1. java.io Package
Sujit Kumar
Zenolocity LLC
Copyright 2012-2024 @

2. java.io Package
• The java.io package contains a fairly large number
of classes that deal with Java input & output.
Most of the classes consist of:
• Byte streams : that are subclasses of the abstract
classes InputStream or OutputStream
• Character streams : that are subclasses of the
abstract classes Reader and Writer
• The methods for reading & writing data from
byte streams and character streams are similar.

3. java.io Package (contd…)
• The Reader and Writer classes read and write
16-bit Unicode characters.
• The InputStream and OutputStream classes
read and write 8-bit bytes respectively.
• Derived classes of the above 4 abstract classes
add additional responsibilities using the
Decorator pattern.

4. How to choose the correct
implementation (derived class)?
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What is your format: text or binary ?
Do you want random access to a file ?
Are you using objects or non-objects ?
What are your sources and sinks of data (like
sockets, files, strings or array of characters) ?
• Do you need to use filtering techniques like
buffering, checksumming ?

5. Byte Stream Abstract Classes
InputStream — byte reader
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read() a single byte or an array of bytes
skip() bytes
mark() and reset() position
close() the stream (always in finally block)
OutputStream —byte writer
• write() a single byte or an array of bytes
• flush() the stream (in case it is buffered)
• close() the stream (always in finally block)

6. Character Stream Abstract Classes
Reader — character reader
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read() a single char or into a char array
skip() chars
mark() and reset() position
close() the stream (always in finally block)
Writer — character writer
• write() a single char or from a char array
• flush() the stream (in case it is buffered)
• close() the stream (always in finally block)

7. Byte Stream Concrete classes
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FileInputStream, FileOutputStream
BufferedInputStream, BufferedOutputStream
ByteArrayInputStream, ByteArrayOutputStream
DataInputStream, DataOutputStream
FilterInputStream, FilterOutputStream
ObjectInputStream, ObjectOutputStream
PipedInputStream, PipedOutputStream
LineNumberInputStream
• PrintStream

8. Character Stream Concrete classes
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FileReader, FileWriter
BufferedReader, BufferedWriter
CharArrayReader, CharArrayWriter
InputStreamReader, InputStreamWriter
FilterReader, FilterWriter
ObjectReader, ObjectWriter
PipedReader, PipedWriter
StringReader, StringWriter
• LineNumberReader
• PrintWriter

9. Applying the Decorator Pattern
• Example: Read each line from a file
• BufferedReader => InputStreamReader =>
FileInputStream
• Wrap an InputStreamReader over a
FileInputStream.
• Then, wrap a BufferedReader over the
InputStreamReader.

10. Applying the Decorator Pattern
(contd…)
• BufferedReader => Reads text from a characterinput stream, buffering characters so as to
provide for the efficient reading of characters,
arrays, and lines.
• FileInputStream => obtains input bytes from a file
in a file system.
• InputStreamReader => bridge from byte streams
to character streams. It reads bytes and decodes
them into characters using a specified charset.

11. Exception Handling
• The constructors and methods of I/O classes can
throw a checked exception.
• The base class for I/O exceptions is
java.io.IOException. Some methods can throw a
subclass of IOException.
For example, the constructors of FileInputStream, FileOutputStream and
FileReader can throw FileNotFoundException.
• The call to close() should always occur in a finally
block (ensures that the I/O channel is closed
properly even if an exception occurs).