Vectors

1
Vectors

• An Array has some limitationsFixed size,
  Homogeneous element types(This is not the case
  with an array of Objects)
• A Vector is another data structure that has some
  advantages and some useful methods.

2
Vectors (2)

• A Vector only stores Objects.(Which can
  represent any other object type)
• A vector has dynamic size allocation. It expands
  its capacity as required.
• Dynamically adjusts when elements are inserted
  or removed.

3
Vector methods

• ConstructorVector v new Vector()Vector v
  new Vector(int size)Vector v new Vector(int
  size, int incr)
• Adding elementsv.addElement(Object x) // Adds
  element at endv.insertElementAt(Object x,int
  index) //adds at index shuffles other
  elements backv.setElementAt(Object o, int indx)
  // overwrites existing element
  Retrieving Datav.elementAt(in
  t index) // gets element at index
• Erasing datav.remove(int index) // Removes
  element at index // Lower
  elements shuffle upv.clear() //
  Clears all elements

4
Vectors use polymorphism

• Vectors only store (Capital O) Objectsthe
  ancestor class of all Java classes.
• Since a superclass can represent any class below
  it in the inheritance hierarchy, an Object can
  represent an object of any Javaclass. Storing
  Objects. Vector v new Vector
  v.addElement(Test) v.addElement(new
  Date()) v.addElement(new Coordinate(2.3,4.5)

5
Objects in Vectors

• The objects are automatically cast to Objects.
  (Implicit casting).
• A Vector only returns Objects. Before objects can
  be used they must be cast back to their original
  type. (explicit cast). String s1
  (String)v.Element(0)?String s1
  (String)v.Element(1)?But an explicit cast must
  match previous implicit cast, else RunTime error!

6
Reading from text files

• This section is a short introduction to file
  handling. More later.
• File I/O is handled by data stream objects
• Reading from a text file ( a file of ASCII
  characters) uses a FileReader objectFileReader
  fr new FileReader(Filename)

7
Text files(2)

• A BufferedReader object provides methods to read
  the data from a FileReader object.BufferedReader
  br new BufferedReader(fr)
• Use the readLine() method to read each line of
  the file as a String.String str
  br.readLine()Each line can be broken into
  smaller strings using the StringTokenizer.

8
Text Files (3)

• Consider a file data.txt that contains the
  following lines. John Smith LMIF Sue
  Jones LBCF James Wilson LMIF Mary
  Collins LMIFTo open and read the data from the
  fileFileReader fr new FileReader(data.txt)
  BufferedReader br new BufferedReader(fr)String
  s br.readLine()while(s!null)
  System.out.println(s) s br.readLine()

9
Text Files (4)

• Data Streams must have exception handling (Error
  checking) in place. Requires Try and Catch
  blocks.try FileReader fr new
  FileReader(data.txt) BufferedReader br new
  BufferedReader(fr) String str br.readLine()
  while(str!null) System.out.println(str)
  strbr.readLine() br.close()catc
  h(IOException e) System.out.println(I/O
  error)

10
Writing to a text file

• Use FileWriter object to open the file.
  FileWriter fw new FileWriter(filename)
• The PrintWriter object provides methods to write
  Strings to the file. PrintWriter pw new
  PrintWriter(fw)
• Write to file using the println() method.
  pw.println(This goes to the file)
• Uses the same format as System.out.println()

11
Writing to a file (2)

• try
• FileWriter fw new FileWriter(out.txt)Print
  Writer pw new PrintWriter(fw)pw.println(This
  is the first line)pw.println(This is the next
  line)pw.println(This is the last line)
• pw.close()
• catch (IOException e)
• The data is not written to the file until the
  PrintWriter object is closed or is flushed.

12
Object Serialization

• Designed as a way of storing Graphics objects so
  components hidden by a window can be restored
  when the window closes.Called Serialization
  because it allows us to pick up where we left off
  after some interruption is finished. Like a TV
  serial.
• Serializable objects can be stored and retrieved
  from a file as single entities.An array is an
  object. Object Serialization allows us to store
  the array and all its contents in one step.

13
Object Serialization (2)

• Consider an array of Student objects. The
  Students contain contain Course objects as data
  members and the Course objects contain other
  objects as their data members.

Course 1
Course 2
Student 1
Course 3
To file
Student 2
Array
Course 8
Student 3
Course 3
Course 5
Course 2
Course 7
Serialization lets us handle the array as a
single object.All its internal structure is
carried with it without needingspecial treatment.
14
Object Serialization (3)

• The classes that define the objects we wish to
  save using OS must implement the Serializable
  interface. This is a marker interface that
  enables the process.
• OS works through ObjectDataStream
  objects.ObjectInputStream and ObjectOutPutStream.
  These connect to the file through DataStream
  objects DataInputStream and DataOutputStream.
• Object Serialization only stores and retrieves
  Objects, like a Vector, so Polymorphism, implicit
  and explicit casting are involved.

15
Object Serialization (4)

• When loading an object using OS then the class of
  the Object and the classes of all other objects
  that may be contained within the Object must be
  known on the system to which the the Object is
  loaded.
• This must be checked by including
  aClassNotFoundException catchin addition to the
  IOException catch.

16
Object Serialization Example

• This example stores an array of Coordinate
  objects.The Coordinate class must implement
  Serializable.
• import java.io.
• public class Coordinate implements
  Serializable double x, y public
  Coordinate(double x, double y)
• this.xx this.yy public String
  toString() return Coord x x ,
  y

17
OS Example (2). Saving Objects

• import java.io.
• public class SaveCoord
• Coordinate ca new Coordinate10public
  static void main (String args)
• SaveCoord sc new SaveCoord() sc.ca0new
  Coordinate(2.3,4.5)
• sc.ca1new Coordinate(3.1,6.5)
• sc.ca2new Coordinate(4.3,1.5)
• try FileOutputStream d new
  FileOutputStream(d.dat)ObjectOutputStream o
  new ObjectOutputStream(d)o.writeObject(sc.ca)
• o.close()
• catch (IOException x)

18
OS Example (3). Loading Objects

• import java.io.
• public class LoadCoord
• Coordinate ca
• public static void main(String args)
• LoadCoord lc new LoadCoord()try
  FileInputStream d new FileInputStream(d.dat)
  ObjectInputStream in new ObjectInputStream(d)
  lc.ca (Coordinate ) in.readObject()// Cast
• in.close() catch (IOException e)
• catch (ClassNotFoundException cnf)
• //All classes used by the object must be
  knownSystem.out.println(lc.ca0)
• System.out.println(lc.ca1)System.out.println
  (lc.ca2)