Creating Objects: Constructors

1
Creating Objects Constructors

• Constructor class method(s) used to create an
  object
• Every class has one or more
• Name of a constructor method is always the same
  as the name of the class
• Constructors may or may not have arguments
• Use the keyword new to create an object
• Creation of a new object is an operation
• Keywords (such as new) or symbols that perform
  operations are called operators

2
Constructing a String object

• The new operation returns a reference to a new
  String object
• Need a reference variable to receive the result
  of the operation
• String name
• name new String(Horace)
• or
• String name new String(Horace)

Reference variable
constructor
operator
argument
3
Reading Interactive Data

• We can use javax.swing package and JOptionPane
  class
• JOptionPane class has a method call
  showInputDialog that takes a string (as prompt)
  and allows the user to enter a string
• Eg s1 JOptionPane.showInputDialog("Enter an
  integer")
• S1 is a string that can now be used.

4
Reading Interactive Data - an example

• import javax.swing.JOptionPane
• public class Example1
• public static void main(String
  args)
• String s1, s2
• int n1, n2, max
• // prompt for integers
• s1 JOptionPane.showInputDia
  log("Enter an integer")
• s2 JOptionPane.showInputDia
  log("Enter another integer")
• // convert these strings to
  integers
• n1 Integer.parseInt(s1)
• n2 Integer.parseInt(s2)
• // find the maximum
• max Math.max(n1,n2)
• // print the message and the
  max value
• JOptionPane.showMessageDialog
  (null, "The biggest is " max)

5
Disk Files

• Advantages
• Persistence data in a file lasts longer than
  data on the monitor
• Capacity more information can be stored on a
  disk than displayed at one time on a monitor
• Attributes
• Contents (data)
• A file name
• Operations
• Writing to a file (creating and storing data for
  the first time)
• Deleting a file
• Renaming a file
• Overwriting a file
• Obtaining contents of a file

6
Modeling disk files

• Java provides a predefined class to model disk
  files, called File
• Constructor for the file class
• Accepts the files name (a String reference) as
  the argument
• Example
• File f1 new File(myData.txt)
• IMPORTANT
• We have created a reference to a file object
• This does not create a file on the disk
• If this file is not on the disk, we cannot do
  anything with this object

7
If the file is on the disk

• The File object provides two methods that model
  some of the operations we listed earlier
• delete()
• File f new File(junk.txt)
• f.delete()
• renameTo()
• File f1, f2
• f1 new File(junk.txt)
• f2 new File(garbage.txt)
• f1.renameTo(f2)
• Download FileStuff.zip demo

8
Writing output to disk

• To create a new file, or overwrite an existing
  file, we need a pathway, or stream
• Java has a predefined class to model this stream,
  FileOutputStream
• The constructor for FileOutputStream uses a
  reference to a File object as its argument
• File f new File(stuff.txt)
• FileOutputStream fstream new
  fileOutputStream(f)
• Opens the file so that it can receive data
• Creates a new file if the file does not exist
• However, it does not provide any convenient
  methods for output

9
Using PrintStream methods for disk files

• We want a PrintStream object to be associated
  with a disk file and not a monitor
• The PrintStream constructor takes a reference to
  a FileOutputStream as its argument
• The PrintStream object is associated with the
  FileOutPutStream
• Using println() and print() methods on this
  PrintStream object means that output will go to
  the disk file
• File f new File(stuff.txt)
• FileOutputStream fstream new
  FileOutputStream(f)
• PrintStream target new PrintStream(fstream)
• target.println(This is a new disk file.)

10
Catching exceptions (first pass)

• In Karel we had runtime errors that would halt
  program flow
• Try to pick a beeper when none were there
• To to move through a wall that was in the way
• A similar type of problem may be trying to access
  a file that is unavailable
• We will add the line throws Exception to the
  heading of the main() method of our program class
• public static void main(String args) throws
  Exception

11
Throwing exceptions and methods

• Any method that can throw an exception must
  acknowledge it in the header with a throws clause
• A throws clause consists of the keyword throws
  and a list of all Exceptions thrown in the method
• Example
• public static void main(String args) throws
  Exception

12
Throwing Exceptions

• To handle the unexpected, Java uses the throw
  statement
• throw reference
• Reference is an object of a subclass of the class
  Exception
• See the API and the Java Interlude on page ??? of
  the text
• The throw statement usually takes the following
  form
• throw new Exception-class(String-argument)
• This is called throwing an exception

13
Handling the unexpected

• By default the throws statement launches a chain
  of method terminations from the method that
  executes the throw to the main() method,
  terminating the program itself
• Often the program can respond to the unexpected
  and recover, by catching the Exception
• Breaking the cascade of method throwing along the
  invocation chain

14
try catch()

• To catch an Exception, the statements containing
  invocations of Exception-throwing methods are
  surrounded by braces and preceded with the
  keyword try
• try
• someObject.someMethod()
• Following is the keyword catch and a declaration
  of an reference variable that will refer to the
  thrown Exception object in parentheses
• catch (Exception e) statements
• Finally comes a group of statements surrounded by
  braces that will be executed when the Exception
  reaches this method

15
Implementing a trycatch in file output

• We could also implement the trycatch mechanism
  in the following way
• File f new File("stuff.txt")
• try
• FileOutputStream fstream new
  FileOutputStream(f)
• PrintStream target new PrintStream(fstream)
  
• target.println("This is a new disk file.")
• catch (FileNotFoundException e)
• System.out.println("File not found.")
• We will not worry about the details of this
  implementation at this time

16
Creating or Overwriting a File Summary

1. Create a File object to represent the file
2. Create a FileOutputStream object to represent the
   pathway to the file (using the File reference as
   an argument)
3. Create a PrintStream object to provide a
   convenient output pathway to the file (using the
   FileOutputStream reference as an argument)
4. Use print() or println() methods of PrintStream
   as needed
5. Be sure to watch for exceptions appropriately

17
Input from keyboard, file, or network connection

• Sequence of steps similar to output
• Construct an object that models some sort of
  input stream
• FileInputStream
• BufferedInputStream
• Use the input stream object to construct an
  InputStreamReader object
• Models stream of input as a stream of characters
• Does not recognize boundaries between Strings in
  the input
• Use the InputStreamReader object to construct a
  BufferedReader object
• Provides method readLine() reads an entire line,
  creates a String object, and returns a reference
  to that object

18
Input the keyboard

• Java provides a predefined BufferedInputStream
  object to represent a stream of input that comes
  from the keyboard, System.in
• Unlike System.out, System.in cannot be used right
  away (different classes)
• System.in must be used to construct an
  InputStreamReader object
• Which is used as an argument to construct a
  BufferedReader object
• Which can then be sent readLine() messages to
  read lines as Strings

19
Getting a line from the keyboard

• InputStreamReader isr
• BufferedReader keyBoard
• String inputLine
• isr new InputStreamReader(System.in)
• keyBoard new BufferedReader(isr)
• inputLine keyBoard.readLine()
• System.out.print(inputLine)
• System.out.println("s")

20
Interactive Input/Output

• A program that expects input from a keyboard,
  must provide that information to the user as it
  runs
• A general form for interactive input and output
• System.out.print(prompt goes here)
• string reference variable keyboard.readLine()
• possibly compute something
• System.out.println(output string goes here)

21
Input Disk Files

• Only slightly more involved that obtaining input
  from a keyboard
• Instead of BufferedInputStream object
  (System.in), we need a FileInputStream object
• Construct a FileInputStream object the same as
  constructing a FileOutputStream object
• File f new File(myData.txt)
• FileInputStream fs new FileInputStream(f)
• Now we can use the FileInputStream object as we
  used System.in in getting data from the keyboard

22
Getting a line from a disk file

• File f new File(myData.txt)
• FileInputStream fs new FileInputStream(f)
• InputStreamReader isr
• BufferedReader input
• isr new InputStreamReader(fs)
• input new BufferedReader(isr)
• String inputLine
• inputLine input.readLine()
• System.out.println(inputLine)

23
Network Computing

• Network concepts
• Computer network group of computers that can
  directly exchange information with each other
• Internet a computer network of networks. Allows
  computers on one network to exchange information
  with another computer on a different network
• THE Internet
• Each computer has its own unique Internet
  address
• Internet addresses on the Net are Strings
• Information on the Net is organized into units
  called network resources (usually a file of some
  type)
• Each resource is uniquely identified by its URL
  (Universal Resource Locator)
• Form of URL protocol//internet address/file name

24
Network Input

• Reading input from the World Wide Web is as
  simple as reading data from a disk file
• Obtain an InputStream that models a stream of
  input from a Web site
• Class URL provided by Java to model URLs
• The constructor for the URL class takes a String
  argument
• URL u new URL(http//www.whitehouse.gov)
• It also provides a method, openStream, that takes
  no arguments, but returns an InputStream
• InputStream ins u.openStream()

25
openStream()

• Does a lot of work behind the scenes
• Sets up communication software on your machine
• Contacts the remote machine
• Waits for response
• Sets up connection
• Constructs an InputStream object to model the
  connection
• Returns a reference to this object
• We then construct a BufferedReader as we did for
  disk files and keyboards
• Download WHWWW.zip demo