Transcript Text one char at a time from a file

Two Ways to Store Data in a
File
Text
format
Binary format
Text Format
Information stored as a sequence of characters
Characters are stored as their ASCII equivalent
- int value 12345 stored as ‘1’ ‘2’ ‘3’ ‘4’ ‘5’
5 bytes
91 92 93 94 95
A text file is ‘readable’ by humans.
Java classes ‘Reader’ and ‘Writer’ (and
their subclasses) are for use with text
files.
Text I/O
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In Java, characters are stored using UNICODE.
Reader objects read characters in the current
O.S. format, and convert them to UNICODE
storage format.
Writer objects convert the UNICODE stored
data to character encoding used by the current
O.S.
Reader and Writer are abstract classes
Text Input
READER class is an abstract class which is:
extended by
extended by
InputStreamReader which is
extended by FileReader
BufferedReader
Reader class provides abstract methods which will be implemented
specifically by all subclasses
InputStreamReader object reads bytes from an input stream and decodes
them into characters, using the platform’s charset (ASCII for Windows).
These characters are converted to UNICODE for storage.
read() reads one character (number of bytes depends on charset)
FileReader object inherits InputStreamReader capabilities, but the stream
is a file.
BufferedReader object is a wrapper for any InputStreamReader (including
FileReader), and provides buffering (ie. handles multiple char reads and
provides a string)
Text one char at a time

Create a InputStreamReader object
Constructor takes an input stream, which may
be keyboard, network connection, any input source
InputStreamReader in = new InputStreamReader(System.in);
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InputStreamReader extends the Reader class,
implementing input methods specifically for files
Use its read method to read a single character
o
returns the next char as an int
o
or the integer -1 at end of input
Test for -1 to determine if a char was read
Reading one char at a time….
InputStreamReader reader = new
InputStreamReader(System.in);
char c;
//Java chars are UNICODE
int next = reader.read() ; //byte read and
if (next != -1)
c = (char)next();
// process c
next = reader.read() ;
}
//See demo testISReader.java
Reading one line at a time….
InputStreamReader in = new
InputStreamReader(System.in);//input is from keyboard
BufferedReader bin = new BufferedReader(in);
String val;
val = bin.readLine();
// get one string from keyboard
while(val != null) { //while user does not enter CTRL Z
System.out.println( val );
System.out.flush();
//output any lines waiting to be
printed
val = bin.readLine();
}
//See demo testBISReader.java
Text one char at a time from a file
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Create a FileReader object
FileReader extends the Reader class, implementing
input methods specifically for files
Use its read method to read a single character
o
returns the next char as an int
o
or the integer -1 at end of input
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Test for -1 to determine if a char was read
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Close the file when done
Reading one char at a time from file….
FileReader reader = new
FileReader("input.txt");
char c;
//Java chars are UNICODE
int next = reader.read() ; //byte read and
if (next != -1){
c = (char)next();
next = reader.read();
reader.close();
// see testFileReader.java
Reading Text Line by Line
Create a BufferedReader object (pass a
FileReader object to constructor)
objects of type BufferedReader can
group characters – ‘buffer’ them
method readLine() available, to provide file data 1 line
at a time (the method handles reading the
characters from the FileReader for you)
readLine() returns the next line of file (as a String), or
null if none exists
//Reads first line from file named input.txt
// line is expected to contain a double value
FileReader reader = new FileReader("input.txt");
BufferedReader in = new BufferedReader(reader);
String inputLine = in.readLine();
double x = Double.parseDouble(inputLine);
//Reads and all lines from file
// and writes them to console
import java.io;
public class demo{
public static void main(String[] args)throw IOException{
FileReader reader = new FileReader("input.txt");
BufferedReader in = new BufferedReader(reader);
String line = in.readLine();
While (line != null){
System.out.println(line);
line = in.readLine();
}
in.close();
}
}
Text Output
WRITER class is an abstract class which is:
extended by
extended by
OutputStreamWriter which is
extended by FileWriter
PrintWriter
Writer class provides abstract methods which will be implemented
specifically by all subclasses
OutputStreamWriter object writes bytes to an output stream and encodes
them into characters, using the platform’s charset (ASCII for Windows).
These characters are converted from stored UNICODE
write(char) writes one character (number of bytes written depends
on charset) Object is buffered, may be flushed
FileWriter object is inherits OutputStreamWriter capabilities, but the stream
is a file
PrintWriter object is a wrapper for any OutputStreamWriter, and provides
buffering (ie. handles multiple char writes and provides a string)
Write a Character to File
FileWriter class implements Writer methods
specifically for files.
FileWriter writer = new
FileWriter("output.txt");
char c =‘a';
writer.write(c);
writer.close();
Writing Strings to Text Files
A PrintWriter object handles the ‘unbuffering’
of data for output file writer
Create PrintWriter object
(pass FileWriter
object to constructor)
PrintWriter class provides ‘println’ method
which accepts String and uses FileWriter to
print one char at a time.
FileWriter writer = new FileWriter(“output.txt”)
PrintWriter out = new PrintWriter(writer);
//use PrintWriter object to output data to
file output.txt
FileWriter writer = new FileWriter(“output.txt”)
PrintWriter out = new PrintWriter(writer);
out.println(29.95);
out.println(new Rectangle(5,10,15,25));
out.println("Hello, World!");
StringTokenizer Class
When reading a line of text, we get a single long string. Suppose
our line of text looked something like:
John|Doe|16|1998
In other words, the string contained know ‘delimiters’, and we
wanted to access the ‘pieces’ between these delimiters.
Java.util package provides a class to help here: StringTokenizer
Methods:
StringTokenizer(String theline, String delimiters)
boolean hasMoreTokens()
String nextToken()
int countTokens()
see file CountTHE.java
Binary Format
More compact and efficient
int 12345 stored using binary representation:
00000000 00000000 0010000 0011100
00
00
48
57
4 bytes
Java abstract classes InputStream and OutputStream
(and their subclasses) provide methods for reading
and writing these types of files
Binary File I/O
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In Java, an object from which we can read a
sequence of bytes is called an input stream.
An object to which we can write a sequence of
bytes is called an output stream.
I/O streams have beginning, end, and are read
sequentially.
InputStream and OutputStream are abstract
classes.
InputStream class has an abstract method
abstract int read()
// reads and returns one byte
An abstract method is an method which MUST be implemented by
any extending class. The idea is that the extending class provides
specifics…
OutputStream class has an abstract method
abstract void write(int b);
// writes one byte to output
Java provides many stream classes which extend from
InputStream and OutputStream
that let you work with data in the forms that
you normally use……….
Byte Streams
InputStream is extended by
FileInputStream which is used for byte based
input from a file
OutputStream is extended by
FileOutputStream which is used for byte based
output to a file
Both of these classes implement read and write methods as
specified in their abstract super classes.
see file ByteIO.java
Input Byte Streams
InputStream is extended by FilterInputStream
FilterInputStream is extended by DataInputStream
FilterInputStream acts as a ‘wrapper’ for an InputStream objects, which it
uses as its basic source of data. Methods of FilterInputStream objects
simply pass requests to the InputStream objetct.
Each subclass of FilterInputStream transforms the data along the way, or
provides some additional functionality. For example, DataInputStream
‘wraps’ a InputStream object and assembles bytes read into numerical
types.
** readInt, readDouble, readBoolean are just some of the methods
available with a DataInputStream object
See file DataByteIO.java
Input Byte Streams
InputStream is extended by FilterInputStream
FilterInputStream is extended by BufferedInputStream
Another subclass of FilterInputStream is BufferedInputStream.
BufferedInputStream ‘wraps’ a InputStream object and provides ‘buffering’ for
efficiency. BufferedInputStream overloads ‘read’ so that in addition to one byte at a
time, one byte array can be read.
// buffered input
BufferedInputStream bin = new BufferedInputStream(new InputStream(“file.txt”));
// add another layer so that DataInputStream object will not do individual reads
// for each byte, but use the buffered reads
DataInputStream dbin = new DataInputStream(bin);
Output Byte Streams
FilterOutputStream extends OutputStream
(Subclasses may be superclasses themselves)
PrintStream extends FilterOutputStream
FilterOutputStream acts as a ‘wrapper’ for an OutputStream object,
which it uses as its basic depository of data. Methods of
FilterOutputStream simply pass requests to the OutputStream objetct.
PrintStream ‘wraps’ a OutputStream and provides an accurate display
of data types to the output, in addition to buffering. PrintStream also
provides the overloaded println,
which accepts String and byte[] parameters, as well as byte.
System.out, is an object of type PrintStream, which is provided by the
System class in the java.lang input. System.out ‘wraps’ the
OutputStream object representins the console.
FILE OBJECTS provide methods which allow the creation, deletion, and
other file manipulation/status methods…
FileInputStream and FileReader constructors are overloaded to accept a File
object.
The file object will allow you to check the status of a file before you open
it!!
File f = new File(“in.txt”);
if ( ! f.exists() )
System.out.println(“file does not exist”);
else if ( ! F.canRead() )
System.out.println(“file cannot be read”);
else {
FileInputStream in = new FileInputStream(f);
BufferedInputStream bufIn = new BufferedInputStream(in);
int bb = bufIn.read();
A JFileChooser Dialog
File Dialogs
•
Use JFileChooser to let a user supply a file name through a file dialog
•
Construct a file chooser object
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Call its showOpenDialog or showSaveDialog method
(Specify null or the user interface component over which to pop up the dialog )
•
If the user chooses a file:
JFileChooser.APPROVE_OPTION is returned
•
If the user cancels the selection:
JFileChooser.CANCEL_OPTION is returned
•
If a file is chosen, use GetSelectedFile method
to obtain a File object describing the file
Code to Use a JFileChooser
JFileChooser chooser new JFileChooser();
FileReader in;
if (chooser.showOpenDialog(null) == JFileChooser.APPROVE_OPTION) {
File selectedFile = chooser.getSelectedFile();
in = new FileReader(selectedFile);
}
else
in = null;
// see file testDialog.java
Mark and Reset
The abstract classes InputStream and Reader provide:
void mark (int byteLimit)
* Marks the current position in this input stream. A subsequent call to the
reset method repositions this stream at the last marked position so that
subsequent reads re-read the same bytes.
* byteLimit is the number of bytes that can be read before this mark
becomes invalid
void reset()
* repositions stream to position when mark was last called
boolean markSupported () indicates if a stream supports these methods
int[] intList;
intList = new int[100];
//this file contains one number 66
FileReader file = new FileReader("number.txt");
BufferedReader ifile = new BufferedReader(file);
// all array elements are initialized to 66
ifile.mark(5);
for (int i = 0; i<100; i++){
String sVal = ifile.readLine();
ifile.reset();
intList[i] = Integer.parseInt(sVal);
}
Object Output
Another byte (binary) I/O class which extends from
OutputStream is ObjectOutputStream
ObjectOutputStream class can save entire objects to disk !
Objects that are written to an object stream must belong to a
class that implements the Serializable interface.
class Coin implements Serializable
{
...
}
** Serializable interface has no methods.
Serializable
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Objects that are written to an object stream
must belong to a class that implements the
Serializable interface.
class Coin implements Serializable
{
...
}
** Serializable interface has no methods.
ObjectOutputStream methods
Stream does not just write objects ……..
write( ) - writes a byte
writeInt(int) - writes a 32 bit int
writeDouble(double) - writes a 64 bit double
writeChar(char) - writes a 16 bit char
writeObject(Object) - writes the specified object to
the output stream (if Serializable)
close() - closes stream
Writing an Object to a File
OutputStream os = new FileOutputStream("data.txt");
ObjectOutputStream out = new ObjectOutputStream(os);
ArrayList<Box> boxlist = new ArrayList<Box> ();
boxlist.add(new Box(5));
boxlist.add(new Box(17));
boxlist.add(new Box(20));
out.writeObject(boxlist);
out.close();
Object Input
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Another byte (binary) I/O class which extends from
InputStream is
ObjectInputStream
ObjectInputStream class can read from a file, written
by ObjectOutputStream
(Data must by read with respect to datatype and order
with which it was written)
ObjectInputStream methods
read( ) - reads a byte
readInt() - reads a 32 bit int
readDouble() - reads a 64 bit double
readChar() - reads a 16 bit char
readObject( ) - reads the specified object to
the input stream (if
Serializable)
close() - closes stream
Reading from File containing Objects
public static void main(String[] args) throws
IOException,ClassNotFoundException {
InputStream is = new FileInputStream("data.txt");
ObjectInputStream in = new ObjectInputStream(is);
ArrayList<Box> boxlist = (ArrayList<Box>) in.readObject();
for (int i = 0; i< boxlist.size(); i++)
System.out.println(boxlist.get(i));
in.close();
Random .vs. Sequential Access
• Sequential access
o
A file is processed a byte at a
time.
• Random access
o
Allows access at arbitrary
locations in the file