Java IO

JAVA IO
Presented By: Utsab Neupane

Communication between the computer and the
outside world

1. An essential part of most programs
2. Computers spend a lot of time performing nothing
but I/O operations
3. Very complicated business to deal with
4. Ranks No.1 in the performance killer list

1. Console I/O
2. Keyboard I/O
3. File I/O
4. Network I/O
5. …and possibly more

1. The Java I/O API
a. The java.io package
b. Since JDK 1.0
c. Extensible

➢ an abstraction that either produces or consumes
information.
➢ linked to a physical device by the Java I/O system.
➢ All streams behave in the same manner, even if the
actual physical devices to which they are linked differ.
➢ Clean way to deal with i/o without having every part of
your code understand the difference between a
keyboard and a network.

1. Stream provides a sort of abstraction.
2. Recall System.out and System.in
3. System.in object of InputStream
4. System.out and System.err is object of
PrintStream class.
(Will be described later)

Reading Information into some Program

Writing information from a program.

1. Byte Stream
2. Character Stream

Byte Stream
perform input and output of 8-bit bytes

Defined by using two Class
Hierarchy
Two abstract Classes
1. InputStream
2. OutputStream

1. Always Close Streams
a. helps to avoid serious resource leaks
2. When not to use Byte Stream
a. represents a kind of low-level I/O that you should avoid
b. usually with the keyboard as the source and the monitor as the
destination which contains character data
c. Byte streams should only be used for the most primitive I/O.

So,Why talk about byte
streams?

Because all other
stream types are built
on byte streams.

Character Stream
perform input and output of character
(Wrapper class for Byte Stream)

1. Provides convenient means for handling input
and output of character.
2. Use Unicode
3. In some cases more efficient than byte stream

1. For most applications, I/O with character streams is no more
complicated than I/O with byte streams.
2. Input and output done with stream classes automatically
translates to and from the local character set.
3. A program that uses character streams in place of byte
streams automatically adapts to the local character set and
is ready for internationalization — all without extra effort by
the programmer.

Defined by using two Class
Hierarchy
Two abstract Classes
1. Reader
2. Writer

1. CopyCharacters is very similar to CopyBytes.
2. most important difference is that
CopyCharacters uses FileReader and
FileWriter for input and output in place of
FileInputStream and FileOutputStream.
3. Notice that both CopyBytes and
CopyCharacters use an int variable to read to
and write from.

However,
1. In CopyCharacters, the int variable holds a
character value in its last 16 bits.
2. In CopyBytes, the int variable holds a byte
value in its last 8 bits.

1. Most of the examples we've seen so far use
unbuffered I/O.
2. Each read or write request is handled directly by the
underlying OS.
3. Make a program much less efficient, since each
such request often triggers disk access, network
activity, or some other operation that is relatively
expensive

1. To reduce this kind of overhead, the Java platform
implements buffered I/O streams.
2. Buffered input streams read data from a memory
area known as a buffer; the native input API is called
only when the buffer is empty.
3. Similarly, buffered output streams write data to a
buffer, and the native output API is called only when
the buffer is full.

A program can convert
an unbuffered stream
into a buffered stream
using the wrapping
idiom

Simply wrap up stream classes with Buffered Stream Class.
inputStream = new BufferedReader(new FileReader("oldFile.txt"));
outputStream = new BufferedWriter(new FileWriter("newFile.txt"));

Four Buffered Stream Classes to wrap
unbuffered streams:
1. BufferedInputStream
2. BufferedOutputStream
3. BufferedReader
4. BufferedWriter

1. File class is provided by java.io package.
2. An abstract representation of file and
directory pathnames.
3. Note File object is not for reading / writing
files.
4. Used for obtaining information associated
with file like permission, time and date, path.

1. Directory too is treated as file and have
additional method list() to list the filename in
directory.
public File(String pathname)
public File(String parent, String child)
public File(File parent, String child)
public File(URI uri)

1. To Get Paths
a. getAbsolutePath(),getPath(), getParent(), getCanonicalPath()
2. To Check Files
a. isFile(), isDirectory(), exists()
3. To Get File Properties
a. getName(), length(), isAbsolute(), lastModified(), isHidden()
4. To Get File Permissions
a. canRead(), can Write(), canExecute()
5. To Know Storage information
a. getFreeSpace(), getUsableSpace(), getTotalSpace()

6. Utility Functions
➢ Boolean createNewFile() //created new File
➢ Boolean renameTo(File nf); // renames the file and returns
true if success
➢ Boolean delete(); //deletes the file represented by path of
file (also delete directory if its empty)
➢ Boolean setLastModified(long ms) //sets timestamp(Jan 1,
1970 UTC as a start time)
➢ Boolean setReadOnly() //to mark file as readable (also
can be done writable, and executable.)

A mechanism where an object can be
represented as a sequence of bytes.
Includes the object's data as well as information
about the object's type and the types of data
stored in the object.

With deserialization the type information and
bytes that represent the object and its data can
be used to recreate the object in memory.
the entire process is JVM independent.
(means an object can be serialized on one
platform and deserialized on an entirely different
platform.)

1. ObjectInputStream
2. ObjectOutputStream

The value of SSN value is 0.
What happened? Anything missed?

The value of the SSN field was 11122333 when the
object was serialized, but because the field is
transient.
So,this value was not sent to the output stream.

1.Performance
2. Hard to keep track of changed object
(For more information : http://programmers.stackexchange.
com/questions/191269/java-serialization-advantages-and-
disadvantages-use-or-avoid)

1. Gives you a brief overview of data
compression
2. Describes the java.util.zip package
3. Shows how to use this package to compress
and decompress data
4. Shows how to compress and decompress
serialized objects to save disk space
5. Shows how to compress and decompress
data on the fly to improve the performance of
client/server applications

1. used to get input from console
2. provides methods to read text and password

1. contains key and value pair both as a string
2. used to get property value based on the
property key
3. subclass of Hashtable
4. provides methods to get data from properties
file and store data into properties file
5. Moreover, can be used to get properties of
system

1. The problem with the Properties class is that
it makes assumptions about where user
preferences should be stored
2. The basic assumption is that the file system
would be used to store this information but
this raises several issues

Where in the file system should this
data be stored?

In the application directory, perhaps?

What if the user did not have write
access to that directory?

What if the user was running on a
network computer?

Preferences
Introduced in Java 1.4

1. allow for the easy storage of user preferences
without the developer having to worry about
where this information will be stored
2. API determines the appropriate place
3. For example, when running an application in
Windows, the preferences will be stored in
the Windows registry.

Preferences prefs = Preferences.userRoot();
Preferences prefs = Preferences.systemRoot();

1. http://docs.oracle.
com/javase/7/docs/api/java/io/package-summary.html
2. https://docs.oracle.com/javase/tutorial/essential/io/
3. https://docs.oracle.
com/javase/tutorial/jndi/objects/serial.html
4. http://docs.oracle.
com/javase/7/docs/api/java/io/Console.html
5. http://www.oracle.
com/technetwork/articles/java/compress-1565076.html

Questions
1. What class and method would you use to read a few pieces of data that are at known
positions near the end of a large file?
2. When invoking format, what is the best way to indicate a new line?
3. How would you determine the MIME type of a file?
4. What method(s) would you use to determine whether a file is a symbolic link?
Exercises
1. Write an example that counts the number of times a particular character, such as e,
appears in a file. The character can be specified at the command line.
2. A file which has some int value among with other string value . Write a program that gets
the int piece of data. What is the int data?

1. Program 1 The first programming exercise is to write a simple program which
reads byte data from a file from an InputStream and display it in console and
writes each byte to a file using OutputStream.
2. Program 2 Copy your first program to another file. Open it in an editor and
change the class name to correspond to the new file name. Modify the
program to read input one line at a time using a BufferedReader and print the
line in reverse (you could use a StringBuilder to reverse it).
3. Program 3 Write a program which reads input one line at a time using a
Scanner and prints it out converted to upper case.

Java IO

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