Javaio
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This document provides an overview of Java I/O streams and how to open, use, and close them. It discusses how opening a stream creates a connection to external data, using a stream involves reading or writing data, and closing streams is important to release resources. It then gives examples of opening a FileReader stream, reading lines with a BufferedReader, and includes code for a LineReader class to simplify reading text files and a LineWriter class to write text files.
Javaio
- 1. 7-Jun-21 Simple Java I/O Part I General Principles
- 2. 2 Prologue “They say you can hold seven plus or minus two pieces of information in your mind. I can’t remember how to open files in Java. I’ve written chapters on it. I’ve done it a bunch of times, but it’s too many steps. And when I actually analyze it, I realize these are just silly design decisions that they made. Even if they insisted on using the Decorator pattern in java.io, they should have had a convenience constructor for opening files simply. Because we open files all the time, but nobody can remember how. It is too much information to hold in your mind.” —Bruce Eckel, http://www.artima.com/intv/aboutme2.html
- 3. 3 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
- 4. 4 How to do I/O import java.io.*; Open the stream Use the stream (read, write, or both) Close the stream
- 5. 5 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
- 6. 6 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
- 7. 7 Example of opening a stream A FileReader is a used to connect to a file that will be used for input: FileReader fileReader = new FileReader(fileName); The fileName specifies where the (external) file is to be found You never use fileName again; instead, you use fileReader open use close
- 8. 8 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
- 9. 9 Example of using a stream int charAsInt; charAsInt = fileReader.read( ); The fileReader.read() method reads one character and returns it as an integer, or -1 if there are no more characters to read The meaning of the integer depends on the file encoding (ASCII, Unicode, other) You can cast from int to char: char ch = (char)fileReader.read( ); FileReaderExample1.java open use close
- 10. 10 Manipulating the input data Reading characters as integers isn’t usually what you want to do A BufferedReader will convert integers to characters; it can also read whole lines The constructor for BufferedReader takes a FileReader parameter: BufferedReader bufferedReader = new BufferedReader(fileReader); open use close
- 11. 11 Reading lines String s; s = bufferedReader.readLine( ); A BufferedReader will return null if there is nothing more to read FileReaderExample2.java open use close
- 12. 12 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! Java will normally close your streams for you when your program ends, but it isn’t good style to depend on this open use close
- 13. 7-Jun-21 Simple Java I/O Part II LineReader and LineWriter
- 14. 14 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 of formatted text files Compilers, in general, work only with text
- 15. 15 My LineReader class class LineReader { BufferedReader bufferedReader; LineReader(String fileName) {...} String readLine( ) {...} void close( ) {...} }
- 16. 16 Basics of the LineReader constructor Create a FileReader for the named file: FileReader fileReader = new FileReader(fileName); Use it as input to a BufferedReader: BufferedReader bufferedReader = new BufferedReader(fileReader); Use the BufferedReader; but first, we need to catch possible Exceptions
- 17. 17 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); }
- 18. 18 readLine String readLine( ) { try { return bufferedReader.readLine( ); } catch(IOException e) { e.printStackTrace( ); } return null; }
- 19. 19 close void close() { try { bufferedReader.close( ); } catch(IOException e) { } }
- 20. 20 How did I figure that out? I wanted to read lines from a file I thought there might be a suitable readSomething method, so I went to the API Index Note: Capital letters are all alphabetized before lowercase in the Index I found a readLine method in several classes; the most promising was the BufferedReader class The constructor for BufferedReader takes a Reader as an argument Reader is an abstract class, but it has several implementations, including InputStreamReader FileReader is a subclass of InputStreamReader There is a constructor for FileReader that takes as its argument a (String) file name
- 21. 21 The LineWriter class class LineWriter { PrintWriter printWriter; LineWriter(String fileName) {...} void writeLine(String line) {...} void close( ) {...} }
- 22. 22 The constructor for LineWriter LineWriter(String fileName) { try { printWriter = new PrintWriter( new FileOutputStream(fileName), true); } catch(Exception e) { System.err.println("LineWriter can’t " + "use output file: " + fileName); } }
- 23. 23 Flushing the buffer When you put information into a buffered output stream, it goes into a buffer The buffer may or may not be written out right away If your program crashes, you may not know how far it got before it crashed Flushing the buffer forces the information to be written out
- 24. 24 PrintWriter Buffers are automatically flushed when the program ends normally Usually it is your responsibility to flush buffers if the program does not end normally PrintWriter can do the flushing for you public PrintWriter(OutputStream out, boolean autoFlush)
- 25. 25 writeLine void writeLine(String line) { printWriter.println(line); }
- 26. 26 close void close( ) { printWriter.flush( ); try { printWriter.close( ); } catch(Exception e) { } }