Held: Tuesday, 1 April 2014

Back to Outline 33 - An Introduction to Linear Structures. On to Outline 35 - Priority Queues and their Basic Implementation.

Summary

We consider our second important linear structure, the queue.

Related Pages

• Reading: Linear Structures
• Reading: Queues
• Lab: Array-based Queues
• Repository: https://github.com/Grinnell-CSC207/linear
• EBoard 34 (Source 34) (HTML 34) (PDF 34)

Overview

• Wrappers, Adapters, Delegation, and such.
• Design patterns and terminology.
• Lab.

Administrivia

• Today's writeup: Exercise 5.
• I will reserve time at the start of class for questions on the exam.
• Reading for Wednesday: Priority Queues
• Review session tonight at 7. Earnest will discuss the wonders of DNF, kth-smallest on arrays, and more.
• Consider running for the SEPC.
• Get ready to vote on T-Shirts (the Curried Lamb-Duh T-shirt has disappeared; you should encourage the SEPC to add it).
• Extra credit:
  • Convocation, Wednesday, noon: Elizabeth Kolbert (the Sixth Extinction)
  • Technology in the liberal arts symposium on Thursday.
  • CS extras Thursday: ???
  • CS table Friday: TBD

Wrappers, Adapters, and Delegation

What interesting (or not so interesting) design ideas did you get from ReportingLinearStructure.java?

Here are some of the things I see as important ideas:

• We already know that we can add functionality to a particular class by subclassing.
• ReportingLinearStructure.java shows another mechanism for adding functionality, one that works for an arbitrary class that implements an interface.
• The basic strategy:
  • Our wrapper/adapter has a field that contains an object that implements the interface. I call this the wrapped object or wrapped class.
  • Our wrapper/adapter also implements the interface.
  • Most of the methods are implemented by delegating the actual work to the wrapped object.
  • But the wrapper can add functionality or modify the behavior.

Here's my really bad ASCII art diagram.

          Wrapper
         +-------------------+
Client ---> fun: wrapped.fun |
         |                   |
         |  wrapped          |
         |    +---------+    |
         |    | fun     |    |
         |    +---------+    |
         +-------------------+

Uses

• As you saw, one use of this pattern is to provide some automatic logging of your program. When you want to log everything an object does, you wrap that object with something that logs each call. Once you're satisfied, you remove the wrapper.
• It's also useful to add some analysis capabilities, such as a count of the number of times each method is called.
• Or we can change some of the behavior. For example, we might have something that calls put twice for each call. (I have no idea why you would want such behavior.)
• A talented Java programmer can probably write a generic logging or counting wrapper. I don't have the time or depth of knowledge of Java introspection.

Terminology and Design Patterns

• As object-oriented (and other) programmers have found common approaches to problems, they've worked to find common names for these approaches to make it easier to talk to others about the approach.
• Often, people use the term "design patterns" to refer to this approach.
  • Design patterns are a bit more specific, in the sense that a good design pattern also tells you a lot about when to use the pattern.
  • Most of the members of this department like design patterns primarily as a use of common terminology.
• I've had trouble finding the right term for this approach, which is a common pattern.
• I call these "wrappers", but I use "wrapper" for a lot of things.
• They use delegation (or the delegate pattern).
• They are kind of like the adapter pattern, although adapters are usually used to make objects meet new interfaces. (You've seen me use adapters to get built-in Java classes meet my interfaces.)

Lab

• Read code.
• Fix bugs.
• Fix more bugs.
• Write an interator.