Assigned: Friday, 30 September 2005
Due: 11:00 a.m., Wednesday, 5 October 2005
Summary: In this assignment, you will further explore
the polymorphic text composition presented in the
reading and
lab on
polymorphism.
Purpose: To enhance your understanding of polymorphism
and of object-oriented design.
Contents:
In the
reading and
lab on
polymorphism, you explored a number of related classes that permitted
you to build interesting two-dimensional layouts of characters. You
were able to combine and recombine text objects because of Java's
support of polymorphism. That is, if we treat every combination
of text as an object that implements TextBlock (and that is likely to be
constructed from other TextBlocks), then we can combine these objects
arbitrarily.
The textual layout example also had a hidden agenda: To help you think
about problems in a more object-oriented fashion. In particular, most
beginning programmers, when asked to do some form of textual composition,
focus on the methods that would compose pieces of text. In this
example, we built objects that composed pieces of text. The
advantages of using objects included the ability to reuse a composed piece
of text and a better ability to take advantage of polymorphism.
However, we left some aspects of the example unexplored in
both the reading and the lab. In particular, few of you had
the opportunity to build your own composition classes, we did
not consider possible effects of mutating textblocks, and we
did not write the traditional toString method.
1. Implement each of the following textual composition classes:
-
Truncate which builds (from a textblock and a specified
width) a new textblock that truncates the underlying textblock to the
desired width.
-
Center which builds (from a textblock and a specified
width) a new textblock that centers the underlying textblock within
the specified width.
-
RightJustify which builds (from a textblock and a specified
width) a new textblock that right-justifies the underlying textblock within
the specified width.
- Some composition of your own design.
2. The basic building block of text compositions is the TextLine.
Suppose we added a setContents(String newline) method to that
class which changes the underlying line.
a. What changes would we have to make to the other composition classes
to support that change?
b. Make those changes (and indicate within the code that you have made
them).
3. As some of you noted, although we think of TextBlocks as representing
text, they do not provide a toString method. Part of the
problem with providing such a method is that toString is
generally expected to return a single-line string. Let's explore how
we might do so.
a. TextLine's toString could simply return the line.
b. BoxedBlock's toString could return the line surrounded by square braces, as in "[Hello]".
c. HCompose's toString could return the two parts surround by parentheses and separated by a plus, as in "(Hello+Goodbye)".
d. VCompose's toString could return the two parts surround by parentheses and separated by a slash, as in "(Hello/Goodbye)".
e. You may, but need not, design similar ways to represent the additional text composition mechanisms described in part 1 of this assignment.
Implement all of these toString methods.
Create a directory for the package, share the directory and its contents,
and send me the location of that directory.
You should work in groups of two or three on this assignment.
When you ask questions, I'll try to put the answers here.
Friday, 30 September 2005 [Samuel A. Rebelsky]
;
Check with Bobby