chapter 7 : building a text-based user interface

An Introduction to Programming and Object

Oriented Design using Java2nd Edition. May 2004

Jaime NiñoFrederick Hosch

Chapter 7 : Building a Text-based user interface.

2May 2004 Chapter 7

Objectives

After studying this chapter you should understand the following: independence of servers from clients;

likeliness of the user interface to change;

i/o streams;

structure of a simple text-based user interface;

purpose and structure of a while loop;

how existing classes can be composed to define a new class.

3May 2004 Chapter 7

Objectives

Also, you should be able to: use input/output objects to read and write data;

build a simple text-based user interface;

use and trace a basic while statement;

define classes whose implementations contain other classes as components.

4May 2004 Chapter 7

Relating User-interface and Model

Model: objects that solve problem at hand.

User interface: interacts with user, getting input from user, and giving output to user, reporting on status of model.

User Interface Modelcontrols, views

Flexibility in design: identifying these as separate subsystems.

5May 2004 Chapter 7

Stream-based I/O

Data streams: sequence of bytes.

character stream: sequence of characters

input stream: stream is a source of data for an application.

output stream: stream is appliction destination (or sink).

Application… b b b b b b b b b …

… b b b b b b b b b …



6May 2004 Chapter 7

Standard Input and Output streams

standard input:

Application

characters from keyboard

standard error:characters to display

standard output:characters to display

7May 2004 Chapter 7

Writing standard output

System.out: provides methods for writing to standard output.public void println (String s)

Write specified String to standard output and then terminate the line.

public void print (String s)Write the specified String to standard output.

public void println ()Terminate current line by writing line terminator to standard output.

public void flush ()

Flush stream: write buffered output to standard output and flush that stream.

8May 2004 Chapter 7

Input: nhUtilities.basicIO.BasicFileReader

Constructors:

public BasicFileReader ()Create a BasicFileReader attached to standard input.

public BasicFileReader (String fileName)Create a BasicFileReader attached to the named file.

9May 2004 Chapter 7


Command:

public void readInt ()Read a new int from input stream. The digit string is interpreted as a decimal integer. Characters following any white space in input stream must have format of an optionally signed decimal integer.

10May 2004 Chapter 7

readInt

standard input

• • • • 1 2 3 4 5 a b c

BasicFileReader

lastInt = ?

Application

standard input

a b c

BasicFileReader

lastInt = 12345

Application

before executing readInt()

after executing readInt()



Query:

public int lastInt ()int most recently read by readInt.

require: this.readInt() has been successfully performed.



readDouble and readWord skip white space at beginning of stream. readDouble requires characters after white space have format of an

optionally signed double literal. readWord reads a sequence of nonwhite space characters.

Other commands for reading from the input stream:public void readChar ()

Read a new character from this input stream.

public void readDouble ()Read a new double from this input stream.

public void readLine ()Read the rest of the line from this input stream.

public void readWord ()Read a new word from this input stream.



Other queries:public char lastChar ()

Character most recently read by readChar.

require: this.readChar() has been successfully performed.

public double lastDouble ()double most recently read by readDouble.

require: this.readDouble() has been successfully performed.

public String lastString ()String most recently read by readWord or readLine.

require: this.readWord() or this.readLine() has been successfully performed.



Other queries:

public boolean eof ()End of input stream has been reached.


Building an interface for Rectangle

Build an interface that let’s user specify dimensions of the Rectangle and ask for property to be displayed.

class RectangleTUIA simple text-based interface for Rectangles.

public RectangleTUI ()Create a new RectangleTUI instance.

public void start ()Run the interface.



Application creates interface and executes start:

public class RectangleExample {

public static void main (String[] argv) {

(new RectangleTUI()).start();

}

}



Instance variables are initialized in constructor:

RectangleTUI properties:

private Rectangle rectangle; // the modelprivate BasicFileReader in; //standard input

public RectangleTUI () {this.rectangle = null;this.in = new BasicFileReader();

}



To get Rectangle dimensions from user:

private void createRectangle ()Create a new Rectangle with user-provided dimensions.

Uses readIntWithPrompt to get dimensions from user.

private int readIntWithPrompt (String prompt) {System.out.print(prompt); System.out.flush();in.readInt();in.readLine();return in.lastInt();

}



private void createRectangle () {int length;int width;length = readIntWithPrompt(

"Rectangle length (a non-negative integer): ");width = readIntWithPrompt(

"Rectangle width (a non-negative integer): ");this.rectangle = new Rectangle(length,width);

}



RectangleTUI is a client of Rectangle. Constructor creates an instance of Rectangle using input

values from user. But, Rectangle has pre-conditions to keep:

User may enter negative values for length or width.

public Rectangle (int length, int width)Create a new Rectangle with specified length and width.

require: length >= 0 && width >= 0


While statement

conditiontrue

falsestatement

EN D

BE G IN

while ( condition )statement


Re-implementing createRectangle

private void createRectangle () {

//initialize to insure at least one execution of loops:

int length = -1;

int width = -1;

while (length < 0)

length = readIntWithPrompt(

"Rectangle length (a non-negative integer): ");

while (width < 0)

width = readIntWithPrompt(

"Rectangle width (a non-negative integer): ");

// assert: length >=0 && width >= 0

this.rectangle = new Rectangle(length,width);

}



private void displayMenu ()Display the menu to the user.

private void executeChoice (int choice)Perform the indicated action, and display results to the user.



Name operation choices with identifiers:

private static final int LENGTH = 1;

private static final int WIDTH = 2;

private static final int AREA = 3;

private static final int PERIMETER = 4;

private static final int NEW = 5;

private static final int EXIT = 0;

private static final int NO_CHOICE = -1;



public void start () {createRectangle();int choice = NO_CHOICE;while (choice!= EXIT) {

displayMenu();choice= readIntWithPrompt("Enter choice:

");executeChoice(choice);

}}



private void displayMenu () {

System.out.println();

System.out.println("Enter number denoting action to perform:");

System.out.println("Display length............." + LENGTH);

System.out.println("Display width.............." + WIDTH);

System.out.println("Display area..............." + AREA);

System.out.println("Display perimeter.........." + PERIMETER);

System.out.println("Create new rectangle......." + NEW);

System.out.println("Exit......................." + EXIT);

}


Building an interface for Rectangleprivate void executeChoice (int choice) {

System.out.println();if (choice == LENGTH)

System.out.println("Length is " + rectangle.length());else if (choice == WIDTH)

System.out.println("Width is " + rectangle.width());else if (choice == AREA)

System.out.println("Area is " + rectangle.area());else if (choice == PERIMETER)

System.out.println("Perimeter is " + rectangle.perimeter());else if (choice == NEW)

createRectangle();else if (choice == EXIT)

System.out.println("Goodbye.");else

System.out.println(choice + " is not valid.");}


Using composition

Build an application that lets a user access a “locked oracle.” user can get a fortune from oracle only by providing

correct key.

Use classes CombinationLock and Oracle.

Create two new classes, one modeling a locked oracle, and other defining user interface.


Using composition

public class Oracle A dispenser of fortunes. An Oracle gives a fortune only if it is awake;

normal sequence of actions is: wake Oracle; get fortune; put the Oracle back to sleep.

public Oracle () Create new Oracle. This Oracle is initially asleep. ensure: !this.isAwake()

public boolean isAwake () This Oracle is awake.

public String fortune ()The prophecy currently seen by this Oracle. require: this.isAwake()

public void awaken () Wake this Oracle. Oracle will divine a fortune when it wakes.

public void sleep () Put this Oracle to sleep.


Using compositionpublic class LockedOracle

A keyed dispenser of fortunes. A LockedOracle will give a fortune only if the correct key is provided. A LockedOracle must be told to conjure a fortune before the fortune can be retrieved.

public static final String NO_FORTUNEString indicating no fortune has been conjured.

public LockedOracle (int key)Create new LockedOracle with the specified key.require: 0 <= key && key <= 999

public String fortune ()The prophecy currently seen by this LockedOracle. If a fortune has

not been conjured, the String NO_FORTUNE is returned.

public void conjureFortune (int keyToTry)Prophesy. This LockedOracle will make a prophecy only if the

correct key is presented.require: 0 <= keyToTry && keyToTry <= 999


Using composition

Composition: process of defining a new class by putting together existing classes.

An instance of the new class, the composite, references instances of the existing classes, its components.

CompositeClass ComponentClasshas-a


Implementing LockedOracle

LockedOracle class defined to be a composite, with CombinationLock and Oracle components.

LockedOracle

Oracle

CombinationLock



Instance variables are initialized in constructor:

private CombinationLock lock;private Oracle oracle;private String fortune; //current prophesy

public static final String NO_FORTUNE ="Sorry, no fortune for you.";

public LockedOracle (int key) {lock = new CombinationLock(key);lock.close();oracle = new Oracle();fortune = NO_FORTUNE;

}



Command conjureFortune first attempts to open lock, and wakes oracle only if it succeeds:

public String fortune () {

return this.fortune;

}

public void conjureFortune (int keyToTry) {lock.open(keyToTry);if (lock.isOpen()) {

oracle.awaken();fortune = oracle.fortune();oracle.sleep();lock.close();

} else fortune = NO_FORTUNE;

}


Implementing the User interface

class LockedOracleTUI

A simple text-based interface for a LockedOracle.

public LockedOracleTUI (LockedOracle oracle)

Create a new interface for the specified LockedOracle.

public void start ()

Run the interface.



Application creates an oracle and an interface, and executes interface’s start method:

public class OracleExample { public static void main (String[] argv) { LockedOracle oracle = new LockedOracle(123); LockedOracleTUI ui = new LockedOracleTUI(oracle); ui.start() }}



Instance variables and constructor.

private LockedOracle oracle;private BasicFileReader in;

public LockedOracleTUI (LockedOracle oracle) {this.oracle = oracle;this.in = new BasicFileReader();

}



Specify private helper methods:

private boolean readYes (String prompt)Read a yes or no response from the user. Return true if user keys “yes.”

private int readKey (String prompt)Read and return a legal key.

ensure:0 <= this.readKey() && this.readKey() <= 999



public void start () {String fortune;boolean goOn = true;while (goOn) { goOn = readYes("Fortune? (Key yes or no): "); if (goOn) { int key = readKey("Enter key (0-999): "); oracle.conjureFortune(key); fortune = oracle.fortune(); System.out.println(fortune); System.out.println(); }}System.out.println("Good-bye.");

}


Summary

Built a simple text-based user interface.

We reviewed the relationship between client and server:

a client is dependent on its server,

a server is independent of its client.


Summary

In designing a system, it is preferable to make stable components independent of those likely to require change.

Since user interface is considerably less stable than model, we favor designs in which the user interface acts as client to the model.

In cases where model must collaborate with the user interface, need to minimize interface for this collaboration.


Summary

Introduced i/o streams.

A stream is a sequence of bytes, sometimes viewed as characters, to which an application can append data (an output stream) or from which an application can read data (an input stream).

Java has an extensive library of classes to deal with streams.

Use only basic functionality from the predefined object System.out for output, BasicFileReader class from nhUtilities.basicIO package for

input.


Summary

Presented two simple applications with their text-based user interfaces.

Introduced while statement or while-loop: a Java construct that specifies a sequence of actions to be repeated until a condition fails to hold.


Summary

In design of LockedOracle composed existing classes to construct a new class.

The relation between a composite class and its component classes is called the has-a relation.

LockedOracle class wraps Oracle, adapting its specification to that required by the system: the adapter pattern.

chapter 7 : building a text-based user interface

Documents

character stream

readintstandard input

public void flush flush

public void readword

public void readint

public void readchar

beginning of stream

public void readdouble