Object Diagrams

• the right (approximately) half of the drawing area is a single region bordered by a rectangle. The upper left hand corner of this region is at coordinates (260, 0), its width is 240 pixels, and it height is 500 pixels.

• the left (approximately) half of the drawing area is subdivided into two equal size regions each of which is bordered by a rectangle. The upper left corner of the top subregion is at coordinates (0,0). The upper left corner of the botton subregion is at coordinates (0, 260). Each subregion has a width and height of 240 and 240, respectively.

Using the classes mentioned earlier , the drawing area could be implemented by a Canvas object, each of the three drawing regions by Rectangle objects, and the border width would be handled by associating an appropriately initialized Pen object with the Canvas object.

The class diagram shown earlier does not convey some essential information about the structure of the drawing area. For example, the class diagram does not indicate:

• the Shapes known by the Canvas are specifically Rectangles,
• there are exactly 3 Rectangle objects known by the Canvas,
• the location of each Rectangle object, and
• the value of the Pen object's width.

An object diagram for the example is shown below. The drawingArea object is an instance of the Canvas class and has pointers to three Rectangle objts (upperLeft, lowerLeft, atRight). Each of the Rectangle objects has a composed Location object. Since the diagram depicts objects, the actual values of each Rectangle object's Location, height and width can be shown in the diagram. Also, the width of the Pen object (aPen) used by the drawingArea object can be shown explicitly.