$3\mathrm{.}1$ Exercises

Figure $3\mathrm{.}1$: Simple editor.

Exercise $3\mathrm{.}1.$ Figure $3\mathrm{.}1$ is a partial diagram of an interactive editor.

First, try to interpret the class diagram without reading any further. Write down your interpretation of the class without going to the second item.

Second, the diagram shows the following:

$($a$)$ A sheet is a collection of lines and boxes.

$($b$)$ A line is a series of line segments that connects two boxes.

$($c$)$ Each line segment is specified by two points.

$($d$)$ A point may be shared by a vertical and horizontal $($up to two$)$ line segments.

$($e$)$ A selection is a collection of lines and boxes that have been highlighted.

$($f$)$ A buffer is a collection of lines and boxes that have been cut or copied.

Revise the class diagram and use generalization to express the constraint that a line or a box belong to exactly one buffer or one selection or one sheet. Add a base class to Buffer, Selection, and Sheet. Discuss the merits of the revision.