CMPT 270 Files and I/O Streams PDF

CMPT 270 Developing Object Oriented Systems Files and I/O Streams Jason T Bowey University of Saskatchewan Jason T Bowey CMPT 270 1/19  Learning Objectives ˆ Understand Java's Input and Output Steam objects ˆ Become familiar with Java Charac...

CMPT 270 Developing Object Oriented Systems Files and I/O Streams Jason T Bowey University of Saskatchewan Jason T Bowey CMPT 270 1/19  Learning Objectives ˆ Understand Java's Input and Output Steam objects ˆ Become familiar with Java Character and Byte stream classes for handling text and binary les ˆ Employ Java applications that interact with les and streams: ˆ Encrypting and decrypting data ˆ Reading a PNG le Jason T Bowey CMPT 270 2/19  Streams ˆ A Stream is an object that enables the ow of data between a program and some I/O device or le ˆ If the data ows into a program, then the stream is called an input stream ˆ If the data ows out of a program, then the stream is called an output stream ˆ Java stream objects are broadly categorized into two groups: ˆ Java Byte Streams - used to perform input and output for 8-bit bytes ˆ Java Character Streams - perform input and output for 16-bit unicode characters Jason T Bowey CMPT 270 3/19  Streams ˆ Input streams can ow from the keyboard or from a le ˆ System.in is an input stream that connects to the keyboard Scanner keyboard = new Scanner(System.in); ˆ Output streams can ow to a screen or to a le ˆ System.out is an output stream that connects to the screen System.out.println("Output stream"); Jason T Bowey CMPT 270 4/19  File ˆ Files are very commonly used to store infomation/data ˆ There are two ways to store data in a le: ˆ Text Format - data represented as a sequence of characters (Character streams) ˆ Binary Format - data represented as a sequence of bytes (Byte stream) Jason T Bowey CMPT 270 5/19  Text Files ˆ Designed to be read by human beings ˆ Computers use binary bit patterns to represent both numbers and characters ˆ Each character or symbol is represented with a unique number called a code point (e.g., the letter 'A' is mapped to 65) ˆ Dierent encoding techniques are adopted to cover a wide range of characters in various languages Jason T Bowey CMPT 270 6/19  Binary Files ˆ Consists of a sequence of binary digits and are designed to be read by programs ˆ An advantage of binary les is that they are more ecient to process than text les ˆ Unlike most binary les, Java binary les are platform independent Jason T Bowey CMPT 270 7/19 Data Representation in les Jason T Bowey CMPT 270 8/19 Java Classes for Handling Text Files Jason T Bowey CMPT 270 9/19 Java Classes for Handling Binary Files Jason T Bowey CMPT 270 10/19  Why Two Sets of Classes? ˆ FileInputStream and FileOutputStream used Java Byte Streams ˆ FileReader and FileWriter classes use Java Character Streams ˆ Though internally, FileReader uses FileInputStream and FileWriter uses FileOutputStream Jason T Bowey CMPT 270 11/19  Reading from a Text File Using Scanner ˆ The Scanner class can be used for reading from the keyboard as well as from a text le ˆ Simple replace System.in in the Scanner's constructor with a suitable steam connected to the le   1 Scanner stream = new Scanner ( 2 new FileInputStream ( filename ));   ˆ Methods of the Scanner class for reading input behave the same whether reading from the keyboard or reading from a text le ˆ e.g., the nextInt method Jason T Bowey CMPT 270 12/19  Text File Input 1. Create an object of the File class   1 File inputFile = new File ( " input. txt " );   2. Create an object of the Scanner class   1 Scanner in = new Scanner ( inputFile );   3. Use the regular Scanner methods   1 while ( in. hasNextLine ()) 2 { 3 String line = in. nextLine (); 4 // process line of text 5 }   Jason T Bowey CMPT 270 13/19  Writing to a Text File The class PrintWriter is a stream class that can be used to write to a text le ˆ An object of the class PrintWriter has the methods print, printf, and println ˆ These are similar to the System.out methods of the same names, but used for text le output instead of screen output ˆ Converts characters into a number following an encoding technique and then translate the number into bits Jason T Bowey CMPT 270 14/19  Text File Output 1. Create an object of the PrintWriter class   1 PrintWriter out = new PrintWriter (" output. txt " )   ˆ If output.txt exists, it will be emptied ˆ If output.txt doesn't exist, it will create an empty le 2. Use PrintWriter methods   1 out. println (" Hello , world !" );   Jason T Bowey CMPT 270 15/19  Closing Files ˆ You must use the close method before le reading and writing is complete - your text may not be saved to the le until you use the close method! ˆ Closing a Scanner  1 while ( in. hasNextLine ()) 2 { 3 String line = in. nextLine (); 4 } 5 in. close ();   ˆ Closing a PrintWriter  1 out. println (" Hello , world !" ); 2 out. close ();   Jason T Bowey CMPT 270 16/19  Closing Files To read binary data from a disk le, you create a FileInputStream object   1 InputStream inputStream = new 2 FileInputStream (" input. bin " );   Use FileOutputStream to write data to disk le in a binary form   1 OutputStream outputStream = new 2 FileOutputStream ( " output. bin " );   Jason T Bowey CMPT 270 17/19  Exercise 1: Reading and Writing to a Text File ˆ Write a program that reads the contents of a text le ands each line of text to a List ˆ Print out the text to the console ˆ Write the text into a new le, in reverse order Jason T Bowey CMPT 270 18/19  Summary ˆ IO Streams can use Characters or Bytes ˆ Use FileInputStream and FileOutputStream for Byte Streams ˆ Use FileReader and FileWriter for Character Streams Jason T Bowey CMPT 270 19/19