Simple Java I/O
Part I
General Principles
26-Jul-16
Prologue
“They say you can hold seven plus or minus two pieces of
information in your mind. I can’t remember how to
open files in Java. I’ve written chapters on it. I’ve done
it a bunch of times, but it’s too many steps. And when I
actually analyze it, I realize these are just silly design
decisions that they made. Even if they insisted on using
the Decorator pattern in java.io, they should have had
a convenience constructor for opening files simply.
Because we open files all the time, but nobody can
remember how. It is too much information to hold in
your mind.”
—Bruce Eckel, http://www.artima.com/intv/aboutme2.html
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Streams
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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:
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A file has a definite length, and therefore an end
Keyboard input has no specific end
3
How to do I/O
import java.io.*;
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Open the stream
Use the stream (read, write, or both)
Close the stream
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Why Java I/O is hard
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open
use
close
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
5
Opening a stream
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open
use
close
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
6
Example of opening a stream

open
use
close
A FileReader is a used to connect to a file that will be
used for input:
FileReader fileReader =
new FileReader(fileName);
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The fileName specifies where the (external) file is to be
found
You never use fileName again; instead, you use
fileReader
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Using a stream

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open
use
close
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
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Example of using a stream
open
use
close
int ch;
ch = fileReader.read( );
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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)
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Manipulating the input data
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open
use
close
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);
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Reading lines
open
use
close
String s;
s = bufferedReader.readLine( );
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A BufferedReader will return null if there is
nothing more to read
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Closing
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open
use
close
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!
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Simple Java I/O
Part II
LineReader and LineWriter
26-Jul-16
Text files
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Text (.txt) files are the simplest kind of files
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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 of formatted text files
Compilers, in general, work only with text
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My LineReader class
class LineReader {
BufferedReader bufferedReader;
LineReader(String fileName) {...}
String readLine( ) {...}
void close( ) {...}
}
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Basics of the LineReader constructor
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Create a FileReader for the named file:
FileReader fileReader =
new FileReader(fileName);
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Use it as input to a BufferedReader:
BufferedReader bufferedReader =
new BufferedReader(fileReader);
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Use the BufferedReader; but first, we need to
catch possible Exceptions
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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);
}
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readLine
String readLine( ) {
try {
return bufferedReader.readLine( );
}
catch(IOException e) {
e.printStackTrace( );
}
return null;
}
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close
void close() {
try {
bufferedReader.close( );
}
catch(IOException e) { }
}
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How did I figure that out?
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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
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The LineWriter class
class LineWriter {
PrintWriter printWriter;
LineWriter(String fileName) {...}
void writeLine(String line) {...}
void close( ) {...}
}
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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);
}
}
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Flushing the buffer
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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
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PrintWriter
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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)
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writeLine
void writeLine(String line) {
printWriter.println(line);
}
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close
void close( ) {
printWriter.flush( );
try { printWriter.close( ); }
catch(Exception e) { }
}
26
Simple Java I/O
Part III
FileDialogs
26-Jul-16
About FileDialogs
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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
28
Typical FileDialog window
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FileDialog constructors
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FileDialog(Frame f)
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FileDialog(Frame f, String title)
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Creates a FileDialog attached to Frame f
Creates a FileDialog attached to Frame f, with the given title
FileDialog(Frame f, String title, int type)
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Creates a FileDialog attached to Frame f, with the given title;
the type can be either FileDialog.LOAD or FileDialog.SAVE
30
Useful FileDialog methods I
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String getDirectory()
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String getFile()
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Returns the selected directory
Returns the name of the currently selected file, or null if no
file is selected
int getMode()
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Returns either FileDialog.LOAD or FileDialog.SAVE,
depending on what the dialog is being used for
31
Useful FileDialog methods II
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void setDirectory(String directory)
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void setFile(String fileName)
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Changes the current directory to directory
Changes the current file to fileName
void setMode(int mode)
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Sets the mode to either FileDialog.LOAD or FileDialog.SAVE
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Using a FileDialog
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Using a FileDialog isn’t difficult, but it is lengthy
See my LineReader and LineWriter classes for
complete examples
33
Simple Java I/O
Part IV
Serialization
26-Jul-16
Serialization
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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
35
Conditions for serializability
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If an object is to be serialized:
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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
36
Implementing Serializable
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To “implement” an interface means to define all the
methods declared by that interface, but...
The Serializable interface does not define any
methods!
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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
37
Writing objects to a file
ObjectOutputStream objectOut =
new ObjectOutputStream(
new BufferedOutputStream(
new FileOutputStream(fileName)));
objectOut.writeObject(serializableObject);
objectOut.close( );
38
Reading objects from a file
ObjectInputStream objectIn =
new ObjectInputStream(
new BufferedInputStream(
new FileInputStream(fileName)));
myObject = (itsType)objectIn.readObject( );
objectIn.close( );
39
What have I left out?
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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 ( );
}
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The End
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