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Files and Streams
Java I/O
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File I/O
I/O streams provide data input/output solutions
to the programs.
A stream can represent many different kinds of
sources and destinations, such as
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disk files,
devices,
other programs,
and memory arrays.
Input Stream
different kinds of data, including
simple bytes, primitive data types,
localized characters, and objects.
Source: java.sun.com
Output Stream
Source:java.sun.com
File I/O
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File I/O provides a simple model for reading and
writing data from/to files.
Streams work with a large variety of data
sources and destinations, including disk files.
Streams don't support all the operations that
are common with disk files.
File Objects
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File class can be used to write platformindependent code to examines and manipulates
files.
File instances represent file names, not files.
The File object corresponding to the file name
might not even exist.
Creating a File object
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A File object contains the file name string used to
construct it.
e.g.
File f = new File(“sample.txt");
File testFile = new File(“c:\\test\\data.txt”);
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The file name string never changes throughout the
lifetime of the object.
But a file name may have different corresponding File
objects.
Example
Manipulating Files
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If a File object names an actual file, a program can use
it to perform a number of useful operations on the file.
These include passing the object to the constructor for
a stream to open the file for reading or writing.
The delete method deletes the file immediately, while
the deleteOnExit method deletes the file when the
virtual machine terminates.
Manipulating Files
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The setLastModified sets the modification
date/time for the file.
The renameTo() method renames the file.
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Note that the file name string behind the File object
remains unchanged, so the File object will not refer
to the renamed file.
Working with Directories
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You can also use File class to work with
directories.
The mkdir method creates a directory.
The mkdirs method does the same thing, after
first creating any parent directories that don't
yet exist.
The list and listFiles methods list the contents
of a directory.
Example
Byte Streams
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Programs use byte streams to perform input and
output of 8-bit bytes.
All byte stream classes are descended from
InputStream and OutputStream.
Two important Streams
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FileInputStream
FileOutputStream.
Using Byte Streams
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You can use FileInputStream and
FileOutputStream as the following
in = new FileInputStream(“input.txt");
out = new FileOutputStream("output.txt");
Example
Important note
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Always close streams!
This practice helps avoid serious resource leaks.
Byte streams should only be used for the most
primitive I/O.
Character Streams
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Characters in Java environment use Unicode
conventions.
Character stream I/O automatically translates
this internal format to and from the local
character set.
Example
• What is the difference between this
and previous example?
Buffered Streams
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To reduce the overhead of multiple request of I/O
access and improve the efficiency of programs , Java
platform provides buffered I/O streams.
Buffered input streams read data from a memory area
known as a buffer;
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the native input API is called only when the buffer is empty.
Similarly, buffered output streams write data to a buffer, and
the native output API is called only when the buffer is full.
Example
Class BufferedInputStream
Random Accessing Files
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he java.io.RandomAccessFile class implements
both the DataInput and DataOutput interfaces
that can be used for both reading and writing
files.
Example
PrintWriter Class
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Provides better methods to write formatted
data to a text-output stream.
But, it does not contain methods for writing raw
bytes, for which a program should use
unencoded byte streams.
Formatting
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In PrintWriter class two levels of formatting are
provided:
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print and println format individual values in a
standard way.
format formats almost any number of values based
on a format string, with many options for precise
formatting.
The format Method
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The format method formats multiple arguments
based on a format string.
The format string consists of static text
embedded with format specifiers; except for
the format specifiers, the format string is
output unchanged.
Example
public class Root2 {
public static void main(String[] args) {
int i = 2;
double r = Math.sqrt(i);
System.out.format("The square root of
%d is %f.%n", i, r);
}
}
Conversions
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d formats an integer value as a decimal value.
f formats a floating point value as a decimal
value.
n outputs a platform-specific line terminator.
x formats an integer as a hexadecimal value.
s formats any value as a string.
tB formats an integer as a locale-specific month
name.
...
Conversions
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Except for %% and %n, all format specifiers must
match an argument. If they don't, an exception
is thrown.
In the Java programming language, the \n
escape always generates the linefeed character
(\u000A). Don't use \n unless you specifically
want a linefeed character. To get the correct
line separator for the local platform, use %n.
Summary
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The java.io package contains many classes that your programs can
use to read and write data.
Most of the classes implement sequential access streams.
The sequential access streams can be divided into two groups:
those that read and write bytes and those that read and write
Unicode characters.
Each sequential access stream has a speciality, such as reading
from or writing to a file, filtering data as its read or written, or
serializing an object.
One class, RandomAccessFile, implements random input/output
access to a file. An object of this type maintains a file pointer,
which indicates the current location from which data will be read
or to which data will be written.
Questions
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What class would you use to read a few pieces
of data that are at known positions near the
end of a large file?
In a format call, what's the best way to indicate
a new line?
How would you append data to the end of a
file?
Source: java.sun.com