1. OO Design and Programming II I/O: Reading and Writing
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2. What I/O? Basics first Write output on the screen
Write output to a file
Get input from the command line
Get input from a file
Get input from the screen (swing)
As you can master the basics, move to advanced I/O
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3. File Class abstraction machine-independent
deals with most of the machine-dependent complexities of files and path names
filename is a string
Constructors
Methods: canRead(); canWrite(); exists(); isFile(); isAbsolute();
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5. File Class Example Examples\BJ_FileClassTest
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6. I/O Stream
Similar to C/C++
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7. java.io package Reading Writing open a stream while more information
read information
close the stream
Writing
open a stream
while more information
write information
close the stream
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8. Character Streams and Byte Streams
What is the difference?
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9. Superclass Reader (character)
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10. Superclass Reader (character)
Reader and Writer - abstract superclasses for character streams
Reader: API and partial implementation for readers
Streams that read 16-bit characters
Subclasses implement specialized streams
Be familiar to searching specifications in the API documentation
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11. To read from a file Use int read() int read( ) throws IOException
Reads a single character from the file and returns an integer value (0 to 65535)
Will block until a character is available, an I/O error occurs, or the end of the stream is reached.
Subclasses that intend to support efficient single-character input should override this method.
read( ) returns -1 when the EOF is encountered
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12. Superclass Writer (character)
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13. Reader
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14. Writer
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15. FileReader To construct a FileReader, use the following constructors:
public FileReader(String filename)
public FileReader(File file)
Ref:
A java.io.FileNotFoundException would occur if you attempt to create a FileReader with a nonexistent file.
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16. How to read with FileReader
// Create an input stream
input = new FileReader("temp.txt");
int code;
/* Repeatedly read a character and display it on the console*/
while ((code = input.read()) != -1)
System.out.print((char)code);
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17. // To catch I/O exceptions
try {
// Create an input stream
// Repeatedly read a character and display it on the console }
catch (FileNotFoundException ex) {
System.out.println("File temp.txt does not exist");
catch (IOException ex) {
ex.printStackTrace();
finally {
input.close(); // Close the stream
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18. FileWriter To construct a FileWriter, use the following constructors:
To construct a FileWriter, use the following constructors:
public FileWriter(String filename)
public FileWriter(File file)
public FileWriter(String filename, boolean append)
public FileWriter(File file, boolean append)
See Examples\BJ_TestFileWriter.java
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19. InputStreamReader
InputStreamReader converts byte streams to character streams:
reads bytes and decodes them into characters using a specified charset.
charset specifiable
by name
or given explicitly,
or the platform's default charset
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20. Superclass Writer (character)
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21. InputStreamReader
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22. BufferedReader/BufferedWriter
For efficient I/O:
BufferedReader: readLine() method
If the end of stream is reached, readLine() returns null.
BufferedWriter: newLine() method to write a line separator.
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23. Coding BufferedReader in =
new BufferedReader(new FileReader("foo.in"));
buffers the input from the specified file.
Without buffering,
each invocation of read() or readLine() could cause bytes to be read from the file, converted into characters, and then returned, which can be very inefficient
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24. Examples of BufferedReader
See BJ_BRread
BJ_BRreadLines
BJ_TinyEditor // reads multiple lines from console and reverberates using InputStreamReader and BufferedReader
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25. PrintWriter/PrintStream
BufferedWriter: outputs characters and strings. PrintWriter and PrintStream both output objects, strings and numeric values as text.
PrintWriter in jdk1.2 and more efficient
replaces PrintStream jdk1.0)
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26. PrintWriter To construct a PrintWriter: public PrintWriter(Writer out)
public PrintWriter(Writer out, boolean autoFlush)
If autoFlush is true, the println methods will cause the buffer to be flushed.
The methods in PrintWriter and PrintStream do not throw an IOException though some constructor may throw.
No need to invoke them from a try-catch block for methods.
Example: BJ_TestPrintWriter.java
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27. Binary I/O Text I/O: read: specific encoding to Unicode
write: Unicode to file specific encoding
Binary I/O: No conversions;
exact byte read or written to file
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28. Superclass InputStream (byte)
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29. Superclass OutputStream (byte)
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30. Using the Streams Input Output character FileReader FileWriter
PrintWriter
binary
FileInputStream
FileOutputStream
PrintStream
PrintOutputStream

31. Q: What is the difference?
Open File for Reading
File inFile = new File("test_in.txt");
FileReader in = new FileReader(inFile);
File inFile = new File("test_in.txt");
FileInputStream in = new FileInputStream(inFile);
Q: What is the difference?
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32. FileReader vs. FileInputStream
reads streams of characters
FileInputStream
reads streams of raw bytes
example: image data
Can also read characters
See BJ_ShowFile // reads from file and displays
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33. Open File for Writing File outFile = new File("test_out.txt");
FileWriter out = new FileWriter( outFile );
File outFile = new File("test_out.txt");
FileOutputStream out = new FileOutputStream(outFile);
File outFile = new File(“test_out.txt”);
PrintStream outfile = new PrintStream(outFile);
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34. Close File
Infile.close( );
outFile.close( );
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35. Command Line Arguments
public static void main(String[ ] args) {
if( args.length == 2 ) {
String firstArgument = args[0];
String secondArgument = args[1];
} else {
System.out.println(“java command <args1> <args2>”); }
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36. Source Code Review and Demo Copy Files
BJ_Copy_File project containing:
CopyFileWOException and CopyfileWithException
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37. Combination of multiple input streams Source Code Review and Demo
BJ_Concatenate
Exercise : run Concatenate.java
Don’t forget ListOfFiles.java
Add a println statement to trace program execution and
make sure you understand the execution sequence
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38. Note the difference between
read()
Return int
Check for -1 to detect EOF
readLine()
Return a string
Check for null to detect EOF
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39. read( ) int c = in.read( ); int c; while( (c = in.read()) != -1 ) {
……. }
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40. readLine( ) String str = infile.readLine( ); While( true ) {
if( str == null ) break; …… }
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