design abstraction

8/12/2019 Design Abstraction

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1

The Software Challenge

People may come and go, but software may remain

A software product is often 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 may evolve

New features may be added, environments may

change, so initial specification may be incomplete


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The Software Specification Challenge

Software specification is not easy

It should be generated at the beginning of project and

maintained up-to-date while the software goes

through changes

It should be clarified through extensive interaction

between the users and the system analyst, and then

approved by the users

It should be clear and understandable to any

programmer


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3

The Software Life CycleThe Life and

Death of Software

Software products go through several stages as they

mature from initial concept to finished product

The sequence of stages is called a life cycle


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4

Software Life Cycle Models

Waterfall model: simplest way of organizing activities

that transform software from one stage to another

Activities are performed in sequence and the results of

one flows into the next


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Waterfall Model

1 Requirements (requirements are determined)

2 Analysis (requirements are analyzed)

3 Design (components are designed)

4 Implementation (components are implemented)5 Testing (components are assembled and tested as a

whole)

5


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Software Life Cycle Models

Waterfall model is s imp le but u nworkable

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 theuser can fully express his or her requirements


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Software Life Cycle Activities

1) Requirements

Specification

2) Design

a. Architectural Design

b. Component Design

c. Detailed Design

3) Implementation

4) Test

a. Unit Test

b. Integration Test

c. Acceptance Test

5) Installation

6) Maintenance

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Design Principles in Software Life Cycle

Activities

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

UML diagrams are a design tool to illustrate the

interactions between

Classes

Classes and external entities


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Requirements Analysis, Use Cases, and

Sequence Diagrams

First step in analysis is to study the problem of input and

output requirements carefully to make sure they are

understood and make sense

Use case: list of the user actions and system responses

for a particular sub-problem in the order that they are

likely to occur

Sequence diagram: shows all the objects involved in

this use case across the horizontal axis, time is shown

along the vertical axis


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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


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Using Abstraction to Manage Complexity

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 beachieved by a procedure from its implementation


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Using Abstraction to Manage Complexity

(contd)

Data abstraction: specify the data objects for a

problem and the operations to be performed on them

without concern for their representation in memory

Representation of a data object is irrelevant to other

classes that access to it only via its methods

Information hiding: concealing the details of a class

implementation from users of the class


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An Example: Telephone Directory

Maintain a collection of telephone directory entries,

where each entry is referred to by a unique name.

Can read from a file, save to a file, lookup, add, remove,

and change phone number

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Dependencies Among Possible Actions

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Object Relations

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Things you already know (about) ...

Java programs (you know and love)

Classes and objects (you can create and use)

Inheritance (you understand and can extend)

Abstract classes (you remember what they are) Interfaces (your contractual obligations)

Interfaces are a key idea in CSC220

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Interfaces

The interface specifies the names, parameters, and

return values of the ADT methods without specifying how

the methods perform their operations and without

specifying how the data is internally represented

Each class that implements an interface must provide

the definitions of all methods declared in the interface


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Interfaces

You cannot instant iate an interface; that is, cantinvoke

newINTERFACE_NAME

You can declare a variable that has an interface type

and use it to reference an actual object

A Java interface is a contract between the interface

designer and the programmer who codes a class that

implements the interface


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Public Methods of PhoneDirectory Interface

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Method Action

public void loadData(String fileName) Loads the data from the data file whose name

is given by fileName

public String addOrChangeEntry(String name,

String number)

Changes the number of the individual with the

name to the number

public String lookupEntry(String name) Searches the directory for the name

public String removeEntry(String name) Removes the entry with the given name

public void saveData(String fileName) Saves the data in a load-able format in the

data file whose name is given by fileName


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The PDUserInterface Interface

There is only one required public method,

processCommands, which takes input from the user and

executes the command

Two different implementations:

PDGUI: a GUI-based implementation

PDConsoleUI: a console-based implementation


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PDGUI: Implementation as GUI

Uses JOptionPanedialog windowsString[] commands = {"Add/Change Entry", "Look Up Entry, "Remove Entry", "Save Directory", "Exit};

do {

choice = JOptionPane.showOptionDialog(

null, // No parent

Select a Command, // Prompt message

PhoneDirectory, // window title

JOptionPane.YES_NO_CANCEL_OPTION, // Option typeJOptionPane.QUESTIONE_MESSAGE, // Message type

null, // Accompanying icon

command, // Choice names (array)

command[commands.length1]); // Default command

switch (choice) {

case 0: doAddChangeEntry(); break;

case 1: doLookupEntry(); break;

case 2: doRemoveEntry(); break;

case 3:

case 4: doSave(): break;

default: // Do nothing.

}

} while (choice != commands.length1);


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PDConsoleUI: Implementation Using a

Console

Uses System.out to display the menu of choices and

results.

It also uses a Scanner object (scIn) created out of

System.in to read data from the keyboard.

// Constructor

/** Default constructor. */

public PDConsoleUI() {

scIn = new Scanner(System.in);

}

choice = scIn.nextInt(); // Get next choice.

scIn.nextLine(); // Skip trailing newline.


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ArrayBasedPD that Implements

PhoneDirectory Interface, Private Data Fields

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Data Field Attribute

private static final int INITIAL_CAPACITY The initial capacity of the array (or the array

size). It has the final attribute so change is

not allowed.

private int capacity The current capacity (or the array size), so

the capacity changesprivate int size The number of entries held in the directory

private DirectoryEntry[] theDirectory The directory realized as an array of

DirectoryEntry objects

private String fileName The name of the data file

private boolean modified A boolean variable that maintains whetherany change has been made to any entry

since the last time the data were saved or

loaded. Data is automatically saved at

closing if this variable is true.


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The Private Methods of the

ArrayBasedPD Class

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Private Method Action

private int find(String name) Searches the array for the name and

returns the position of the name; -1

indicates that the name was not found

private void add(String name, String

number)

Adds to the array a new entry with the given

name and number

private void reallocate() Creates a new array with twice the capacity

of the current one with the same entries


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DirectoryEntry Class

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Data Field Attribute

private String name The name of an individual

private String number The phone number of the individual

Constructor Action

public DirectoryEntry(String name, String

number)

Create an entry with the name and the

number

Method Action

public String getName() Retrieves the name of an individual

public String getNumber() Retrieves the number of an individual

public void setNumber(String number) Sets the number of an individual to the give

number


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Implementing and Testing the Array-Based

Phone Directory

The main loop of ReadData has two notable points

The whileand if statements combine an assignment with a

conditional statement

The breakstatement allows exiting of the while loop without

storing an entry

// Read each name and number and add the entry to the array.

BufferedReader in = new BufferedReader(new FileReader(new File (this.filename)))

while ((name = in.readLine()) != null) { // Read name and number from successive lines.if ((number = in.readLine()) != null) { break; } // No number read, exit loop.

// Add an entry for this name and number.

this.add(name, number);

}


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Code Reuse

A part of program can be reused for other programs

If there is a code for maintaining a phone book (adding,

removing, editing, loading, and storing), some of the

ideas and concepts used for the code can be used for

writing a code for maintaining business cards.

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Maximizing Code Reuse

View the program to be developed as a process of

dealing with data

The data has to be maintained during execution of the

program

The data may be read from and/or stored into files

The data may be generated from input

Various operations on the data may be performed

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Abstract Data Types

Abstract data type (ADT) = the combination of data

together with its methods (how the data objects are

accessed)

ADTs specify a set of required methods, but do not

specify how those methods should be implemented(thatswhy they are called abstract)

Datastructuresquite often refer to ADTs

For frequently used ADTs the most efficient universal

(applicable to the vast majority of programming

languages) implementations are known

design abstraction

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