Held: Monday, 10 November 2014

Back to Outline 37 - An Introduction to Linear Structures. On to Outline 39 - 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 38 (Source 38) (HTML 38)

Overview

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

Administrivia

• New lab partners.
• Extended office hours continue this week.
• Sorry that I could not be here on Friday. I hope you were nice to Mira.
• I'm surprised that no one showed up to the review session last night.
• We'll discuss general issues with the makeup exams later today.
• I probably won't return makeup exams until next week.

Upcoming Work

• Homework 9: Skip Lists due Wednesday.
• Reading for Tuesday.
  • Priority Queues

Cool Upcoming Events on Campus

• Peace studies talk today at 4:15 p.m. in JRC 101 "With All the World’s Violence, Where is Peace?"

Extra Credit

Academic

• Convocation Wednesday at noon: Dean Latham.
• Wednesday at 7:30 p.m. Internship panel with Nelson, Mira, and Lea.

Peer Support

• Ajuna's Radio show Mondays at 8pm. Listen to African music.
• Ezra's Radio show Thursdays at midnight. Radio melodrama.
• Charlie's Friday Night "War in Animated Film" ExCo.

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.