Java IO Input and Output

1
Lecture

• Java I/O - Input and Output
• java.io.

2
7. Java I/O

• I/O in Java
• Stream Classes
• Streams vs Readers
• Files
• Read/Write Example
• Serialization Examples

3
Java I/O Style

• Java uses streams to handle byte and character
  I/O
• A stream is an ordered source of or destination
  for bytes.
• Java library includes pre-defined streams for
  dealing with common I/O operations
• cin or stdin - System.in
• cout or stdout - System.out
• cerr or stderr - System.err
• all streams information is contained in the
  java.io package
• Streams can be connected to one another or to
  files

• Input stream methods
• int read()
• int read(byte)
• int read(byte, int, int)
• close()
• int available()
• skip(long)
• boolean markSupported()
• mark(int)
• reset()
• Output stream methods
• write(int)
• write(byte)
• write(byte, int, int)
• close()
• flush()

4

• Stream Classes
• BufferedInputStream and BufferedOutputStream
• DataInputStream and DataOutputStream
• PipedInputStream and PipedOutputStream
• URL Input Streams

• Streams vs. Readers
• Streams are used for reading/writing ints/chars.
• Readers/Writers are used for 16-bit Unicode
  characters.
• Readers/Writers are in the java.io package.
• Most important stream class InputStreamReader
  and OutputStreamWriter.
• Local character encoding for English-speaking
  world is 8859_1
• User can specify non-local character encodings
  when constructing the Reader/Writer

5

• Java Files and I/O
• Files and directories are represented in Java by
  the File class.
• The File class contains information about the
• file/directory
• The File object can point to an absolute or
  relative directory.
• The file separator is different depending on your
  platform (Unix, Microsoft Windows, etc.)
• Java represents this by the static String
  separator in the java.io.File class

• File methods
• long lastModified()
• long length()
• boolean delete()
• boolean exists()
• boolean canRead()
• boolean canWrite()
• boolean isFile()
• boolean isDirectory()
• boolean isAbsolute()
• String getName()
• String getPath()
• String getAbsolutePath()
• String getParent()
• boolean renameTo(File)

6

• import java.util.
• import java.io.
• public class ReadWrite
• public static void main( String args )
• try
• FileOutputStream f new FileOutputStream("m
  yfile.txt")
• OutputStreamWriter osw new
  OutputStreamWriter(f)
• BufferedWriter bw new BufferedWriter(osw)
  
• bw.write("This is some text\n")
• bw.write("second line\n")
• bw.write("and third line\n")
• bw.close()
• osw.close()
• f.close()
• catch(FileNotFoundException ef)
• System.out.println("Could not open file")
• catch(IOException ei)
• ei.printStackTrace()

• Read / Write Example
• Note use of try-catch
• Example Output
• gt java ReadWrite
• Line 1This is some text
• Line 2second line
• Line 3and third line
• gt

7

• import java.util.
• import java.io.
• public class Serialize
• public static void main( String args )
• Wrapper w
• try
• FileInputStream f new FileInputStream("myf
  ile.ser")
• ObjectInputStream s new
  ObjectInputStream(f)
• w (Wrapper)s.readObject()
• System.out.println("Read Wrapper stamped "
  w.timestamp )
• s.close()
• f.close()
• catch(FileNotFoundException ef)
• System.out.println("Could not open file")
• catch(IOException ei)
• ei.printStackTrace()
• catch(ClassNotFoundException ec)
• System.out.println("Could not extract
  Wrapper from myfile.ser")

• Serialization Example
• persistent objects of a sort
• save to file or squirt down a network link
• Example Output
• gt rm myfile.ser
• gt java Serialize
• Could not open file
• New Wrapper stamped Tue Jan 12 075447
  GMT1030 1999
• gt java Serialize
• Read Wrapper stamped Tue Jan 12 075447
  GMT1030 1999
• New Wrapper stamped Tue Jan 12 075450
  GMT1030 1999
• gt java Serialize
• Read Wrapper stamped Tue Jan 12 075450
  GMT1030 1999
• New Wrapper stamped Tue Jan 12 075453
  GMT1030 1999
• gt

8
7. Exercises

• Create a Java application called Viewer that can
  display the contents of any text file. The user
  should specify the file on the command line,
  using either a relative path or absolute path.
• Create a Java application to copy a text file to
  a new file with a different name.
• Modify the driver program you wrote in a previous
  section so that it has the ability to serialize
  and save the People array.
• Add to your driver the ability to read in an
  existing database file and print out the
  contents.
• Add a text-based menu system to your driver
  program giving the user options to load/save
  database files, and to add/search database
  entries.

9
Summary

• Java I/O is done using Streams and with
  Readers and Writers
• System.in/out etc
• Serialization is built into the Java language
  see transient keyword

• Streams are interoperable and can be stacked
  on top of one another to achieve desired user
  functionality
• See java.io package