Simple Java I/O. Part I General Principles

Transcription

1 Simple Java I/O Part I General Principles

2 Streams All modern I/O is stream based A stream is a connec8on to a source of data or to a des8na8on for data (some8mes 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 characteris8cs: A file has a definite length, and therefore an end Keyboard input has no specific end

3 How to do I/O import java.io.*; Open the stream Use the stream (read, write, or both) Close the stream

4 open use close Why Java I/O is hard Java I/O is very powerful, with an overwhelming number of op8ons Any given kind of I/O is not par8cularly difficult The trick is to find your way through the maze of possibili8es

5 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 connec8on to that external place Once the connec8on is made, you forget about the external place and just use the stream

6 open use close Example of opening a stream A FileReader is 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

7 open use close Using a stream Some streams can be used only for input, others only for output, s8ll 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

8 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 -1 if there are no more characters to read The meaning of the integer depends on the file encoding (ASCII, Unicode, other)

9 open use close Manipula8ng 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);

10 open use close Reading lines String s; s = bufferedreader.readline( ); A BufferedReader will return null if there is nothing more to read

11 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 8me 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!

12 Simple Java I/O Part II LineReader and LineWriter

13 Text files Text (.txt) files are the simplest kind of files text files can be used by many different programs FormaYed text files (such as.doc files) also contain binary formazng informa8on Only programs that know the secret code can make sense formayed text files Compilers, in general, work only with text

14 My LineReader class class LineReader { BufferedReader bufferedreader; LineReader(String filename) {...} String readline( ) {...} } void close( ) {...}

15 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

16 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); }

17 readline String readline( ) { try { return bufferedreader.readline( ); } catch(ioexception e) { e.printstacktrace( ); } return null; }

18 close void close() { try { bufferedreader.close( ); } catch(ioexception e) { } }

19 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

20 The LineWriter class class LineWriter { PrintWriter printwriter; LineWriter(String filename) {...} void writeline(string line) {...} } void close( ) {...}

21 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); } }

22 Flushing the buffer When you put informa8on into a buffered output stream, it goes into a buffer The buffer may not be wriyen out right away If your program crashes, you may not know how far it got before it crashed Flushing the buffer is forcing the informa8on to be wriyen out

23 PrintWriter Buffers are automa8cally 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)

24 writeline void writeline(string line) { printwriter.println(line); }

25 close void close( ) { printwriter.flush( ); try { printwriter.close( ); } catch(exception e) { } }

26 Simple Java I/O Part III Serializa=on

27 Serializa8on You can also read and write objects to files Object I/O goes by the awkward name of serializa8on Serializa8on in other languages can be very difficult, because objects may contain references to other objects Java makes serializa8on (almost) easy

28 Condi8ons for serializability If an object is to be serialized: The class must be declared as public The class must implement Serializable The class must have a no argument constructor All fields of the class must be serializable: either primi8ve types or serializable objects

29 Implemen8ng the Serializable interface To implement an interface means to define all the methods declared by that interface, but... The Serializable interface does not define any methods! Ques8on: What possible use is there for an interface that does not declare any methods? Answer: Serializable is used as flag to tell Java it needs to do extra work with this class

30 open use close Wri8ng objects to a file ObjectOutputStream objectout = new ObjectOutputStream( new BufferedOutputStream( new FileOutputStream(fileName))); objectout.writeobject(serializableobject); objectout.close( );

31 open use close Reading objects from a file ObjectInputStream objectin = new ObjectInputStream( new BufferedInputStream( new FileInputStream(fileName))); myobject = (itstype)objectin.readobject( ); objectin.close( );

32 What have I leb out? Encrypted files, compressed files, files sent over internet connec8ons,... Excep8ons! All I/O involves Excep8ons! try { statements involving I/O } catch (IOException e) { e.printstacktrace ( ); }