Core JFC Java Foundation Classes 2nd edition - Read

Logical Scrolling and Forcing Visibility of an Area 577
Listing 5-19 A custom scroll pane that can handle logical
scrolling (continued)
}
private int [] positions;
private JLabel[] labels; // Pictures, one per JLabel
private int picturesPerRow; // Pictures in each row
private int rows; // Number of rows
correct orientation. The makeVisible method helps to provide the functionality
offered by the button panel on the main frame, as you'll see later.
The rest of this class is the implementation of the PictureScrollBar
class, which is derived from JScrollBar and overrides the getUnitIncrement
and getBlocklncrement methods to provide the logical scrolling.
The getUnit Increment method is trivial—it just returns 1, causing the
viewport to move by f pixel in the appropriate direction. All of the real work
of this class is in getBlocklncrement. You've already seen a description of
how this method works in principle. Since this class could manage either a
vertical or a horizontal scrollbar, there are two slightly different pieces of
code, in the getlmagePositions method, that achieve the same thing in a
slightly different way. In the case of a horizontal scrollbar, the x coordinates
of the picture columns are found by invoking getLocation on the three pictures
on the top row; since the pictures are arranged in columns, this returns
the x coordinates of all the pictures. Similarly, the vertical scrollbar finds the y
locations of the pictures in the left-hand column.
Since the pictures won't move once they have been placed on the panel,
the getBlocklncrement method only invokes getlmagePositions the
first time it is called, saving the resulting array of offsets in the positions
array for future use. Using this array and the current value of the scrollbar,
obtained using getValue, the index of the picture that occupies the top-left
corner of the viewport can be located. As described earlier, it is then a simple
matter to locate the picture that should next be made visible and, with that
information and the values in the positions array, the distance by which
the scrollbar should be moved can be calculated with a simple subtraction.
Forcing Visibility of Part of a Scrolled Area
The last piece of the PictureScrollPane class is the makeVisible
method. When the user presses one of the nine buttons at the top right of the