Last Time

1
Last Time

• Misc. useful classes in Java
• String
• StringTokenizer
• Math
• System

2
Stuff

• Assignment 1 is posted (finally!).

3
Today

• Wrapper classes
• JFileChooser
• Text File I/O
• The File class

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Wrapper Classes

• Sometimes it is necessary for a primitive type
  value to be an Object, rather than just a
  primitive type.
• Some data structures only store Objects.
• Some Java methods only work on Objects.
• Wrapper classes also contain some useful
  constants and a few handy methods.

5
Wrapper Classes - Cont.

• Each primitive type has an associated wrapper
  class
• Each wrapper class Object can hold the value that
  would normally be contained in the primitive type
  variable, but now has a number of useful static
  methods.

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Integer Wrapper Class - Example

• Integer number new Integer(46)//Wrapping
• Integer num new Integer(908)
• Integer.MAX_VALUE // gives maximum integer
• Integer.MIN_VALUE // gives minimum integer
• Integer.parseInt(453) // returns 453
• Integer.toString(653) // returns 653
• number.equals(num) // returns false
• int aNumber number.intValue() // aNumber is 46

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Aside - Why an equals Method for Objects?

• The String class also has equals and
  equalsIgnoreCase.
• These wrapper classes also have an equals method.
• Why not use the simple boolean comparators (,
  !, etc.) with Objects?
• These comparators just compare memory addresses.
• How are you going to sort Objects?

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Aside - Why an equals Method for Objects?, Cont.

• can only compare memory addresses when Objects
  are compared.
• Most Data Container Objects will have both an
  equals method and a compareTo method.
• The equals method tests for equality using
  whatever you define as equal.
• The compareTo method returns a postive or
  negative int value (or zero to indicate equal),
  again depending on how you define one Object to
  be greater or less than another.

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Wrapper Classes Cont.

• The Double wrapper class has equivalent methods
• Double.MAX_VALUE // gives maximum double value
• Double.MIN_VALUE // gives minimum double value
• Double.parseDouble(0.45E-3) // returns 0.45E-3
• parseDouble is only available in Java 2 and newer
  versions.
• See the Java documentation for more on Wrapper
  classes.

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Character Wrapper Class

• Many useful methods to work on characters
• character is a char
• getNumericValue(character)
• isDigit(character)
• isLetter(character)
• isLowerCase(character)
• isUpperCase(character)
• toLowerCase(character)
• toUpperCase(character)

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Built - In GUI Windows

• We will learn to build our own GUI windows, but
  you should be aware of the GUI Windows already
  built into Java
• JOptionPane
• JColorChooser
• JFileChooser
• These are all built to perform common tasks and
  are very easy to use.
• Imported from the javax.swing package.
• See BuiltInDemo.java.

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JFileChooser

• A built in file browser/selector dialog box.
• The demo only used the chooser in the most simple
  way.
• For example, you can specify a starting folder
  and add as many file extension filters as you
  like.
• The chooser returns a File object, from which you
  can obtain much information about the file.

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JFileChooser Window
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JFileChooser Example Code

• (At the top
• import java.swing.JFileChooser)
• JFileChooser chooser new JFileChooser()
• int result chooser.showOpenDialog(null)
• if (result JFileChooser.APPROVE_OPTION)
• System.out.println(chooser.getSelectedFile())

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JFileChooser, Cont.

• Upon completion of the dialog, the
  getSelectedFile() method returns a File object.
• This object can easily be used with file I/O
  code

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Simple Alternative

• Prompt the user for a filename as a String, using
  the console window.

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File I/O

• Files provide a convenient way to store and
  re-store to memory larger amounts of data.
• We will use arrays to store the data in memory,
  and well talk about these things later.
• Three kinds of file I/O to discuss
• Text
• Binary
• Random access
• For now, well stick with text I/O.

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Text File Output in Java 5.0

• Use the PrintWriter class. (As usual), you must
  import the class
• import java.io.PrintWriter
• In your program
• PrintWriter fileOut new PrintWriter(outFilename)
  
• (outFilename is a String filename we obtained
  somewhere else)

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Text File Output in Java 5.0, Cont.

• Unfortunately the instantiation of the
  PrintWriter object can cause a FileNotFoundExcepti
  on to be thrown and you must be ready to catch
  it
• try
• writeFile new PrintWriter(outputFile)
• catch (FileNotFoundException e)
• System.out.println(e.getMessage())
• System.exit(0)
• // end try catch

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Aside - File Paths in Strings

• Sometimes you might have to include a path in the
  filename, such as C\Alan\CISC212\Demo.txt
• Dont forget that if you have to include a \ in
  a String, use \\, as in
• C\\Alan\\CISC212\\Demo.txt

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Text File Output in Java 5.0, Cont.

• The PrintWriter constructor can also accept a
  File object (such as provided from JFileChooser!)

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Text File Output in Java 5.0, Cont.

• The Object fileOut, owns a couple of familiar
  methods print() and println().
• When you are done writing, dont forget to close
  the file with
• fileOut.close()
• Way easy!!

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Text File Input in Java 5.0

• Use the FileReader and Scanner classes. Our
  usual import statements
• import java.util.Scanner
• import java.io.FileReader
• import java.io.FileNotFoundException
• Well get to that last one in a minute.

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Text File Input in Java 5.0, Cont.

• In my program
• fileIn new FileReader("Test.txt")
• Scanner fileInput new Scanner(fileIn)
• Scanner class constructor can also accept a File
  object directly.
• Unfortunately the FileReader constructor (whats
  a constructor anyways?) throws a kind of
  exception that I cannot ignore - so the code
  above cannot be used exactly in this way.

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Text File Input in Java 5.0, Cont.

• This works
• FileReader fileIn null
• try
• fileIn new FileReader("Test.txt")
• catch (FileNotFoundException e)
• // Do something clever here!
• Scanner fileInput new Scanner(fileIn)

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Without using FileReader

• You can also send a File object to the Scanner
  class when you instantiate it instead of a
  FileReader object.
• You will still need to do this in a try catch
  block as shown in the previous slide.
• See the demo program TextFileReaderDemo.java

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Text File Input in Java 5.0, Cont.

• We are going to have to talk about try/catch
  blocks soon! But for now, lets get back to file
  input.
• To get the file contents, and print them to the
  console, for example
• while (fileInput.hasNextLine())
• System.out.println(fileInput.nextLine())

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Aside - Scanner Class Tokenizer

• The Scanner class has a built in String
  Tokenizer.
• Set the delimiters using the useDelimiter(delimite
  r_String) method.
• Obtain the tokens by calling the next() method.
• The hasNext() method will return false when there
  are no more tokens.

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The File Class

• File is a class in the java.io. package.
• It contains useful utility methods that will help
  prevent programs crashing from file errors.
• For example
• File myFile new File(test.dat)
• myFile.exists() // returns true if file exists
• myFile.canRead() // returns true if can read
  from file
• myFile.canWrite() // returns true if can write
  to file

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The File Class, Cont.

• myFile.delete() // deletes file and returns true
  if successful
• myFile.length() // returns length of file in
  bytes
• myFile.getName() // returns the name of file
  (like test.dat)
• myFile.getPath() // returns path of file (like
  C\AlanStuff\JavaSource)

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Binary and Random Access

• Binary files contain data exactly as it is stored
  in memory you cant read these files in
  Notepad!
• Text file is sequential access only.
• What does that mean?
• Random access can access any byte in the file at
  any time, in any order.
• More about Binary and Random File Access later!