Computer Systems and Elements of Programming Java programming: Lecture 5

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Computer Systems
and Elements of Programming Java programming
Lecture 5

• Dr Niki Trigoni
• Department of Computer Science
• and Information Systems
• Birkbeck College, University of London
• Email niki_at_dcs.bbk.ac.uk
• Web Page http//www.dcs.bbk.ac.uk/niki

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Review of lecture 4

• Inheritance allows us to define one class as an
  extension of another
• A subclass is a class that extends (inherits
  from) another class. It inherits all fields and
  methods from its superclass.
• Classes linked through inheritance relationships
  form an inheritance hierarchy.
• The constructor of a subclass must always invoke
  the constructor of its superclass as its first
  statement. If omitted, Java will attempt to
  insert a constructor call automatically.
• All classes with no explicit superclass have
  Object as their superclass.
• Subclass objects may be used wherever objects of
  a superclass are expected.

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Overview of lecture 5

• Putting together arrays and inheritance
• Dynamic method lookup
• The main method and its parameters
• Reading and writing files
• String handling

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Putting together arrays and inheritance

• Say that we would like to define a class Database
  with a field that contains an array of
  Multimedia_Item objects
• Remember that

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Putting together arrays and inheritance
public class Database // Fields private
MultiMedia_Item items private int
noOfItems // Constructor public
Database() noOfItems 0
items new MultiMedia_Item 100
What is the type of field items?
Explain where and how the field items is
initialized.
Explain where and how the field items is
initialized.
The array items will store objects of which class?
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Putting together arrays and inheritance
public class Database // Fields //
Constructor // Methods public void addItem
(MultiMedia_Item newItem) if (noOfItems
lt items.length) itemsnoOfItems
newItem noOfItems
Why did we set the type of the input parameter
to be MultiMedia_Item ?
Why do we need the if statement?
What does the method do if the array items is
full?
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Putting together arrays and inheritance
public class Database // Fields //
Constructor // Methods public void
printItems () int i for (i0
iltnoOfItems i) itemsi.print()

Why do we need a for statement to print the
field items?

• Notice in the for loop the
• Init stmt
• Continue condition
• Incrementor stmt

Can you replace the for stmt with a while stmt?
Which method do we assume that the
class MultiMedia_Item has?
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Review of method print

• Remember how we defined the following method in
  class MultiMedia_Item
• class MultiMedia_Item
• // Fields and constructor omitted
• public void print()
• System.out.print(Title title)
• if (gotIt)
• System.out.println()
• else
• System.out.println()
• System.out.println(Duration playingTime)
• System.out.println(Comment comment)

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Review of method overriding

• Remember how we overrode the method print in the
  subclass CD?
• class CD extends MultiMedia_Item
• // Fields and constructor omitted
• public void print()
• super.print()
• System.out.print (Artist artist)
• System.out.println (NoOfTracks
  numberOfTracks)

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Review of method overriding

• Remember how we overrode the method print in the
  subclass Video?
• class Video extends MultiMedia_Item
• // Fields and constructor omitted
• public void print()
• super.print()
• System.out.print (Director director)

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Dynamic method lookup
public class DatabaseManager public static
void main (String argv) Database db
new Database () CD item1 new CD
(Intensive Care, Robbie Williams, 12, 45)
db.addItem (item1) Video item2 new
Video (Broken Flowers, 90, Jim Jarmusch)
db.addItem (item2) db.printItems()

• Within printItems, we call the method print() on
  each MultiMedia_Item object in the
• items array. Which implementation of method
  print() will be invoked on
• the first object of the array?
• the second object of the array?

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Dynamic method lookup

• The print() method is invoked on the i-th object
  as follows
• The object db.itemsi is accessed.
• The most specific class of the object is found
• The class is CD for the (first) object
  db.items0
• The class is Video for the (second) object
  db.items1
• The implementation of the method print() of the
  class (determined in the previous step) is
  selected
• The method print() of class CD for the first item
• The method print() of class Video for the second
  item
• What would happen if the method print() was not
  defined in class Video?
• The method print() of the immediate superclass
  (Item) would be invoked.

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More complex scenario

• Suppose that we want to write a program that
• Creates a new Database object
• Reads an input text file with details about a
  number of cds and videos, and adds corresponding
  CD and Video objects to the field items of the
  Database object
• Prints the contents of items to an output text
  file.
• The program should have two input parameters
• The name of the input text file
• The name of the output text file

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The main method and its parameters
public class DatabaseManager public static
void main (String argv) if
(argv.length ! 2)
System.out.println (Error This program has two
arguments) System.out.println (
- inputFileName - outputFileName)
return String inputFileName
argv0 String outputFileName
argv1
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The main method (continued)
public class DatabaseManager public static
void main (String argv) // continuing
method main from previous page //
. Database db new
Database() db.readItemsFromFile
(inputFileName) db.writeItemsToFile
(outputFileName) System.out.println(Job
done!) // It remains to define two
methods of class Database // - public void
readItemsFromFile (String inputFile) // -
public void writeItemsToFile (String outputFile)

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Reading and writing files

• To read and write files, we use objects of class
• BufferedReader. For example, BufferReader has a
  method (readLine) that allows us to read a line
  of a given file.
• FileWriter. For example, FileWriter has a method
  (write) that allows us to write a string to a
  given output file.
• Opening, reading, writing and closing files may
  cause errors (for example if we try to read a
  non-existing file, or we try to write to a file
  that we do not have access to).
• For example, the method readLine of BufferReader
  throws an exception (an object of class
  Exception) if we try to read beyond the end of
  file.
• Wherever we use methods that can possibly throw
  exceptions, we must catch potential exceptions,
  i.e. write code how to react to these errors.

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Reading text from input file
try catch
(Exception e) System.out.println(Exc
eption e.toString() )
import java.io. public class Database public
void readItemsFromFile (String inputFile)
FileReader fileReader new
FileReader ( inputFile ) BufferedReader
reader new BufferedReader (fileReader) Strin
g line reader.readLine() while (line !
null) addTextLineItem (line) line
reader.readLine() reader.close()

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Writing text to output file
import java.io. public class Database public
void writeItemsToFile (String outputFile)
int i try FileWriter
writer new FileWriter ( outputFile ) for
(i0 iltnoOfItems i) writer.write(
itemsi.toString() \r\n ) writer.close()
catch (Exception e)
System.out.println(Exception e.toString() )

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Input file format

• CD - ltcd_titlegt - ltcd_artistgt -
  ltcd_no_of_tracksgt
• Video - ltvideo_titlegt - ltvideo_directorgt
• Example of a file with two lines
• CD-Intensive Care-Robbie Williams-12
• Video-Broken Hearts-Jim Jarmusch

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String handling
public class Database public void
addTextLineItem (String line) if
(line null) return String wordArray
line.split(-) // check the first word
to see if it is CD or Video if
(wordArray0.equals(CD)) CD cd
new CD() cd.setTitle( wordArray1) cd.set
Artist (wordArray2) Integer
noOfTracksObject new Integer (wordArray3)
cd.setNoOfTracks (noOfTracksObject.intValue())
items.addItem (cd) else
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String handling
public class Database public void
addTextLineItem (String line) //
continued from previous page // else
if (wordArray0.equals (Video))
Video video new Video() video.setTitle
(wordArray1) video.setDirector
(wordArray2) items.addItem (video)
else System.out.println (First word
of a line must be CD or Video)
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String handling
public class Item public String toString()
String str new String(Item-
getTitle()) return str
public class CD extends Item public
String toString() String str new
String(CD- getTitle()-) str.concat
(getArtist()) return str
// write similar method (toString()) for class
Video
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Summary of lecture 5

• If an array is defined to store objects of a
  class C, we can store in it objects of any class
  that inherits from C.
• Java decides dynamically how to implement a
  method invocation on an object. The
  implementation selected depends on the specific
  class that the object is instance of.
• It is possible to pass user values to a Java
  application through command-line arguments of the
  main method
• The classes BufferedReader, FileReader and
  FileWriter can be used to read and write files.
• The class String includes many methods that allow
  us to handle String objects like equals, concat,
  etc.

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Learning outcomes of lecture 5

• Be able to handle polymorphic arrays, i.e. arrays
  that include objects of different subclasses of a
  given class
• Be able to explain the meaning of dynamic method
  lookup and demonstrate it with the use of an
  example
• Be able to read and write data from files
• Be able to pass user values to a Java application
  (command-line arguments of method main)
• Be able to handle (compare, concatenate, etc.)
  String objects using the methods of class String.