Software Design – Overview – Characteristics – Cohesion & Coupling – Layered design – Approaches

Chapter 1: Introduction to Software Design 2
Outline
• The software challenge and the software life cycle
• Activities of each phase of the software life cycle
• Using top-down design and object-oriented design
• Managing complexity:
• Data abstraction
• Procedural abstraction
• Information hiding
• Class diagrams document interactions between classes

Outline (2)
• Abstract data types:
• Role in modeling
• Implementing them with classes and interfaces
• Use cases: tool to document interaction with a user
• Software design process example:
• Design and implementation of an array-based
telephone directory
• Sequence diagrams: tool for documenting the interaction
between multiple classes used in a program

The Software Challenge
• Software is ...
• Used for a long time
• Updated and maintained
• By people who did not write it
• Initial specification may be incomplete
• Specification clarified through extensive interaction
between user(s) and system analyst(s)
• Requirements specification needed at the beginning of
any software project
• Designers and users should both approve it!

Things Change!
• Users’ needs and expectations change
• Use reveals limitations and flaws
• Desire for increased convenience, functionality
• Desire for increased performance
• Environment changes
• Hardware, OS, software packages (“software rot”)
• Need to interact with clients, parent org., etc.
• Law and regulations change
• Ways of doing business
• Style, “cool” factor

The Software Life Cycle
• Software goes through stages as it moves from initial
concept to finished product
• The sequence of stages is called a life cycle
• Must design and document software:
• In an organized way for:
• Understanding and ...
• Maintenance (change) after the initial release
• The maintainer is not necessarily the author!
• ... and even authors forget
• ... and no one can keep all details in mind at once

Software Life Cycle Models:
The Waterfall Model
• Simplest way to organizing activities in stages
• Activities are:
• Performed in sequence
• Result of one flows (falls) into the next
• The Waterfall Model is simple ... but unworkable
• Fundamental flaw: Assumption that each stage can
and must be completed before the next one occurs
• Example: User may need to see finished product to
express true requirements!

Waterfall Model

Waterfall Model (2)

Other Software Life Cycle Models
• Common theme among models: stages or cycles
• Unified Model:
• Cycles are called phases and iterations
• Activities are called workflows
• The four phases of the Unified Model:
• Inception
• Elaboration
• Construction
• Transition

Other Software Life Cycle Models (2)

Software Life Cycle Activities
Activities essential for successful development:
• Requirements specification
• Architectural, component, & detailed designs
• Implementation
• Unit, integration, and acceptance testing
• Installation and maintenance

Software Life Cycle Activities Defined

Software Life Cycle Activities (more)
• Requirements Specification
• System analyst works with 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

Software Life Cycle Activities (continued)
• Design
• Top-down: break system into smaller subsystems
• Object-oriented: identify objects and their interactions
• UML diagrams: tool to show interactions between:
• Classes (inside the system)
• Classes and external entities

Example of Top-Down: Stepwise Refinement

Example of Object-Oriented: Class Diagram

Using Abstraction to Manage Complexity
• An abstraction is a model of a physical entity or activity
• Models include relevant facts and details
• Models exclude matters irrelevant to system/task
• Abstraction helps programmers:
• Complex issues handled in manageable pieces
• Procedural abstraction: distinguishes ...
• What to achieve (by a procedure) ...
• From how to achieve it (implementation)
• Data abstraction: distinguishes ...
• Data objects for a problem and their operations ...
• From their representation in memory

Using Abstraction to Manage Complexity (2)
• If another class uses an object only through its methods,
the other class will not be affected if the data
representation changes
• Information hiding: Concealing the details of a class
implementation from users of the class
• Enforces the discipline of data abstraction

Abstract Data Types, Interfaces, and
Pre- and Post-conditions
• A major goal of software engineering: write reusable code
• Abstract data type (ADT): data + methods
• A Java interface is a way to specify an ADT
• Names, parameters, return types of methods
• No indication of how achieved (procedural abstraction)
• No representation (data abstraction)
• A class may implement an interface
• Must provide bodies for all methods of the interface

Abstract Data Types, Interfaces, and
Pre- and Postconditions (2)

Abstract Data Types, Interfaces, and Pre-
and Postconditions (continued)
• You cannot instantiate (new) an interface
• But you can:
• Declare a variable that has an interface type
• Use it to reference an actual object, whose class
implements the interface
• A Java interface is a contract between
• The interface designer and ...
• The coder of a class that implements the interface
• Precondition: any assumption/constraint on the method
data before the method begins execution
• Postcondition: describes result of executing the method

Requirements Analysis:
Use Cases, and Sequence Diagrams
• Analysis first step: study input and output requirements:
• Make sure they are understood and make sense
• Use case:
• User actions and system responses for a sub-problem
• In the order that they are likely to occur
• Sequence diagram:
• Shows objects involved across the horizontal axis
• Shows time along the vertical axis
• See page 26 for an example; shows:
• User, PDApplication, PhoneDirectory, BufferedReader,
PDUserInterface object + a number of method calls

Design of an Array-Based Phone Directory
• Case study shows:
• Design
• Implementation
• Testing of a software-based phone directory
• In UML class diagrams:
+ sign next to a method/attribute means it is public
- sign next to a method/attribute means it is private

Design of Array-Based Phone Directory
Classes/interfaces to design include:
• PDUserInterface: interface; later we consider:
• Console (command line) UI class
• Graphical (JOptionPane) UI class
• PDApplication: main / driving class
• PhoneDirectory: interface
• ArrayBasedPD: class implementing PhoneDirectory
• DirectoryEntry: class, for one item in the directory

Design of Array-Based Phone Directory (2)

Design of DirectoryEntry
• Simple class, similar to Person in Java review:
• Two private fields, for name and number
• Two-argument constructor
• Get methods for both fields
• Set method for number (only)

The PhoneDirectory Interface
/**
* The interface for the telephone directory.
* @author Koffman & Wolfgang
*/
public interface PhoneDirectory {
...
}
• Shows syntax of an interface
• Shows a javadoc comment and the @author tag

PhoneDirectory.loadData
/** Load the data file containing the
* directory, or establish a connection with
* the data source.
* @param sourceName The name of the file
* (data source) with the phone directory
* entries
*/
void loadData (String sourceName);
• Shows syntax of method in an interface (note ;)
• Shows a javadoc comment with the @param tag
• Since returns void, no @return tag

PhoneDirectory.lookupEntry
/** Look up an entry.
* @param name The name of the person
* to look up
* @return The number, or null if name
* is not in the directory
*/
String lookupEntry (String name);
• Shows a javadoc comment with the @return tag
• I prefer a space before the ( in a declaration (not a call)

PhoneDirectory.addOrChangeEntry
/** Add an entry or change an existing entry.
* @param name The name of the person being
* added or changed
* @param number The new number to be assigned
* @return The old number or, if a new entry,
* null
*/
String addOrChangeEntry (String name,
String number);
• Shows a javadoc comment with two @param tags

PhoneDirectory.removeEntry
/** Remove an entry from the directory.
* @param name The name of the person to be
* removed
* @return The current number. If not in
* directory, return null
*/
String removeEntry (String name);

PhoneDirectory.save
/** Method to save the directory.
* pre: The directory is loaded with data.
* post: Contents of directory written back to
* the file in the form of name-number pairs
* on adjacent lines;
* modified is reset to false.
*/
void save ();
• Illustrates pre/post conditions

Design of ArrayBasedPD.loadData
Input: a file name; Effect: read initial directory from the file
1. Create a BufferedReader for the input
2. Read the first name
3. while the name is not null
4. Read the number
5. Add a new entry using method add
6. Read the next name

Design of
ArrayBasedPD.addOrChangeEntry
Input: name and number; Effect: change number of existing
entry, or make new entry if there was none
1. Call method find to see if the name is in the directory
2. if the name is in the directory
3. change number with DirectoryEntry.setNumber
4. Return the previous value of the number
else
5. Add a new entry using method add
6. Return null

• Remaining method designs proceed along the same
lines
• The class diagram changes, showing private fields and
methods added ....

Implementing and Testing the Array-Based
Phone Directory: ArrayBasedPD.java
import java.io.*;
/** This is an implementation of the
* PhoneDirectory interface that uses an
* array to store the data.
* @author Koffman & Wolfgang
*/
public class ArrayBasedPD
implements PhoneDirectory {
...
} // note: import, javadoc, implements

ArrayBasedPD Data Fields (1)
// Data Fields (with javadoc comments)
/** The initial capacity of the array */
private static final int INITIAL_CAPACITY = 100;
/** The current capacity of the array */
private int capacity = INITIAL_CAPACITY;
/** The current size of the array (number of
directory entries) */
private int size = 0;

ArrayBasedPD Data Fields (2)
/** The array to contain the directory data */
private DirectoryEntry[] theDirectory =
new DirectoryEntry[capacity];
/** The name of the data file that contains the
directory data */
private String sourceName = null;
/** Boolean flag indicates if the directory was
modified since it was loaded or saved. */
private boolean modified = false;

ArrayBasedPD.loadData
public void loadData (String sourceName) {
// Remember the source name.
this.sourceName = sourceName;
try { ...
} catch (FileNotFoundException ex) {
// Do nothing — no data to load.
return;
} catch (IOException ex) {
System.err.println(“Directory load failed.");
ex.printStackTrace();
System.exit(1);
}
}

ArrayBasedPD.loadData (2): Inside try
BufferedReader in = new BufferedReader(
new FileReader(sourceName));
while (true) {
String name, number;
// read name and number from succeeding lines
if ((name = in.readLine()) == null) break;
if ((number = in.readLine()) == null) break;
// insert entry (if got both name and number)
add(name, number);
}
in.close(); // should always close input
• Slightly different loop approach from the text
• Same assign-in-if-condition “hack”

ArrayBasedPD.loadData (3): alternate
boolean more = true;
while (more) {
more = false;
String name = in.readLine();
if (name != null) {
String number = in.readLine();
if (number != null) {
add(name, number);
more = true;
}
}
}
• Nested if statements not as pleasant (what if 7 inputs?)
• Control variables tend to be harder to understand/get right

ArrayBasedPD.addOrChangeEntry
public String addOrChangeEntry (String name,
String number) {
String oldNumber = null;
int index = find(name);
if (index > -1) {
oldNumber = theDirectory[index].getNumber();
theDirectory[index].setNumber(number);
}
else {
add(name, number);
}
modified = true;
return oldNumber;
}

ArrayBasedPD.save
public void save() {
if (!modified) return; // save not needed
try {
// Create PrintWriter for the file.
PrintWriter out = new PrintWriter(
new FileWriter(sourceName));
...
} catch (Exception ex) {
System.err.println(“Directory save failed");
ex.printStackTrace();
System.exit(1);
}
}

ArrayBasedPD.save (2)
// Write each directory entry to the file.
for (int i = 0; i < size; i++) {
// Write the name.
out.println(theDirectory[i].getName());
// Write the number.
out.println(theDirectory[i].getNumber());
}
// Close the file.
out.close();
modified = false;

Implementing and Testing the Array-Based
Phone Directory

ArrayBasedPD.find
private int find (String name) {
for (int i = 0; i < size; i++) {
if (theDirectory[i].getName().equals(name)) {
return i;
}
}
return -1; // Name not found.
}

ArrayBasedPD.add
private void add (String name, String number) {
if (size >= capacity) {
reallocate();
}
theDirectory[size++] =
new DirectoryEntry(name, number);
}
• Differs from text in use of ++
• Note that size means number of names stored,
• while capacity means the number the array can hold

ArrayBasedPD.realloc
private void reallocate () {
capacity *= 2;
DirectoryEntry[] newDirectory =
new DirectoryEntry[capacity];
System.arraycopy(theDirectory, 0,
newDirectory, 0,
theDirectory.length);
theDirectory = newDirectory;
}
Arguments to arraycopy are:
• fromDir, fromIndex
• toDir, toIndex
• number of elements to copy

Testing ArrayBasedPD
• Empty data file
• Data file with only one name-number pair
• Data file with odd number of lines
• Data file with more pairs than initial array size
• Retrieve names not in directory as well as ones that are
• After a change, verify the new information
• Check that after changes, the changes, plus all new
information, are in the newly written file
• Note: This code does not check for empty strings!

Implementing PDUserInterface
• Text offers two classes that implement the UI interface:
• PDGUI: Uses JOptionPane for graphical UI
• PDConsoleUI: Uses console stream I/O
(System.in and System.out)
• Text gives good recipes here that you can use as models
• We will not cover them in detail here

Software Design – Overview – Characteristics – Cohesion & Coupling – Layered design – Approaches

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