Class 21: An Initial Problem: Laying Out Text

Held: Monday, February 28, 2005

Summary: Today we begin our investigation of some new issues in object-oriented programming by visiting a sample problem domain: Text layout.

Related Pages:

• EBoard.
• Sample Code.

Assignments

• Exam 1 (due Friday).

Notes:

• I hope Friday's class went well.
• Are there questions on the exam?

Overview:

• Object-oriented design, revisited.
• Motivating problem: Text layout.
• Basic classes: Lines and Chunks.

Object-Oriented Design, Revisited

• As you may recall, I suggested that there are a few basic concepts central to object-oriented programming:
  • Encapsulation, which we think of in two ways
    • Grouping methods and data
    • Protecting the data from inappropriate use
  • Inheritance, in which we develop new classes from old
  • Polymorphism, in which we use related classes in the same way
• We also try to think in terms of objects
• Believe it or not, but you've already seen at least one instance of polymorphism: Both InputStreamReaders and FileReaders can be used as input to a BufferedReader.
• This week, we're going to consider polymorphism, inheritance, and other object-oriented design concepts in more depth.
• To motivate this exploration, we are going to consider a simple, yet potentially interesting, example: laying out text on the screen.

Our First Few Classes: Lines and Chunks of Text

• We'll begin with a simple first class: A line or portion of a line of text (essentially, a string, but it may help to think of it separately, so we'll leave it separate).
  • We will most frequently display this line
  • But we may also combine it with other lines in various ways
• What simple things do we expect to be able to do with a line of text?
  • Create it
  • Get its contents (as a string)
  • Get its width (in number of characters)
• We'll extend this to a chunk of text that can span multiple lines. (Class name: SimpleTextChunk)
• What basic fields and operations should we supply?
  • Create it (how?)
  • Get its width (in number of characters)
  • Get the number of lines
  • Get a particular line