Software Design
Lectures
Filters

Context

Goal: a Java utility that can apply multiple filters to a text stream

Like connecting Unix commands with pipes

Plan:

Write some use cases to clarify requirements

Implement a prototype

Critique the prototype

I've looked ahead, and know that configuration will be its biggest shortcoming

Learn some useful things about:

XML

Reflection

Come back and finish the program off

What's a Use Case?

A short story about a particular person accomplishing a single task with a piece of software

Use cases are:

An interface between customers and developers

A way for developers to keep track of what they're supposed to be doing

And make sure it's internally consistent

A guide for testing

Typically start a development cycle by writing new use cases

Not "what do we want to do to the software?"

But "what do we want the software to do?"

Once use cases are done, go back and extract:

Visible system features (screens, buttons, etc.)

Invisible features (e.g. database of marks)

Format:

Each use case is a single sequence of actions

Refer to other use cases to handle other sequences

Alternate user action with system response

Example

Software is a web-based system for checking grades on assignments

Angela is a student in CSC207

Title: "Compare Mark To Average When Grades Available"

Goal: compare her mark on Assignment 3 to the class average

Steps:

Open web browser

Browser displays Angela's usual home page

Go to course home page

Browser displays course home page

Click link marked "Grades"

Browser displays marks page, which contains a four-column table

Each row is assignment number, assignment name, due date, and average grade

Note: if assignment has not been graded, that cell in the table is grayed out

See "Compare Mark To Average When Grades Not Available"

Bottom of page has link marked "Log In"

Angela clicks on "Log In" link

Browser displays page with username/password fields

Angela fills in CDF user ID and password correctly and presses "Submit" button

Otherwise see "Incorrect User Name Or Password"

Browser displays marks page

Table now has fifth column showing Angela's mark for each assignment

And "Logout" link at bottom of page

This seems like a lot of detail for something so simple

But when your system can do several thousand things, you need a way to keep track of them

And a way to communicate those capabilities to customers before you start construction

Use Cases for Filter Program

We may ask you to write these as an exercise...

Think about what you do with the Unix command-line tools

Requirements

Specify filters using:

Command-line arguments

Configuration file

Use:

Built-in filters

Filters supplied by users

This turns out to be the biggest issue

Both types of filters may need specific arguments

E.g. pattern to search for in each line

Input and output:

Input:

Standard input

Multiple input files

Multiple input files and standard input

Output:

Standard output

Single output file

Do not support both, or multiple output files, for now

Output must be incremental

Cannot read all input, then process, then produce all output

Filters:

Produce some output before reading any input

Produce one line of output for each line of input

Produce multiple lines of output for each line of input

Produce one or more lines of output for some lines of input, but none for others

Produce one or more lines of output after input exhausted

Error situations:

Bad command-line arguments

Badly formatted configuration file

Bad configuration of individual filters

Non-existent filters

Unreadable input files

Unwriteable output file

Filter throws exception

Prototype Design

Use a fixed set of filters to start with

Each identified by a name and zero or more configuration parameters

Parameters passed to filter as strings

Since the alternative is guessing from string contents what data type to use

Main program:

Reads configuration

Creates filters

Reads input from source(s) a line at a time

Passes data through filters in order

Writes output

Handles exceptions

A Few Thoughts

Must handle command-line arguments and configuration files

Both are lists of strings

Can copy relevant command-line arguments into a list

So have MultiFilter.configureFilters() take a list of strings as its argument

May want to trace behavior of filters when testing

Use a Boolean flag to turn debugging output on and off

Can handle output by putting a special "filter" after all user-specified filters

Copies its input to System.out or an open file

Exercise for the reader: explain why we don't handle input the same way

Connecting Filters

Two options:

A list, with MultiFilter responsible for moving one filter's output to the next filter's input

A recursive chain, with each filter responsible for passing its output to the next filter in the chain

Problem with option 1: what happens if a filter produce multiple lines of output for a single line of input?

E.g. a filter that duplicates each line of input

Could hand a (possibly empty) list of output lines back to MultiFilter for each line of input

But that seems clumsy

And if a filter produces a lot of output for some input lines, it is also inefficient

So use option 2

Each filter has a reference to the next in the chain

MultiFilter feeds input to the first one

Recursive calls inside filters' filter() methods do the rest

Testing

Figuring out how to test something is part of designing it

If you can't think of a way to test it, maybe you ought to use a different design

Things to test:

Configuration of individual filters

Behavior of individual filters

Overall configuration

Both from file and from command-line

Behavior of chained filters

Look at combinations of all types of behaviors

How to test:

Want to use JUnit

Do not want to depend on external files

So give MultiFilter a setInput() method

Give it a BufferedReader wrapped around a StringReader

And an output filter that writes to a list of strings

Can then store test input in test class, capture output, and compare

$Id: filter.html,v 1.1.1.1 2004/01/04 05:02:31 reid Exp $