Transcript session02PartII

UML Exercise
A "draw" utility program lets users draw several geometric objects on a
diagram. A geometric object may be a Circle, Rectangle, Square or
Triangle. A diagram may also contain a text object. Each geometric
object provides a draw() function to draw itself. They also contain
information about thickness of border lines (unsigned short), and
shading (filling) (a byte value for color).
A Circle holds its center and radius and provides functions to get and set
these values, in addition to a function that returns the area. A Triangle
holds the coordinates of the three sides and provides functions to get
and set these values. A Rectangle and Square also hold the coordinates
of their four sides and provide functions to get and set these values. A
Text Object has a string, coordinates of the first character, color of the
text, its font name, point size and if it is bold, italics, underlined or
stricken. All these parameters may be modified and accessed.
In addition to the above geometric objects, the diagram may also hold
sheets. A sheet is an object that appears grayed out on the diagram and
may hold a title. A user may double click on a sheet and it opens a
window of its own. The difference between a sheet and a diagram is
that a sheet may be contained in a diagram. A sheet may hold any of the
geometric objects described above, including other sheets.
Venkat Subramaniam
UML2-1
Object Copying
• Let’s first look at some thing simple and
fundamental
• How do we copy an object
• Consider an example of a Car with an
Engine
Venkat Subramaniam
UML2-2
Venkat’s past recommendation
• Before reading Bloch’s Effective Java!
• Writing a Copy Constructor is a bad idea
• Why?
• Leads to extensibility issues
Venkat Subramaniam
UML2-3
Bloch’s Recommendation
• Cloning comes with its own problems
Further
Reading: [1]
– No constructor called when object cloned
– If a class has final fields, these can’t be given a
value within close method!
• Bloch’s recommendation:
"... you are probably better off providing
some alternative means of object copying
or simply not providing the capability." He
goes on to say "A fine approach to object
copying is to provide a copy constructor."
• I agree with the part “simply not providing the
capability”
• But providing a copy constructor has problems
mentioned earlier?
• What’s the solution?
Venkat Subramaniam
UML2-4
A combined approach
• Implement the clone method
Further
Reading: [2]
– but not the way it is generally done in Java
• Write a protected copy constructor
• From the clone method, invoke the
protected copy constructor
Venkat Subramaniam
UML2-5
References
1. Effective Java, Joshua Bloch
2. “Why copying an object is a terrible thing to do?”
(downloadable from
http://www.AgileDeveloper.com/download.aspx)
Venkat Subramaniam
UML2-6