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Semester 2/Database Systems/Week 8/Week 8 Database Systems.md Normal file
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# [Undoing & Redoing Transactions](Semester%202/Database%20Systems/Week%207/Week%207%20Database%20Systems.md####Undoing%20Transactions)
# Deferred Update Protocol
Updates not written until after transaction has reached commit point.
If transaction fails before committing, no modification is made to database, hence no undoing is required.
May be necessary to redo updates of committed transactions as effect may not have reached database.
## Procedure
Redo transaction is both \<T1 start> and \<T1 commit> are present in the log
### Example
![](Pasted%20image%2020240305130824.png)
Failure in different places:
![](Pasted%20image%2020240305131110.png)
In case(a), no commit is present, so no redo or undo is required.
case(b), T0 commits, but T1 does not, hence T0 is redone.
case(b), T0 and T1 commits, so both are redone.
# Immediate Update Protocol
Updates applied as they occur.
Following a failure, redo updates of committed transactions.
Undo effects of transactions uncommitted.
If no commit record, transacation was active and must be undone.
Undo operations performed in reverse order in log.
In recovery, use log to undo or redo.
## Protocol
Undo
- If <Ti,start> is in the log but <Ti,commit> is not then restore the value of all data items updated by Ti to their old values, going backwards from the last log record for Ti.
Redo
- If <Ti,start> and <Ti,commit> are both in the log then set the value of all data items updated by Ti to the new values, going forward from the first log record for Ti
### Example
![](Pasted%20image%2020240305134506.png)
# Checkpoints
After failure, may not know how far back to search log.
Checkpoints limit log searching, made automatically by DBMS
Creation of checkpoints schedules at predetermined intervals.
Definition: Checkpoints are points of sync between database and log file. All buffers are written to secondary storage.
## Procedure
1. Write to secondary storage
- All log records
- All modified buffer blocks to database.
- Checkpoint record to log file which contains identities of transactions active at time of checkpoint
2. Resume processing.
![](Pasted%20image%2020240305134734.png)
### Example 1 ( Immediate )
- Starts at t0, fails at tf.
- T1 and T6 have to be undone
- In absence of other info, recovery manager has to redo T2, 3, 4, and 5.
- ![](Pasted%20image%2020240305134835.png)
### Example 2 ( serial with checkpoint, Immediate )
- Dotted lines = checkpoints
- T1 is safe since updates are written to disk
- T4 needs to be undone
- T2 and 3 need to be redone
- ![](Pasted%20image%2020240305134944.png)
### Example 3 ( concurrent with checkpoint, Immediate )
- T2 and 3 safe since written to disk
- T1 and 6 need to be undone.
- Since checkpoint, redo T4 and 5.
- ![](Pasted%20image%2020240305135043.png)
# Purpose of Recovery Log File
- Scan log backwards
- Create undo / redo lists
- Undo List: Transactions active at time of crash
- Perform undo(T) for every transaction in list
- Stop when reaching \<T, start> for every transaction in list.
- Redo List: Transactions committed after last checkpoint
- Redo for each transaction in list.
## Example of Log Use ( Immediate )
![](Pasted%20image%2020240305135229.png)

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Semester 2/Programming 2/Project/Part 2/AudioVisual.class
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Semester 2/Programming 2/Project/Part 2/AudioVisual.java
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/**
* Write a description of class AudioVisual here.
* Subclass of LibraryItem to create objects of audio and visual items in a library.
*
* @author (your name)
* @version (a version number or a date)
* @George Wilkinson
* @1.0
*/
import java.util.Scanner;
import java.util.ArrayList;
import java.util.NoSuchElementException;
public abstract class AudioVisual extends LibraryItem
{

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Semester 2/Programming 2/Project/Part 2/Book.class
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Semester 2/Programming 2/Project/Part 2/Book.ctxt
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#BlueJ class context
comment0.params=
comment0.target=Book()
comment0.text=\n\ Constructor\ for\ objects\ of\ class\ Book\n
comment0.text=\n\ Constructor\ for\ objects\ of\ class\ Book\n\ Since\ all\ field\ variables\ initialise\ as\ null,\ nothing\ should\ happen\ here.\n
comment1.params=
comment1.target=void\ printDetails()
comment2.params=detailScanner
comment2.target=void\ readItemData(java.util.Scanner)
numComments=3
comment1.target=java.lang.String\ getAuthor()
comment1.text=\n\ Return\ value\ of\ @author\n
comment2.params=
comment2.target=java.lang.String\ getIsbn()
comment2.text=\n\ Return\ value\ of\ @isbn.\n
comment3.params=author
comment3.target=void\ setAuthor(java.lang.String)
comment3.text=\n\ Set\ value\ of\ @author.\n
comment4.params=isbn
comment4.target=void\ setIsbn(java.lang.String)
comment4.text=\n\ Set\ value\ of\ @isbn\n
comment5.params=
comment5.target=void\ printDetails()
comment5.text=\n\ Print\ to\ terminal,\ relevant\ details\ of\ current\ object.\n
comment6.params=detailScanner
comment6.target=void\ readItemData(java.util.Scanner)
comment6.text=\n\ Passed\ a\ scanner\ object,\ set\ field\ variables\ to\ corresponding\ values.\n
numComments=7

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Semester 2/Programming 2/Project/Part 2/Book.java
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* Subclass of LibraryItem that emulates a Book item.
*
* @George Wilkinson
* @1.0
* @2.3
*/
import java.util.Scanner;
@@ -15,15 +15,54 @@ public class Book extends PrintedItem
/**
* Constructor for objects of class Book
* Since all field variables initialise as null, nothing should happen here.
*/
public Book(){}
/*
* Return value of @author
*/
public String getAuthor()
{
return author;
}
/*
* Return value of @isbn.
*/
public String getIsbn()
{
return isbn;
}
/*
* Set value of @author.
*/
public void setAuthor( String author )
{
this.author = author;
}
/*
* Set value of @isbn
*/
public void setIsbn( String isbn )
{
this.isbn = isbn;
}
/*
* Print to terminal, relevant details of current object.
*/
public void printDetails() {
System.out.println( "ISBN: " + isbn +
"\nAuthor: " + author );
super.printDetails();
}
/*
* Passed a scanner object, set field variables to corresponding values.
*/
public void readItemData( Scanner detailScanner ){
if ( detailScanner != null ) {
this.author = detailScanner.next().trim();

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Semester 2/Programming 2/Project/Part 2/CD.ctxt
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#BlueJ class context
comment0.params=
comment0.target=CD()
comment0.text=\n\ Constructor\ for\ objects\ of\ class\ CD\n
comment0.text=\n\ Constructor\ for\ objects\ of\ class\ CD\n\ Since\ all\ field\ variables\ initialise\ as\ null,\ nothing\ should\ happen\ here.\n
comment1.params=
comment1.target=java.lang.String\ getArtist()
comment1.text=\n\ Return\ value\ of\ @artist.\n
comment2.params=
comment2.target=int\ noOfTracks()
comment2.text=\n\ Return\ value\ of\ @noOfTracks\n
comment3.params=artist
comment3.target=void\ setArtist(java.lang.String)
comment3.text=\n\ Set\ value\ of\ @artist.\n
comment4.params=noOfTracks
comment4.target=void\ setNoOfTracks(int)
comment4.text=\n\ Set\ value\ of\ @noOfTracks.\n
comment5.params=
comment5.target=void\ printDetails()
comment5.text=\n\ Print\ to\ terminal,\ relevant\ details\ of\ current\ object.\n
comment6.params=detailScanner
comment6.target=void\ readItemData(java.util.Scanner)
comment6.text=\n\ Passed\ a\ scanner,\ set\ the\ relevant\ details\ to\ their\ corresponding\ field\ variables.\n
numComments=7

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Semester 2/Programming 2/Project/Part 2/CD.java
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/**
* Write a description of class CD here.
* Subclass of AudioVisual, to create objects of a CD.
*
* @author (your name)
* @version (a version number or a date)
* @George Wilkinson
* @1.0
*/
import java.util.Scanner;
@@ -15,35 +15,54 @@ public class CD extends AudioVisual
/**
* Constructor for objects of class CD
* Since all field variables initialise as null, nothing should happen here.
*/
public CD(){}
/*
* Return value of @artist.
*/
public String getArtist()
{
return artist;
}
/*
* Return value of @noOfTracks
*/
public int noOfTracks()
{
return noOfTracks;
}
/*
* Set value of @artist.
*/
public void setArtist( String artist )
{
this.artist = artist;
}
/*
* Set value of @noOfTracks.
*/
public void setNoOfTracks( int noOfTracks )
{
this.noOfTracks = noOfTracks;
}
/*
* Print to terminal, relevant details of current object.
*/
public void printDetails() {
System.out.println( "Artist: " + artist +
"\nTrack Count: " + noOfTracks );
super.printDetails();
}
/*
* Passed a scanner, set the relevant details to their corresponding field variables.
*/
public void readItemData( Scanner detailScanner ){
if ( detailScanner != null ) {
this.artist = detailScanner.next().trim();

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Semester 2/Programming 2/Project/Part 2/DVD.class
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Semester 2/Programming 2/Project/Part 2/DVD.ctxt
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#BlueJ class context
comment0.params=
comment0.target=DVD()
comment0.text=\n\ Constructor\ for\ objects\ of\ class\ DVD\n
comment0.text=\n\ Constructor\ for\ objects\ of\ class\ DVD\n\ Since\ all\ field\ variables\ initialise\ as\ null,\ nothing\ should\ happen\ here.\n
comment1.params=
comment1.target=java.lang.String\ getDirector()
comment1.text=\n\ Return\ value\ of\ @director.\n
comment2.params=director
comment2.target=void\ setDirector(java.lang.String)
comment2.text=\n\ Set\ value\ of\ @director.\n
comment3.params=
comment3.target=void\ printDetails()
comment3.text=\n\ Print\ relevant\ details\ of\ the\ current\ object.\n
comment4.params=detailScanner
comment4.target=void\ readItemData(java.util.Scanner)
comment4.text=\n\ Passed\ a\ scanner\ object,\ give\ the\ relevant\ variable\ a\ value.\n
numComments=5

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Semester 2/Programming 2/Project/Part 2/DVD.java
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@@ -1,9 +1,9 @@
/**
* Write a description of class DVD here.
* Subclass of AudioVisual to create objects of DVD items.
*
* @author (your name)
* @version (a version number or a date)
* @George Wilkinson
* @1.0
*/
import java.util.Scanner;
@@ -11,26 +11,40 @@ import java.util.Scanner;
public class DVD extends AudioVisual
{
private String director;
/**
* Constructor for objects of class DVD
* Since all field variables initialise as null, nothing should happen here.
*/
public DVD(){}
/*
* Return value of @director.
*/
public String getDirector()
{
return director;
}
/*
* Set value of @director.
*/
public void setDirector( String director )
{
this.director = director;
}
/*
* Print relevant details of the current object.
*/
public void printDetails() {
System.out.println( "Director: " + director );
super.printDetails();
}
/*
* Passed a scanner object, give the relevant variable a value.
*/
public void readItemData( Scanner detailScanner ){
if ( detailScanner != null ) {
this.director = detailScanner.next().trim();

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Semester 2/Programming 2/Project/Part 2/Library.ctxt
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@@ -10,7 +10,7 @@ comment2.target=void\ storeUser(LibraryUser)
comment2.text=\n\ Appends\ a\ LibraryUser\ to\ the\ userList.\n
comment3.params=prefix\ length
comment3.target=java.lang.String\ generateUserID(java.lang.String,\ int)
comment3.text=\n\ Returns\ a\ random\ unique\ user\ ID\ by\ specifying\ \n\ @prefix\ -\ arbitrary\ alphanumeric\ prefix\n\ @length\ -\ length\ of\ numeric\ ID\n\ \n\ Example\:\ length\ \=\ 6,\ expected\ result\ should\ be\ under\ 999,999\ and\ above\ 99,999.\n\ If\ we\ just\ use\ 10^(length),\ this\ would\ generate\ any\ number\ under\ 1,000,000.\n\ This\ is\ an\ issue\ since\ any\ integers\ below\ 100,000\ can\ be\ used,\ which\ would\ be\ incorrect.\n\ By\ using\ the\ offset\ of\ a\ factor\ of\ 10\ below\ the\ desired\ length\ we\ can\ generate\ between\ 0\ and\ 899,999.\n\ After\ this,\ we\ can\ add\ 100,000\ back\ to\ the\ number\ to\ ensure\ a\ baseline\ length\ is\ maintained.\n\ \n\ Note\:\ I\ am\ aware\ that\ this\ is\ overengineered,\ and\ that\ several\ random\ integers\ of\ amount\ @length\ could\ be\ used,\ \n\ but\ this\ is\ considerably\ more\ efficient\ since\ there\ is\ no\ iteration\ involved\ in\ the\ creation\ of\ the\ ID.\ O(1)\n
comment3.text=\n\ Returns\ a\ random\ unique\ user\ ID\ by\ specifying\ \n\ @prefix\ -\ arbitrary\ alphanumeric\ prefix\n\ @length\ -\ length\ of\ numeric\ ID\n\ and\ returning\ a\ unique\ user\ id\n\ @uuid\ -\ a\ unique\ string\ starting\ with\ @prefix,\ and\ concat.\ with\ a\ random\ number\n\ \n\ Example\:\ length\ \=\ 6,\ expected\ result\ should\ be\ under\ 999,999\ and\ above\ 99,999.\n\ If\ we\ just\ use\ 10^(length),\ this\ would\ generate\ any\ number\ under\ 1,000,000.\n\ This\ is\ an\ issue\ since\ any\ integers\ below\ 100,000\ can\ be\ used,\ which\ would\ be\ incorrect.\n\ By\ using\ the\ offset\ of\ a\ factor\ of\ 10\ below\ the\ desired\ length\ we\ can\ generate\ between\ 0\ and\ 899,999.\n\ After\ this,\ we\ can\ add\ 100,000\ back\ to\ the\ number\ to\ ensure\ a\ baseline\ length\ is\ maintained.\n\ \n\ Note\:\ I\ am\ aware\ that\ this\ is\ overengineered,\ and\ that\ several\ random\ integers\ of\ amount\ @length\ could\ be\ used,\ \n\ but\ this\ is\ considerably\ more\ efficient\ since\ there\ is\ no\ iteration\ involved\ in\ the\ creation\ of\ the\ ID.\ O(1)\n
comment4.params=
comment4.target=void\ writeUserData()
comment4.text=\n\ A\ method\ to\ output\ all\ user\ data\ to\ a\ file\ using\ a\ fileDialog\ to\ specify\ file\ location.\n\ \n\ Note\:\ Potentially\ could\ implement\ the\ HashSet\ made\ for\ GenerateUserID\ to\ avoid\ unnecessary\ recursion.\n

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Semester 2/Programming 2/Project/Part 2/Library.java
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@@ -3,7 +3,7 @@
* Class to create objects of a Library, which can hold and report on assets held within
*
* @George Wilkinson
* @2.0
* @3.0
*/
// Import all required libraries. Not using .* as it is not good practice due to potential conflicts.

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@@ -6,9 +6,10 @@ comment1.params=
comment1.target=java.lang.String\ getItemCode()
comment10.params=
comment10.target=void\ printDetails()
comment10.text=\n\ \ \ \ public\ void\ printDetails()\n\ \ \ \ {\n\ \ \ \ \ \ \ \ System.out.println(\ title\ +\ "\ with\ an\ item\ code\ "\ +\ itemCode\ +\ "\ has\ been\ borrowed\ "\ +\ timesBorrowed\ +\ "\ times.");\n\ \ \ \ \ \ \ \ if(\ onLoan\ )\n\ \ \ \ \ \ \ \ \ \ \ \ System.out.println(\ "This\ item\ is\ at\ present\ on\ loan\ and\ when\ new\ cost\ "\ +\ cost\ +\ "\ pence.\\n"\ );\n\ \ \ \ \ \ \ \ else\ \n\ \ \ \ \ \ \ \ \ \ \ \ System.out.println(\ "This\ item\ is\ at\ present\ not\ on\ loan\ and\ when\ new\ cost\ "\ +\ cost\ +\ "\ pence.\\n"\ );\n\ \ \ \ }\n
comment10.text=\n\ Print\ to\ terminal,\ relevant\ field\ variables\n
comment11.params=detailScanner
comment11.target=void\ readItemData(java.util.Scanner)
comment11.text=\n\ Passed\ a\ scanner\ object,\ assign\ values\ to\ relevant\ field\ variables\n
comment2.params=
comment2.target=int\ getCost()
comment3.params=

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Semester 2/Programming 2/Project/Part 2/LibraryItem.java
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@@ -3,7 +3,7 @@
* Superclass of items / assets stored in a Library.
*
* @George Wilkinson
* @2.1
* @3.1
*/
import java.util.Scanner;
@@ -12,7 +12,7 @@ import java.util.NoSuchElementException;
public abstract class LibraryItem
{
// instance variables - replace the example below with your own
// instance variables
private String title;
private String itemCode;
private int cost;
@@ -81,25 +81,15 @@ public abstract class LibraryItem
/*
* Field Mutator End
*/
// Output to console the details of the fields in a human-readable format.
/*
public void printDetails()
{
System.out.println( title + " with an item code " + itemCode + " has been borrowed " + timesBorrowed + " times.");
if( onLoan )
System.out.println( "This item is at present on loan and when new cost " + cost + " pence.\n" );
else
System.out.println( "This item is at present not on loan and when new cost " + cost + " pence.\n" );
}
/*
* Print to terminal, relevant field variables
*/
public void printDetails()
{
System.out.println("Item Code: " + itemCode +
"\nTitle: " + title +
"\nCost: £" + ( ( float )( cost ) )/100 +
"\nCost: £" + ( ( float )( cost ) )/100 + // Convert cost in pence to £pounds.pence to be more readable.
"\nBorrowed " + getTimesBorrowed() + " times." );
if( getOnLoan() )
System.out.println( "On Loan");
@@ -107,6 +97,9 @@ public abstract class LibraryItem
System.out.println( "Available to Loan");
}
/*
* Passed a scanner object, assign values to relevant field variables
*/
public void readItemData( Scanner detailScanner ) {
if ( detailScanner != null ) {

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@@ -4,14 +4,18 @@ comment0.target=LibraryUser()
comment0.text=\n\ Constructor\ for\ objects\ of\ class\ LibraryUser\n
comment1.params=
comment1.target=java.lang.String\ getUserID()
comment1.text=\n\ Accessor\ start\ -\ return\ values\ of\ corresponding\ variables.\n
comment10.params=title
comment10.target=void\ setTitle(java.lang.String)
comment11.params=writer
comment11.target=void\ writeData(java.io.PrintWriter)
comment11.text=\n\ Passed\ a\ PrintWriter,\ append\ field\ variables\ in\ a\ set\ format,\ and\ flush\ the\ buffer.\n
comment12.params=
comment12.target=void\ printDetails()
comment12.text=\n\ Print\ to\ terminal,\ all\ relevant\ fields.\n
comment13.params=detailScanner
comment13.target=void\ readData(java.util.Scanner)
comment13.text=\n\ Passed\ a\ scanner\ object,\ assign\ relevant\ values\ to\ field\ variables.\n
comment2.params=
comment2.target=java.lang.String\ getSurname()
comment3.params=
@@ -22,6 +26,7 @@ comment5.params=
comment5.target=java.lang.String\ getTitle()
comment6.params=userID
comment6.target=void\ setUserID(java.lang.String)
comment6.text=\n\ Accessor\ End\n\ Mutator\ Start\ -\ Set\ value\ of\ corresponding\ variables\n
comment7.params=surname
comment7.target=void\ setSurname(java.lang.String)
comment8.params=firstName

31
Semester 2/Programming 2/Project/Part 2/LibraryUser.java
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@@ -1,9 +1,9 @@
/**
* Write a description of class LibraryUser here.
* Class LibraryUser to create a user of a library.
*
* @author (your name)
* @version (a version number or a date)
* @George Wilkinson
* @1.0
*/
import java.util.Scanner;
@@ -11,7 +11,7 @@ import java.io.PrintWriter;
public class LibraryUser
{
// instance variables - replace the example below with your own
// instance variables
private String userID, surname, firstName, otherInitials, title;
/**
@@ -19,6 +19,9 @@ public class LibraryUser
*/
public LibraryUser(){}
/*
* Accessor start - return values of corresponding variables.
*/
public String getUserID(){
return userID;
}
@@ -39,8 +42,11 @@ public class LibraryUser
return title;
}
/*
* Accessor End
* Mutator Start - Set value of corresponding variables
*/
public void setUserID( String userID ){
this.userID = userID;
}
@@ -61,18 +67,31 @@ public class LibraryUser
this.title = title;
}
/*
* Mutator End
*/
/*
* Passed a PrintWriter, append field variables in a set format, and flush the buffer.
*/
public void writeData( PrintWriter writer )
{
writer.print(userID + ", " + surname + ", " + firstName + ", " + otherInitials + ", " + title + "\n");
writer.flush();
}
/*
* Print to terminal, all relevant fields.
*/
public void printDetails()
{
System.out.println( "ID: " + userID +
"\nName: " + title + " " + firstName + " " + otherInitials + " " + surname );
}
/*
* Passed a scanner object, assign relevant values to field variables.
*/
public void readData( Scanner detailScanner )
{
if ( detailScanner != null ) {

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@@ -1,8 +1,17 @@
#BlueJ class context
comment0.params=
comment0.target=Periodical()
comment0.text=\n\ Constructor\ for\ objects\ of\ class\ Periodical\n\ Since\ all\ field\ variables\ initialise\ as\ null,\ nothing\ should\ happen\ here.\n
comment1.params=
comment1.target=void\ printDetails()
comment2.params=detailScanner
comment2.target=void\ readItemData(java.util.Scanner)
numComments=3
comment1.target=java.lang.String\ getPublicationDate()
comment1.text=\n\ Return\ value\ of\ @publicationDate\n
comment2.params=publicationDate
comment2.target=void\ setPublicationDate(java.lang.String)
comment2.text=\n\ Set\ value\ of\ @publicationDate\n
comment3.params=
comment3.target=void\ printDetails()
comment3.text=\n\ Print\ to\ terminal,\ relevant\ field\ variable's\ values.\n
comment4.params=detailScanner
comment4.target=void\ readItemData(java.util.Scanner)
comment4.text=\n\ Passed\ a\ scanner,\ assign\ relevant\ values\ to\ field\ variables.\n
numComments=5

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Semester 2/Programming 2/Project/Part 2/Periodical.java
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@@ -3,7 +3,7 @@
* Subclass of LibraryItem to create a periodical ( e.g. newspaper, tabloid ).
*
* @George Wilkinson
* @1.0
* @2.3
*/
import java.util.Scanner;
@@ -12,13 +12,39 @@ public class Periodical extends PrintedItem
{
private String publicationDate;
/**
* Constructor for objects of class Periodical
* Since all field variables initialise as null, nothing should happen here.
*/
public Periodical(){}
/*
* Return value of @publicationDate
*/
public String getPublicationDate()
{
return publicationDate;
}
/*
* Set value of @publicationDate
*/
public void setPublicationDate( String publicationDate )
{
this.publicationDate = publicationDate;
}
/*
* Print to terminal, relevant field variable's values.
*/
public void printDetails() {
System.out.println( "Publication Date: " + publicationDate );
super.printDetails();
}
/*
* Passed a scanner, assign relevant values to field variables.
*/
public void readItemData( Scanner detailScanner ){
if ( detailScanner != null ) {
this.publicationDate = detailScanner.next().trim();

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Semester 2/Programming 2/Project/Part 2/PrintedItem.java
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@@ -1,14 +1,12 @@
/**
* Write a description of class PrintedItem here.
* Subclass of LibraryItem to create objects of printed items in a library.
*
* @author (your name)
* @version (a version number or a date)
* @George Wilkinson
* @1.0
*/
import java.util.Scanner;
import java.util.ArrayList;
import java.util.NoSuchElementException;
public abstract class PrintedItem extends LibraryItem
{