Java I/O: Understanding Streams, Reading and Writing Files in Java, Slides of Java Programming

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Simple Java I/O

Part I General Principles

Streams

• All modern I/O is stream-based
• A stream is a connection to a source of data or to a destination for data (sometimes both)
• An input stream may be associated with the keyboard
• An input stream or an output stream may be associated with a file
• Different streams have different characteristics:
  • A file has a definite length, and therefore an end
  • Keyboard input has no specific end

Why Java I/O is hard

• Java I/O is very powerful, with an overwhelming number of options
• Any given kind of I/O is not particularly difficult
• The trick is to find your way through the maze of possibilities

open use close

Opening a stream

• There is data external to your program that you want to get, or you want to put data somewhere outside your program
• When you open a stream, you are making a connection to that external place
• Once the connection is made, you forget about the external place and just use the stream

open use close

Using a stream

• Some streams can be used only for input, others only for output, still others for both
• Using a stream means doing input from it or output to it
• But it’s not usually that simple--you need to manipulate the data in some way as it comes in or goes out

open use close

Example of using a stream

int ch; ch = fileReader.read( );

• The fileReader.read() method reads one character and returns it as an integer, or - if there are no more characters to read
• The meaning of the integer depends on the file encoding (ASCII, Unicode, other)

open use close

Reading lines

String s; s = bufferedReader.readLine( );

• A BufferedReader will return null if there is nothing more to read

open use close

Closing

• A stream is an expensive resource
• There is a limit on the number of streams that you can have open at one time
• You should not have more than one stream open on the same file
• You must close a stream before you can open it again
• Always close your streams!

open use close

Text files

• Text (.txt) files are the simplest kind of files
  • text files can be used by many different programs
• Formatted text files (such as .doc files) also contain binary formatting information
• Only programs that “know the secret code” can make sense formatted text files
• Compilers, in general, work only with text

My LineReader class

class LineReader { BufferedReader bufferedReader;

LineReader(String fileName) {...}

String readLine( ) {...}

void close( ) {...} }

The full LineReader constructor

LineReader(String fileName) { FileReader fileReader = null; try { fileReader = new FileReader(fileName); } catch (FileNotFoundException e) { System.err.println ("LineReader can't find input file: " + fileName); e.printStackTrace( ); } bufferedReader = new BufferedReader(fileReader); }

readLine

String readLine( ) { try { return bufferedReader.readLine( ); } catch(IOException e) { e.printStackTrace( ); } return null; }

How did I figure that out?

• I wanted to read lines from a file
• I found a readLine method in the BufferedReader class
• The constructor for BufferedReader takes a Reader as an argument
• An InputStreamReader is a kind of Reader
• A FileReader is a kind of InputStreamReader

The LineWriter class

class LineWriter { PrintWriter printWriter;

LineWriter(String fileName) {...}

void writeLine(String line) {...}

void close( ) {...} }