UML Exercise

1
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.

2
Object Copying

• Lets first look at some thing simple and
  fundamental
• How do we copy an object
• Consider an example of a Car with an Engine

3
Venkats past recommendation

• Before reading Blochs Effective Java!
• Writing a Copy Constructor is a bad idea
• Why?
• Leads to extensibility issues

4
Blochs Recommendation

• Cloning comes with its own problems
• No constructor called when object cloned
• If a class has final fields, these cant be given
  a value within close method!
• Blochs 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?
• Whats the solution?

Further Reading 1
5
A combined approach
Further Reading 2

• Implement the clone method
• but not the way it is generally done in Java
• Write a protected copy constructor
• From the clone method, invoke the protected copy
  constructor

6
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)