Java I/O

Java/J2EE Programming Training
Simple Java I/O

Classification: Restricted
Agenda
• Streams
• Using a stream
• Manipulating the input data
• Basics of the LineReader constructor
• The LineWriter class
• Flushing the buffer
• PrintWriter
• About FileDialogs
• Typical FileDialog window
• FileDialog constructors
• Useful FileDialog methods I
• Useful FileDialog methods II
• Serialization
• Conditions for serializability
• Writing objects to a file

Streams
• All modern I/O is stream-based
• A stream is a connection to a source of data or to a destination for data
(sometimes both)
• An input stream may be associated with the keyboard
• An input stream or an output stream may be associated with a file
• Different streams have different characteristics:
• A file has a definite length, and therefore an end
• Keyboard input has no specific end

How to do I/O
import java.io.*;
• Open the stream
• Use the stream (read, write, or both)
• Close the stream

Why Java I/O is hard
• Java I/O is very powerful, with an overwhelming number of options
• Any given kind of I/O is not particularly difficult
• The trick is to find your way through the maze of possibilities
open
use
close

Opening a stream
• There is data external to your program that you want to get, or you want to
put data somewhere outside your program
• When you open a stream, you are making a connection to that external
place
• Once the connection is made, you forget about the external place and just
use the stream
open
use
close

Example of opening a stream
• A FileReader is a used to connect to a file that will be used for input:
FileReader fileReader =
new FileReader(fileName);
• The fileName specifies where the (external) file is to be found
• You never use fileName again; instead, you use fileReader

Using a stream
• Some streams can be used only for input, others only for output, still others
for both
• Using a stream means doing input from it or output to it
• But it’s not usually that simple--you need to manipulate the data in some
way as it comes in or goes out

Example of using a stream
int ch;
ch = fileReader.read( );
• The fileReader.read() method reads one character and returns it as an
integer, or -1 if there are no more characters to read
• The meaning of the integer depends on the file encoding (ASCII, Unicode,
other)

Manipulating the input data
• Reading characters as integers isn’t usually what you want to do
• A BufferedReader will convert integers to characters; it can also read whole
lines
• The constructor for BufferedReader takes a FileReader parameter:
BufferedReader bufferedReader =
new BufferedReader(fileReader);

Reading lines
String s;
s = bufferedReader.readLine( );
• A BufferedReader will return null if there is nothing more to read

Closing
• A stream is an expensive resource
• There is a limit on the number of streams that you can have open at one
time
• You should not have more than one stream open on the same file
• You must close a stream before you can open it again
• Always close your streams!

Text files
• Text (.txt) files are the simplest kind of files
• text files can be used by many different programs
• Formatted text files (such as .doc files) also contain binary formatting
information
• Only programs that “know the secret code” can make sense formatted text
files
• Compilers, in general, work only with text

My LineReader class
class LineReader {
BufferedReader bufferedReader;
LineReader(String fileName) {...}
String readLine( ) {...}
void close( ) {...}
}

Basics of the LineReader constructor
• Create a FileReader for the named file:
FileReader fileReader =
new FileReader(fileName);
• Use it as input to a BufferedReader:
BufferedReader bufferedReader =
new BufferedReader(fileReader);
• Use the BufferedReader; but first, we need to catch possible Exceptions

The full LineReader constructor
LineReader(String fileName) {
FileReader fileReader = null;
try { fileReader = new FileReader(fileName); }
catch (FileNotFoundException e) {
System.err.println
("LineReader can't find input file: " + fileName);
e.printStackTrace( );
}
bufferedReader = new BufferedReader(fileReader);
}

readLine
String readLine( ) {
try {
return bufferedReader.readLine( );
}
catch(IOException e) {
e.printStackTrace( );
}
return null;
}

close
void close() {
try {
bufferedReader.close( );
}
catch(IOException e) { }
}

How did I figure that out?
• I wanted to read lines from a file
• I found a readLine method in the BufferedReader class
• The constructor for BufferedReader takes a Reader as an argument
• An InputStreamReader is a kind of Reader
• A FileReader is a kind of InputStreamReader

The LineWriter class
class LineWriter {
PrintWriter printWriter;
LineWriter(String fileName) {...}
void writeLine(String line) {...}
void close( ) {...}
}

The constructor for LineWriter
LineWriter(String fileName) {
try {
printWriter =
new PrintWriter(
new FileOutputStream(fileName), true);
}
catch(Exception e) {
System.err.println("LineWriter can't " +
"use output file: " + fileName);
}
}

Flushing the buffer
• When you put information into a buffered output stream, it goes into a
buffer
• The buffer may not be written out right away
• If your program crashes, you may not know how far it got before it crashed
• Flushing the buffer is forcing the information to be written out

PrintWriter
• Buffers are automatically flushed when the program ends normally
• Usually it is your responsibility to flush buffers if the program does not end
normally
• PrintWriter can do the flushing for you
public PrintWriter(OutputStream out,
boolean autoFlush)

writeLine
void writeLine(String line) {
printWriter.println(line);
}

close
void close( ) {
printWriter.flush( );
try { printWriter.close( ); }
catch(Exception e) { }
}

About FileDialogs
• The FileDialog class displays a window from which the user can select a file
• The FileDialog window is modal--the application cannot continue until it is
closed
• Only applications, not applets, can use a FileDialog; only applications can
access files
• Every FileDialog window is associated with a Frame

Typical FileDialog window

FileDialog constructors
• FileDialog(Frame f)
• Creates a FileDialog attached to Frame f
• FileDialog(Frame f, String title)
• Creates a FileDialog attached to Frame f, with the given title
• FileDialog(Frame f, String title, int type)
• Creates a FileDialog attached to Frame f, with the given title; the type
can be either FileDialog.LOAD or FileDialog.SAVE

Useful FileDialog methods I
• String getDirectory()
• Returns the selected directory
• String getFile()
• Returns the name of the currently selected file, or null if no file is
selected
• int getMode()
• Returns either FileDialog.LOAD or FileDialog.SAVE, depending on what
the dialog is being used for

Useful FileDialog methods II
• void setDirectory(String directory)
• Changes the current directory to directory
• void setFile(String fileName)
• Changes the current file to fileName
• void setMode(int mode)
• Sets the mode to either FileDialog.LOAD or FileDialog.SAVE

Using a FileDialog
• Using a FileDialog isn’t difficult, but it is lengthy
• See my LineReader class (in the Encryption assignment) for a complete
example

Serialization
• You can also read and write objects to files
• Object I/O goes by the awkward name of serialization
• Serialization in other languages can be very difficult, because objects may
contain references to other objects
• Java makes serialization (almost) easy

Conditions for serializability
• If an object is to be serialized:
• The class must be declared as public
• The class must implement Serializable
• The class must have a no-argument constructor
• All fields of the class must be serializable: either primitive types or
serializable objects

Implementing the Serializable interface
•To “implement” an interface means to
define all the methods declared by that
interface, but...
•The Serializable interface does not define
any methods!
•Question: What possible use is there for an
interface that does not declare any methods?
•Answer: Serializable is used as flag to tell Java it
needs to do extra work with this class

Writing objects to a file
ObjectOutputStream objectOut =
new ObjectOutputStream(
new BufferedOutputStream(
new FileOutputStream(fileName)));
objectOut.writeObject(serializableObject);
objectOut.close( );

Reading objects from a file
ObjectInputStream objectIn =
new ObjectInputStream(
new BufferedInputStream(
new FileInputStream(fileName)));
myObject = (itsType)objectIn.readObject( );
objectIn.close( );

What have I left out?
• Encrypted files, compressed files, files sent over internet connections, ...
• Exceptions! All I/O involves Exceptions!
• try { statements involving I/O }
catch (IOException e) {
e.printStackTrace ( );
}

Thank You

Java I/O

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