Transcript Simple Java I/O

Simple Java I/O
Part I
General Principles
11-Apr-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, author of Thinking In Java
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
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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
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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
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Example of opening a stream
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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 charAsInt;
charAsInt = 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)
You can cast from int to char:
char ch = (char)fileReader.read( );
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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!
Java will normally close your streams for you when
your program ends, but it isn’t good style to depend
on this
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Simple Java I/O
Part II
LineReader and LineWriter
11-Apr-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 thought there might be a suitable readSomething method, so
I went to the API Index
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Note: Capital letters are all alphabetized before lowercase in the Index
I found a readLine method in several classes; the most
promising was the BufferedReader class
The constructor for BufferedReader takes a Reader as an
argument
Reader is an abstract class, but it has several implementations,
including InputStreamReader
FileReader is a subclass of InputStreamReader
There is a constructor for FileReader that takes as its
argument a (String) file name
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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 or 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 forces 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) { }
}
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Simple Java I/O
Part III
JFileChoosers
11-Apr-16
About JFileChoosers
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The JFileChooser class displays a window from
which the user can select a file
The dialog window is modal--the application cannot
continue until it is closed
Applets cannot use a JFileChooser, because applets
cannot access files
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Typical JFileChooser window
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JFileChooser constructors
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JFileChooser()
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JFileChooser(File currentDirectory)
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Creates a JFileChooser starting from the user’s directory
Constructs a JFileChooser using the given File as the path
JFileChooser(String currentDirectoryPath)
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Constructs a JFileChooser using the given path
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Useful JFileChooser methods I
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int showOpenDialog(Component enclosingJFrame);
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int showSaveDialog(Component enclosingJFrame);
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Asks for a file to read; returns a flag (see below)
Asks where to save a file; returns a flag (see below)
Returned flag value may be:
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JFileChooser.APPROVE_OPTION
JFileChooser.CANCEL_OPTION
JFileChooser.ERROR_OPTION
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Useful JFileChooser methods II
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File getSelectedFile()
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showOpenDialog and showSaveDialog return a flag telling
what happened, but don’t return the selected file
After we return from one of these methods, we have to ask
the JFileChooser what file was selected
If we are saving a file, the File may not actually exist yet—
that’s OK, we still have a File object we can use
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Using a File
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Assuming that we have successfully selected a File:
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File file = chooser.getSelectedFile();
if (file != null) {
String fileName = file.getCanonicalPath();
FileReader fileReader = new FileReader(fileName);
BufferedReader reader = new BufferedReader(fileReader);
}
File file = chooser.getSelectedFile();
if (file != null) {
String fileName = file.getCanonicalPath();
FileOutputStream stream = new FileOutputStream(fileName);
writer = new PrintWriter(stream, true);
}
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Simple Java I/O
Part IV
Scanner and printf
11-Apr-16
java.util.Scanner
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Java finally has a fairly simple way to read input
First, you must create a Scanner object
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To read from the keyboard (System.in), do:
 Scanner scanner = new Scanner(System.in);
To read from a file, do:
 File myFile = new File("myFileName.txt");
Scanner scanner = new Scanner(myFile);
 You have to be prepared to handle a FileNotFound
exception
You can even “read” from a String:
 Scanner scanner = new Scanner(myString);
 This can be handy for parsing a string
Using the Scanner
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First, you should make sure there is something to scan
 scanner.hasNext()  boolean
 You wouldn’t use this when reading from the keyboard
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You can read a line at a time
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Or, you can read one “token” at a time
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A token is any sequence of nonwhitespace characters
scanner.next ()  String
You must be prepared to deal with exceptions
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scanner.nextLine()  String
Eclipse will tell you what you need to do
nextLine and next return Strings, which you can convert to
numbers or other types if you like
There are also methods to check for and return primitives directly
Scanning for primitives
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You can read in and convert
text to primitives:
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boolean b = sc.nextBoolean();
byte by = sc.nextByte();
short sh = sc.nextShort();
int i
= sc.nextInt();
long l
= sc.nextLong();
float f
= sc.nextFloat();
double d = sc.nextDouble();
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And test if you have
something to read:
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hasNextBoolean()
hasNextByte()
hasNextShort()
hasNextInt()
hasNextLong()
hasNextFloat()
hasNextDouble()
Formatted output
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Java 5 has a printf method, similar to that of C
Each format code is % width code
Some format codes are s for strings, d for integers, f for
floating point numbers
Example:
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double pi = Math.PI;
System.out.printf("%8s %-8s %6d %-6d %8f %-8.2f :) \n",
"abc", "def", 123, 456, pi, pi);
System.out.printf("%8s %-8s %6d %-6d",
"abcdef", "ghijkl", 12345, 6789);
Output:
abc def
abcdef ghijkl
123 456
12345 6789
3.141593 3.14
:)
Simple Java I/O
Part V
Serialization
11-Apr-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
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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
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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
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Writing objects to a file
ObjectOutputStream objectOut =
new ObjectOutputStream(
new BufferedOutputStream(
new FileOutputStream(fileName)));
objectOut.writeObject(serializableObject);
objectOut.close( );
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Reading objects from a file
ObjectInputStream objectIn =
new ObjectInputStream(
new BufferedInputStream(
new FileInputStream(fileName)));
myObject = (itsType)objectIn.readObject( );
objectIn.close( );
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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
“There is no reason anyone would want a
computer in their home.”
--Ken Olson,
President/founder of Digital Equipment Corp.,
1977
“I think there is a world market for maybe five
computers.”
--Thomas Watson
Chairman of IBM,|
1943
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