Program Design

Going from a problem to a program

Chapter 1 in the text — Read it!!

The Software Life Cycle

• In industry, a software product is expected to be used for an extended period of time

– by someone who did not write the program and

– who is not intimately familiar with its internal design

• Software products go through several stages as they mature from initial concept to finished product

• The person who maintains the software is not necessarily the person who writes it

• It is important to design and document software

– in an organized way

– so that it can be easily understood and maintained after the initial release.

Waterfall Model of software life cycle

Problems with the Waterfall Model

• Waterfall model is simple but unworkable

– Fundamental flaw is assumption that each stage can and must be completed before the next one occurs

• Sometimes, it is not until the product is finished that the user can fully express his or her requirements

A closer look at some life cycle activities

• Requirements Specification

– System analyst works with software users to clarify the detailed system requirements

– Questions include format of input data, desired form of any output screens, and data validation

• Analysis

– Make sure you completely understand the problem before starting the design or program a solution

– Evaluate different approaches to the design

• Design

– Top-down approach: breaking a system into a set of smaller subsystems

– Object-oriented approach: identification of a set of objects and specification of their interactions

Using abstraction to help with design

• An abstraction is a model of a physical entity or activity

– Abstraction helps programmers deal with complex issues in a piecemeal fashion

• Procedural abstraction: distinguish what is to be achieved by a procedure from its implementation

• Data abstraction: specify the data objects for a problem and the operations to be performed on them without concern for their representation in memory

• Information hiding: Concealing the details of a class implementation from users of the class

Abstract Data Types

• A major goal of software engineering is to write reusable code

• One way to make code reusable is to encapsulate the data with the methods that manipulate the data

• Abstract data type (ADT): The combination of data together with the methods that manipulate the data.

– By manipulate we mean change the data, retrieve it, print it, etc.

• Then a user of the Abstract Data Type can use the ADT without being concerned with HOW the methods were implemented.

Abstract Data Types & Java Interfaces

• A Java interface is a way to specify an ADT

– The interface specifies the names, parameters, and return values of the ADT methods

– It does not specify how the methods perform their operations and

– Does not specify how the data is internally represented

• Each class that implements an interface must provide the definitions of all methods declared in the interface.

• A Java interface is a contract between the interface designer and the programmer who codes a class that implements the interface.

Pre- and Postconditions

• Precondition: a statement of any assumptions or constraints on the method data before the method begins execution

• Postcondition: a statement that describes the result of executing a method

Designing a program

• First step in analysis is to study the problem of input and output requirements carefully to make sure they are understood and make sense

• This is where we start: we want to write a program to provide users with an electronic phone directory.

• What sorts of questions do we ask the users?

– We are trying to get the requirements here.