Introduction to Software Construction

Chapter 1-3

What Is Software Construction?

What we will cover?

Why Is Software Construction Important? Is a large part of software development Is the central activity in software development With a focus on construction, the individual

programmer's productivity can improve enormously

Construction's product, the source code, is often the only accurate description of the software

Construction is the only activity that's guaranteed to be done

The power of metaphors The chemist Kekulé had a

dream in which he saw a snake grasp its tail in its mouth.

When he awoke, he realized that a molecular structure based on a similar ring shape would account for the properties of benzene.

Further experimentation confirmed the hypothesis

Software Metaphors Metaphors have the virtue of an expected behaviour that is

understood by all.   unnecessary communication and misunderstanding are minimized, allows for quicker learning and a higher level of abstraction for the

problem at hand. A software metaphor

is more like a searchlight than a road map. it doesn't tell you where to find the answer; it tells you how to look for it.

A metaphor serves more as a heuristic than it does as an algorithm. a heuristic process.

This will tell you how to look for a solution, not where to find an algorithm process.

Gives you specific directions for a solution, and where to look to find the solution.

Heuristic vs. algorithm The main difference

the level of indirection from the solution example: Getting to someone's house

Heuristic Find the last letter we mailed you. Drive to the town in the return address. When you get to town, ask someone where our house is. Everyone knows us—someone will be glad to help you. If you can't find anyone, call us from a public phone, and

we'll come get you. Algorithm

Gives you the instructions directly – GPS

Common Software Metaphors (1) Software Penmanship: Writing Code

looks at writing software the same way that most people think that an author writes a book.  You just sit down and start writing from start all the way through to the finish.

along with starting from scratch, just as in writing a story, expect to take at least one version of your program and completely through it away.

this works great if you are working for yourself, however, most employers like to see productive work, not work you throw away.

implies a software-development process that's too simple and rigid to be healthy

Common Software Metaphors (2) Software Farming: Growing a System

like planting seeds and growing crops You design a piece, code a piece, test a piece, and add it to the

system a little bit at a time. By taking small steps, you minimize the trouble you can get into at any one time

a good technique is described with a bad metaphor its suggestion that you don't have any direct control over how

the software develops.

Common Software Metaphors (3) Software Oyster Farming: System

Accretion like a oyster gradually adding coats of

calcium carbonate to a pebble to reduce the irritation.

make the simplest possible version of the system that will run  it doesn't have to perform realistic

manipulations on data, it doesn't have to produce realistic output

however, once the version is complete, we slowly add small working components to the system and test them.   will eventually lead to a fully functioning

system by slowly over time adding little bits of working code to make the operation smooth and pretty.

Common Software Metaphors (4) Software Construction: Building Software

The image of "building" software is more useful than that of "writing" or "growing" software.

It's compatible with the idea of software accretion and provides more detailed guidance.

Building software implies various stages of planning, preparation, and execution that vary in kind and degree depending on what's being built.

Building houses vs. building software In building a house, you won't try to build things you can buy

already built. You'll buy a washer and dryer, dishwasher, refrigerator, and freezer. Unless you're a mechanical wizard, you won't consider building

them yourself. You'll also buy prefabricated cabinets, counters, windows, doors,

and bathroom fixtures. If you're building a software system, you'll do the same

thing. You'll make extensive use of high-level language features rather

than writing your own operating-system-level code. You might also use prebuilt libraries of container classes, scientific

functions, user interface classes, and database-manipulation classes.

It generally doesn't make sense to code things you can buy ready-made.

The Intellectual Toolbox A good craftsman

knows the right tool for the job and knows how to use it correctly.

A good programmer The software practices and techniques that work

become the toolbox.   add what worked as well as what didn't work to

the toolbox to make it more effective

Prerequisites to Construction Before beginning construction of a house, a builder reviews

blueprints, checks that all permits have been obtained, and surveys the house's foundation. A builder prepares for building a skyscraper one way, a housing

development a different way, and a dog-house a third way. No matter what the project, the preparation is tailored to the

project's specific needs and done conscientiously before construction begins.

As with building construction, much of the success or failure of the project has already been determined before construction begins. If the foundation hasn't been laid well or the planning is inadequate,

the best you can do during construction is to keep damage to a minimum.

The importance of these steps cannot be overstated. if this process is poorly done, we will still be lucky if the result is

even working, let alone useful

Emphasizing Quality At the end of the project

emphasis is on testing In the middle of the project

emphasis is on good construction practices At the beginning

plan for a high quality product Your design limits the end results.

by the time you start construction, it is too late to correct a weak or faulty design.

The best we can do in construction is to recognize that the design is bad and go back and try again.   However, this can be an very expensive proposition, but

at times necessary to actually save money.

Cost to fix defects (1)

Cost to fix defects (2)

Problem-Definition Prerequisite The problem definition lays

the foundation for the rest of the programming process you need a clear statement

of the problem before you can even think of a solution

the problem definition must also be general enough that it does not presuppose a solution

the definition should be in the user's language and from the user's point of view

the solution may turn out to be something that is not related to computers at all

The penalty for failing to define the problem

Requirements Prerequisite Ensure :

The user's needs are driving the system, not the programmers

System scope is agreed upon Minimal changes will be needed after construction

begins since requirements are never stable:

use the checklist on pages 33-34 of your text. make sure that the cost of changes is understood establish a good "change control" procedure use an approach that accommodates changes lastly you can always kill the project.

Without good requirements you can have the right general problem but miss the

mark on specific aspects of the problem

Architecture Prerequisite Buy versus build (market research) System architecture

why was a particular architecture chosen? what things did we consider when we made our choices? (Cover your own

butt) Major data structures Functionality

Key algorithms that are present in the user requirements User interface Hardware requirements Nonfunctional

Error processing Robustness Over engineering Fault tolerance Performance usually means the average response time, which is system dependant

Strategy to handle changes

Without good software architecture you may have the right problem but the wrong solution.

It may be impossible to have successful construction

Key Construction Decisions Choice of Programming Language Programming Conventions Your Location on the Technology Wave Selection of Major Construction Practices

Choice of Programming Language

Programming Conventions Allows for more productive coding between

programmers on projects, as it provides a common platform from which the system is built.

Before construction begins, spell out the programming conventions you'll use. Coding-convention details are at such a level of precision that they're nearly impossible to retrofit into software after it's written.

Your Location on the Technology Wave Programmers who program "in" a language

limit their thoughts to constructs that the language directly supports.

If the language tools are primitive, the programmer's thoughts will also be primitive.

Programmers who program "into" a language first decide what thoughts they want to express,

and then they determine how to express those thoughts using the tools provided by their specific language.

Most of the important programming principles depend not on specific languages but on the way you use them

Selection of Major Construction Practices