Lecture 27/28 - IO Streams Reader/ Deeper Look Into Filed

what are the I/O stream

• most input/output is based on I/O streams

• objects that support I/O commands

• System.out for standard output

• System.in for standard input

Two Categories o f data

• Machine formatted data

  • binary form, 0101000101001, each binary digit takes on bit

• Human readable text

  • represented by characters

• To view machine formatted data as human readable txt, an interpreter is required,

  • Microsoft word is an interpreter for word documents

Byte Streams

• for the I/O of machine formatted data

• class name ends with stream

• input stream and output stream should be used in pairs

Character Streams

• for the I/O of human readable text

• class names ends with Reader or Writer

• Reader and writer should be used in pairs

Example of typing into screen

• goes to keyboard

• System.in; Input Stream

• Goes to the Memory.

• System.out; Output Stream

Reading from text file example

• Memory, 1’s and 0’s

• output stream to text file

• text file; input stream to 1’s and 0’s

Main classes of character stream

• Reader and Writer

• All other character classes are subclasses of Reader and Writer

  • FileReader, BufferedReader, FileWriter, BufferedWriter

  • all support same ser of basic operations defined by Reader or Writer

Reader class, goal example

• read a value from a character stream into a variable in java

• translate a value in character form into machine formatted data

• read 3.1415926 from character stream (9 bytes) , translate and assign to a machine formatted float var (4 bytes)

Writer, goal example

• write a value from a variable in Java into a character stream

• translate a machine formatted data into a character stream

• translate machine formatted float variable, pi, into a character stream, and write into output device

Main Classes of Byte Stream

• InputStream and Output Stream

• all other Byte Stream are subclasses of InputStream and OutputStream

  • FileInputStream, BufferedInputStream, FileOutputStream BufferedOutputStream

• Doesn’t have any basic operations that involve translation

• always machine-formatted data

  • difficult for human to inspect what’s going on

  • efficient in time and space

How Should I decide witch to use

• Byte Streams are useful and more efficient for direct machine to machine communication; also efficient in storing data and reading data from files

• Character Streams; are useful when dealing with sequence of characters and when humans want to inspect the data

• as a rule of thumb; if long series of ones and zeroes you need to know what info it is meant to represent and how the info is encoded before you will be ablet o interpret it

Java Reader

• an abstract class for reading character streams

• subclasses must implement read and close()

• we use it’s subclasses

  • BufferedReader: wrapper for InputStreamReader, buffers bytes each time a native I/O is called; more efficent because it frequently accesses RAM instead of hard drive

  • InputStreamReader: reads bytes and decodes them into characters using a specified charset,

Note on the InputStreamReader class

• reads chaarcter from any kind of input stream

• one has to open a file as a byte stream before reading by InputStreamReader

Analogy of Stream, Reader and Writer with milk

FileWriter Class

• write character directly into files

• no need to open file as a byte stream first and then convert it as a character stream

OutputStreamWriter

• write characters into any kind of output stream such as FileInputStream,

• we should open files as one kind of character stream and then use OutputStreamWriter to write into it

Buffered I/O explained

• a buffer collects large amount of data, and does I/O in batches to reduce the number of times to invoke and interrupt a disk drive

• only write to hard drive when buffer is full

• flush(); flushes the output stream and forces any buffered output byte to be written out;

path name and directory

• location of file in file system

• name of directory; name of file;

• abs path; path starting from root

• relative; current folder

Issue with Path In different OS

• Path are formatted slightly different on different OS

• The Class File is an abstraction of the location of files

  • it hides different detailed formats of file path in diff OS

  • unifies diff file path and directory operation in diff OS as the same set of basic operations