Chapter 12. File Input and Output. CS180-Recitation

Transcription

1 Chapter 12 File Input and Output CS180-Recitation

2 Reminders Exam2 Wed Nov 5th. 6:30 pm. Project6 Wed Nov 5th. 10:00 pm. Multitasking: The concurrent operation by one central processing unit of two or more processes. Debbie's exam study group Tue Nov 4th. 5-7 pm. LWSN B131 not B116. Exam 2 review. 2

3 Exam 2 Don't get bogged down in the MCQ section. If you don't know an answer, skip that question and come back to it in the end. Leave plenty of time for programming questions. Programming questions. Syntax not as important as substance. Get the right solution first, then worry about syntax. 3

4 File I/O Why bother? Persistent storage Use over multiple sessions Safety In case of failure (program, computer) What are files? Collections of bytes Can be abstracted and viewed otherwise Different Types? 4

5 Streams All file I/O is treated like a stream Moving flow of data Different classes can work with the stream Provide filters and abstractions Like an assembly line for data File Lots of data FileInputStream bytes DataInputStream int char boolean double String 5

6 Methods of the Class File Figure 10.4 Some methods in class File File.exists, File.canWrite/Read help you avoid exceptions. 6

7 Streams A stream can only flow in one direction. Need input streams and output streams Useful classes: General File String (split) JFileChooser Input FileInputStream DataInputStream ObjectInputStream FileReader BufferedReader Scanner Output FileOutputStream DataOutputStream ObjectOutputStream (FileWriter) PrintWriter 7

8 Example 1 What does the following program do? try String in = "in.txt"; int numlines = 5; BufferedReader reader = new BufferedReader( new FileReader( in ) ); for ( int line = 0; line < numlines; ++line ) System.out.println( reader.readline() ); catch( IOException ioe ) System.err.println( "Crush! Kill! Destroy!" ); What if the number of lines is unknown? Is there anything missing? 8

9 Example 2 try String in = "in.txt"; BufferedReader reader = new BufferedReader( new FileReader( in ) ); String line = reader.readline(); while( null!= line ) System.out.println( reader.readline() ); line = reader.readline(); catch( IOException ioe ) System.err.println( "I can't do that, Dave!" ); Careful, here's a bug! What changes when reading a byte at a time? But we still need to close it! Where? 9

10 Example 3 BufferedReader reader = null; try... catch( IOException ioe )... finally try close() should almost always be called in a finally block! if ( null!= reader ) reader.close(); Why? catch( IOException ioe ) System.err.println( "Error closing file" ); What if dealing with binary data? 10

11 Scanner import java.util.scanner; import java.io.file; import java.io.filenotfoundexception;... try Scanner scanner = new Scanner( new File( in.txt ) ); while( scanner.hasnextline() ) System.out.println( scanner.nextline() ); catch( FileNotFoundException e ) System.err.println( "Unable to open file!" ); 11

12 A similar program: Binary Data Read in as many binary numbers as possible DataInputStream is = null; try String in = "in.txt"; is = new DataInputStream( new FileInputStream( in ) ); while( true ) System.out.println( is.readint() ); catch( EOFException eofe ) System.out.println( "Done reading" ); catch( IOException ioe )... finally... 12

13 Other Concerns Suppose that exception handling and closing are correct. What is wrong with: DataOutputStream dos = new DataOutputStream(... ); dos.writeboolean( true ); dos.writechar( 'I' ); dos.writeutf( like ); dos.writedouble( Math.PI ); // closed... DataInputStream dis = new DataInputStream(... ); double String d = dis.readdouble(); str = dis.readutf(); char c = dis.readchar(); boolean b = dis.readboolean(); 13

14 Object I/O - Text Notice: tostring() is part of any object PrintWriter writer = null; try Thu Nov 02 20:02:39 EST 2006 Today is Thu Nov 02 20:02:39 EST 2006 String outfile = "out.txt"; writer = new PrintWriter ( new FileOutputStream( outfile ) ); Date today = new Date(); writer.println( today.tostring() ); writer.println( "Today is " + today ); catch( FileNotFoundException ioe ) System.err.println( "Exterrrrrrminate!" ); finally... Could use XMLEncoder, but beyond this course 14

15 Object I/O Object I/O can be binary ObjectInputStream and ObjectOutputStream ObjectOutputStream oos = new ObjectOutputStream (new FileOutputStream( <filename> )); oos.writeobject( new Date() );... ObjectInputStream ois = new ObjectInputStream (new FileInputStream( <filename> )); Date adate = (Date)ois.readObject(); 15

16 The Class EOFException Many methods that read from a binary file will throw an EOFException Can be used to test for end of file Thus can end a reading loop Always put these in a try/catch block to avoid program crashes. 16

17 Object Stream Concerns Could an instance of the following class be written: public class SomeClass String astring = sometext ; Why or why not? 17

18 Object Stream Concerns Any object serialized by a stream must implement Serializable Acts as a marker for what can be written public class SomeClass implements Serializable String astring = sometext ; 18

19 Object Stream Concerns Could the following be written? public class ClassOne.. public class ClassTwo implements Serializable private ClassOne c1; 19

20 Object Stream Concerns Can be fixed in one of two ways In ClassOne public class ClassOne implements Serializable.. In ClassTwo public class ClassTwo implements Serializable private transient ClassOne c1; transient qualifier -means that a given attribute won't be saved. -c1 will be null if an instance of ClassTwo is read 20

21 Some Details of Serialization Effects of making a class serializable Affects how Java performs I/O with class objects Java assigns a serial number to each object of the class it writes to the ObjectOutputStream If same object written to stream multiple times, only the serial number written after first time 21

22 Array Objects in Binary Files Since an array is an object, possible to use writeobject with entire array Similarly use readobject to read entire array View array I/O program, listing class ArrayIODemo 22

23 Programming Example A JFrame GUI for manipulating files Note buttons Show first line Remove file Reset Note also the text fields Type in a file name Display first line of file 23

24 Programming Example View JFrame program, listing class FileOrganizer Sample screen output 24

25 Programming Example Sample screen output 25

26 Programming Example Note we did this with a JFrame GUI program Not an applet For security reasons applets are limited in what they can do Designed to be embedded in a Web page, run from another computer Thus applets cannot manipulate files on a remote computer 26