CS5000: Foundations of Programming. Mingon Kang, PhD Computer Science, Kennesaw State University

Transcription

1 CS5000: Foundations of Programming Mingon Kang, PhD Computer Science, Kennesaw State University

2 Files Two types: Text file and Binary file Text file (ASCII file) The file data contains only ASCII values Designed to be read by human beings Binary file Consist of a sequence of binary values Designed to be read by programs More efficient to process than text files

3 Streams Stream in Java Object that enables the flow of data between a program and I/O device Input Stream Data flows into a program Generally from the keyboard or a file Output Stream Data flows out of a program Generally flow to a screen or to a file

4 Streams System.in: input stream Scanner keyboard = new Scanner(System.in) System.out: output stream System.out.println( Output Stream );

5 Text Files Reading text files BufferedReader stream class Writing text files PrintWriter stream class

6 Read a Text File BufferedReader Stream class that read a text file Methods: read( ) and readline( ) Need to import a set of followings import java.io.bufferedreader; import java.io.filereader; import java.io.filenotfoundexception; import java.io.ioexception;

7 Read a Text File No constructor that takes a file name directly. Use FileReader class that convert the file name to an object for the constructor s parameter BufferedReader reader; reader = new BufferedReader(new FileReader(FileName));

8 Path Names If only a file name is used as an argument, it assumes that the file is in the same directory or folder as the one in which the program is run If it is not in the same directory, the full or relative path name have to be given.

9 Path Names Path names are specified depends on the operation systems UNIX /user/etc/data/data.txt Windows C:\\data\\data.txt Reference

10 Read a Text File read() method Read a single character and returns a value of integer type Need a type cast char next = (char)reader.read(); readline() method Check FileRead1.java and FileRead2.java

11 Exception Handling When performing file I/O, many exceptional situations may be thrown. Many of these exception classes are subclasses of the class IOException

12 Exception Handling BufferedReader object may throw two kinds of exceptions FileNotFoundException IOException Both exceptions should be handled

13 Exception Handling Opening a file should be placed inside a try block try{ } catch(filenotfoundexception e){ } catch(ioexception e){ }

14 Write a text file PrintWriter Stream class that write to a text file Import of import java.io.printwriter; import java.io.fileoutputstream; import java.io.filenotfoundexception;

15 Write a text file Connect to the file FileName PrintWriter writer; writer = new PrintWriter(new FileOutputStream(FileName)); For appending: writer = new PrintWriter(new FileOutputStream(FileName, true)); Check JAVA document of PrintWriter

16 Write a text file May throw FileNotFoundException that means the file could not be created for some reasons When a program finishes writing, it should close the stream connection by outputstreamname.close()

17 Write a text file When writing to a file, the data is buffered rather than physically writing to the file as soon as possible. The data is saved in a temporary space When enough data is received in the buffer, the flush() method is invoked, the buffered data is physically written to the file all at once The close() method invokes the method flush.

18 Binary Files Reading binary files ObjectInputStream stream class Writing binary files ObjectOutputStream stream class

19 Write a Binary File ObjectOutputStream Stream class that write a binary file Import of followings import java.io.objectoutputstream; import java.io.fileoutstream; import java.io.ioexception;

20 Write a Binary File Connect to a binary file ObjectOutputStream outputfile = new ObjectOutputStream(new FileOutputStream(FileName)); The constructor for FileOutputStream may throw a FileNotFoundException The constructor for ObjectOutputStream may throw an IOException

21 Write a Binary File ObjectOutputStream methods writeint writedouble writechar writeboolean write flush close

22 Read a Binary File ObjectInputStream Stream class that read a binary file Import of followings: import java.io.objectinputstream; import java.io.fileinputstream; import java.io.ioexception;

23 Read a Binary File Connect to a binary file ObjectInputStream inputfile = new ObjectInputStream(new FileInputStream(FileName)); The constructor for FileInputStream may throw a FileNotFoundException The constructor for ObjectInputStream may throw an IOException

24 Read a Binary File ObjectInputStream methods readint readdouble readchar readboolean close

25 Read a Binary File If the file contains multiple types, each item type must be read in exactly the same order it was written to the file

26 RandomAccessFile Read and write to the same file Enable us to random access to particular parts of a file A random access file consists of a sequence of numbered bytes (like an array) seek() method: move a cursor in a file Good in very large files

27 RandomAccessFile Two constrcutors RandomAccessFile(File file, String mode) RandomAccessFile(String name, String mode)

28 RandomAccessFile Mode r : reading only rw : both reading and writing to the file rws : flushes the contents of the file and metadata (e.g. the modification date of the file) rwd : flushes the contents of the file, but metadata might not change until the file is closed. rw only flushes when flush() is invoked and doesn't change metadata until closing the file. rwd is much slower for writes than rw, and rws is slower again

29 Binary I/O of objects After interface subject, with serialization

30 Reference