Transcript JFileChooser.APPROVE_OPTION

Java I/O
Flexibility and Layering
Copyright © 1999 - 2012 Curt Hill
The Difficulty of File Design
• There are very many different
approaches to file design in
programming languages
• None have mastered it or all others
would have copied them
• C++ has two completely different
approaches
– IOStream use << and >>
– StdIO use printf and scanf
• Random access files are particularly
operating system sensitive
Copyright © 1999 - 2012 Curt Hill
One example: end of file
• Three or more approaches
• Error exit routine
– FORTRAN
– COBOL
• Pre-check
– Pascal
• Post-check
– C++
Copyright © 1999 - 2012 Curt Hill
External Data
• What does data come from? or
Where does it go?
–
–
–
–
–
–
–
A file, such as on disk
Console
Network connection
Pipe
String
Array of bytes
Two or more of above collected
together
• The problem is making these look
similar
Copyright © 1999 - 2012 Curt Hill
Formatting
• How is the data modified from source to
destination?
• No formatting
– Raw bytes or characters
• Collecting
– An integer stored as 4 binary bytes
• Formatting based on desired type
– Digit characters transformed into an integer
Copyright © 1999 - 2012 Curt Hill
Direction
• Files may be processed
in several directions
–Input
–Output
•Append
–Input and Output
Copyright © 1999 - 2012 Curt Hill
Organization and Access
• Organization
– Sequential
• May be buffered for efficiency
– Random Access
• Direct
• Indexed
• Access
– How it is processed
– Must be no stronger than organization
• All are OS dependent
Copyright © 1999 - 2012 Curt Hill
Classes
• There are many classes that capture
these capabilities
• They are then cascaded to give the
desired result
• The beginnings are InputStream and
OutputStream which are abstract base
classes
• To be useful they are subclassed
• These are in java.io.*
Copyright © 1999 - 2012 Curt Hill
InputStream
• Defines the following functions:
–
–
–
–
–
–
–
–
int available()
void close()
void mark(int)
boolean marksupported()
int read(byte [])
int read(byte [], int offset, int len)
void reset()
long skip(long)
Copyright © 1999 - 2012 Curt Hill
InputStream Descendents
• AudioInputStream
• ByteArrayInputStream
• FileInputStream
– Constructor takes a file name string
•
•
•
•
•
•
FilterInputStream
ObjectInputStream
PipedInputStream
SequenceInputStream
StringBufferInputStream
There are also other abstract classes
Copyright © 1999 - 2012 Curt Hill
The Readers
• The InputStream descendents do
not have the convenient methods for
processing input that is people
readable
• The read methods generally only
reads a byte
– This is one half of a character
• There are several Reader objects
that do have nicer methods
Copyright © 1999 - 2012 Curt Hill
Reader
• This is generally not used, but is the
ancestor of several that are quite
handy
• The read method of this object (and
descendents) reads a character, not
a byte
• It has several handy descendents:
– InputStreamReader
– BufferedReader
Copyright © 1999 - 2012 Curt Hill
Readers
• InputStreamReader adds the
functionality of reading from an
InputStream
– Such as a disk file
• BufferedReader adds two things:
– Buffering
– readLine method that reads a whole
line and returns a string
Copyright © 1999 - 2012 Curt Hill
End of File
• Each class has the option to handle
EOF differently
• The DataInputStream does throw an
EOFException
• InputStreamReader has a read
which returns an int
– If EOF it returns a -1
• It also has a ready method which
returns true should there be
somthing left to read
Copyright © 1999 - 2012 Curt Hill
OutputStream
• Another abstract base class
• Defines the following functions:
–
–
–
–
void close()
void flush()
void write(byte [])
void write(byte [], int offset, int len)
Copyright © 1999 - 2012 Curt Hill
Instantiable Output
Streams
•
•
•
•
ByteArrayOutputStream
FileOutputStream
PipeOutputStream
FilterOutputStream
Copyright © 1999 - 2012 Curt Hill
Exceptions
• InputStream and OutputStream objects
throw various kinds of IOExceptions
• Constructors using file names usually
throw FileNotFoundException
• Others I/O throw IOException
• Most operations need to be in try catch
Copyright © 1999 - 2012 Curt Hill
Filters
• FilterInputStream and FilterOutputStream
are also abstract classes
• Designed to be the super classes of all
formatting classes or any other added
functionality
• Subclasses include: BufferXXXStream,
DataXXXStream and others
• Provide the same functionality as
InputStreams or OutputStreams
Copyright © 1999 - 2012 Curt Hill
Buffering
• Buffering separates logical reads or
writes from physical reads or writes
• Devised in the 1950s to save space
on tape
• Today we do it to make access
faster
• Reading an entire track is only
slightly slower than reading a single
sector
Copyright © 1999 - 2012 Curt Hill
Java Buffered Classes
• Two buffering classes:
– BufferedInputStream which takes an
InputStream in its constructor
– BufferedOutputStream which takes an
OutputStream in its constructor
• They provide all the same
functionality as Stream but add
buffering
Copyright © 1999 - 2012 Curt Hill
Output formatting
• PrintStream is a subclass of
FilterOutputStream
• Constructor takes an OutputStream
• Offers the print and println functions with
many overloads
– All the primitives
– Object (uses String.valueOf)
– String
• Handles its own exceptions so try catch is
only needed for Opening
Copyright © 1999 - 2012 Curt Hill
print and println
• These are methods which are
defined in PrintStream
• They format variables and output
them
• They are overloaded to take several
different kinds of arguments
• They will only take one argument
each
– println may have no arguments as well
Copyright © 1999 - 2012 Curt Hill
How the Classes Fit
OutputStream
P
BufferedOutputStream
Application
PrintStream
Copyright © 1999 - 2012 Curt Hill
Example Code
• import java.io.*;
• PrintStream f;
• try {
f = new PrintStream(
new BufferedOutputStream(
new FileOutputStream(
"junk.out")));
} catch (FileNotFoundException e)
{error stuff};
• f.print("Double ");
f.println(d);
Copyright © 1999 - 2012 Curt Hill
Discussion
• Only the creation needs to be
protected by try catch
• PrintStream handles all the errors of
writing itself
• The print and println methods are
similar to those used by System.out
Copyright © 1999 - 2012 Curt Hill
PrintWriter
• Similar to PrintStream
– Not derived from PrintStream
– All the same methods
• Constructor takes an OutputStream
• checkError returns whether an
error occurred
Copyright © 1999 - 2012 Curt Hill
Console Redirection
• The JVM has three standard files
like C
– Standard output
• System.out
– Standard error
• System.err
– Standard input
• System.in
• Each of these may be redirected to a
file or processed by a program
Copyright © 1999 - 2012 Curt Hill
Redirection
• Usually done for the execution of an
existing program
• Three steps
– Start program
– Redirect files
– Process these files
Copyright © 1999 - 2012 Curt Hill
Starting a program
• Uses the Runtime static object
Runtime rt =
Runtime.getRuntime();
• Now start the program
Process proc =
rt.exec(cmd,environment,dir);
• Cmd is an array of string
• Environment is an array of strings
• Dir is string
Copyright © 1999 - 2012 Curt Hill
Standard files of a process
• Use a process method to get the
output file
• Three Process methods are:
– OutputStream getOutputStream();
• Connects to input
– InputStream getErrorStream();
• Connects to error
– InputStream getInputStream();
• Connects to output
Copyright © 1999 - 2012 Curt Hill
Capture the file
• Use this as file:
PrintWriter ps = new
PrintWriter(proc.getOutputStr
eam());
• Any println to this file is piped to the
process input file
Copyright © 1999 - 2012 Curt Hill
File output
• PrintStream is for people readable
text
• What if the data is to be written to a
file and then later read back in?
• No formatting is needed, instead
writing in binary
• Yet it must be reversible and
machine independent
• Uses DataOutputStream
Copyright © 1999 - 2012 Curt Hill
DataOutputStream
• Subclass of FilterInputStream
• Constructor takes an OutputStream
• New functions are:
–
–
–
–
void writeFloat(float)
void writeInt(int)
void writeBytes(String)
void writeChars(String)
• There is no writeString
Copyright © 1999 - 2012 Curt Hill
Input formatting
• DataInputStream is a subclass of
FilterInputStream
• Reads in what a DataOutputStream
wrote
• Constructor takes an InputStream
• Provides functions for reading the
various types:
– byte readByte()
– double readDouble()
• No read string
Copyright © 1999 - 2012 Curt Hill
What about conversion?
• Converting a string of characters
into an integer has mostly been left
out of tradional Java Readers and
Streams
• The way to do this is:
– Read whole lines
– Chop into pieces based on white space
– Use Integer.parseInt (or similar) to
convert to needed types
• In Java 5 the Scanner object was
introduced Copyright © 1999 - 2012 Curt Hill
Scanner
• A Scanner may be constructed
using either a File or InputStream
– A File may be obtained from
JFileChooser
• The scanner by default uses
whitespace as the delimiter between
tokens
• It has a series of hasNextXXX and
nextXXX methods
Copyright © 1999 - 2012 Curt Hill
hasNext methods
• Returns a boolean
• True if the item is the next token
• So:
hasNextInt()
is true if the next token can be
interpreted as an integer
• Most of the types have one:
– hasNextDouble
– hasNextFloat
– hasNextLine
Copyright © 1999 - 2012 Curt Hill
next Methods
• Reads and returns the next item
• Example:
int j = sc.nextInt();
• One for most types:
– nextBoolean
– nextDouble
– nextLine
Copyright © 1999 - 2012 Curt Hill
Scanner Exceptions
• The hasNextXXX will throw the
IllegalStateException if the scanner
is closed
• The nextXXX will throw:
– InputMismatchException if the item
does not conform to specifications
– NoSuchElementException if the
Scanner has no input left
– IllegalStateException if the scanner is
closed
Copyright © 1999 - 2012 Curt Hill
Text files
• Text files in any Microsoft Operating
System always end a line with two
characters
– A carriage return followed by a line
feed
– Both are control characters
• UNIX based systems use only a line
feed
• This may change how the file is
processed on different systems
Copyright © 1999 - 2012 Curt Hill