
This tells us that if we want to write a class which can act as a renderer, it
needs to implement this method. The method will be called with the value
returned by
getValueAt()
, but the
row
and
column
(and
table
) will be
repeated here in case your renderer cares. For example, having the
row
and
column
would allow you to create a table with the third column of the table
in green—your method could check the column number, and if it is 2
(columns are numbered 0, 1, 2, . . . ) set the background color to green for the
Component
that you would return.
