Download ch12 Full Java IO

Simple Java I/O
Part I
General Principles
4-May-17
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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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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Types of Streams
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There are 2 kinds of streams
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byte streams
character streams
Character Streams
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Character streams create text files.
These are files designed to be read with a text editor.
Java automatically converts its internal unicode
characters to the local machine representation (ASCII in
our case).
Byte Streams
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Byte streams create binary files.
A binary file essentially contains the memory
image of the data. That is, it stores bits as they are
in memory.
Binary files are faster to read and write because no
translation need take place.
Binary files, however, cannot be read with a text
editor.
Classes
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Java has 6 classes to support stream I/O
File: An object of this class is either a file or a
directory.
OutputStream: base class for byte output streams
InputStream: base class for byte input streams
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Writer: base class for character output streams.
Reader: base class for character input streams.
RandomAccessFile: provides support for random
access to a file.
Note that the classes InputStream,
OutputStream, Reader, and Writer are abstract
classes.
Three stream objects are automatically created for
every application: System.in, System.out, and
System.err.
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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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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Output to files
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PrintStream: An object in the java.io package
that lets you print output to a file.
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System.out is also a PrintStream.
Any methods you have used on System.out
(such as print, println) will work on every
PrintStream.
Printing into an output file, general syntax:
PrintStream <name> =
new PrintStream(new File("<file name>"));
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If the given file does not exist, it is created.
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If the given file already exists, it is overwritten.
Printing to files, example
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Example:
PrintStream output = new PrintStream(new
File("output.txt"));
output.println("Hello, file!");
output.println("This is a second line of
output.");
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You can use similar ideas about prompting for file names
here.
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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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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Simple Java I/O
Part IV
Scanner and printf
4-May-17
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
4-May-17
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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Simple Java I/O
Part III
JFileChoosers
4-May-17
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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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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