Transcript Introduction to Computer Science I

Technische Universität Darmstadt
Telecooperation/RBG
Introduction to Computer Science I
Topic 20: Streams and Input/Output
Prof. Dr. Max Mühlhäuser
Dr. Guido Rößling
Copyrighted material; for TUD student use only
Dr. G. Rößling
Prof. Dr. M. Mühlhäuser
RBG / Telekooperation
©
Outline
• Introduction to Input/Output Streams and Java
Input/Output (I/O)
• Overview of Processing Streams
• Wrapping Streams and the Decorator Pattern
• User Defined Streams, StreamTokenizer and
Random Access
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Input / Output: Motivation
• So far:
– Program input data is either coded in the program’s source code,
or passed as parameters from the console
• define in Scheme
• Variable declaration and initialization in Java
• Actual arguments passed to functions in Scheme
• args parameters to main
– Program output is shown on the display
• Problems:
– Need to change the program in order to run it with different data
– “Closed world”:
• No interactivity with the outside world
• No influence on the run is possible after the program starts
– Results cannot be stored
• This lecture discusses other ways of input/output
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Input / Output (I/O) - Streams
• Other sources / sinks for program data / results
– Keyboard entry (“standard I/O”),
– Files on local machine ("file I/O")
– Files/processes on the network ("network I/O")
– Main memory ("memory I/O")
• To deal with all these sources/destinations uniformly, the
concept of a data stream is introduced in Java
– Input and output streams (I/O streams for short)
• Streams abstract from I/O “devices”
– Hide details of the implementation and particularities of operating
various I/O devices
• Keyboard, Files, Programs, Network, Memory, …
– Provide unified interfaces for read/write data access
– Java program “talks to“ Java I/O stream objects
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I/O-Streams
• In order to read data, an input stream is attached to a data
source and the data is read element by element
• In order to write data, an output stream is attached to a data sink
and the data is written element by element
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Streams and delayed Lists
• The concept of streams is more general than just I/O
• We saw streams in Scheme
– Delayed lists
– We could create „virtual“ lists
– We could link transparently data with first and rest
• These ideas fit well with I/O
–
–
–
–
Input: on access to the next list item, read the input value
Output: on extension of the list, write the output value
Input stream  delayed list without cons
Output stream  delayed list without first/rest
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Predefined streams in java.io
• Two orthogonal classifications:
• According to the type of data
• According to the structure of the stream
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Classification of streams
• According to the type of the data
– character streams read / write char (16-bit Unicode
character set)
– byte streams read / write byte (8 bit):
• used to deal with binary data, e.g., images, sound, etc.
• According to stream structure
– data streams: data is read/written directly from/to a
concrete source/sink device
– processing streams: data is read from/written to another
stream
• Data is filtered, buffered, manipulated, etc. after reading,
respectively before writing
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Hierarchy of Stream Classes in java.io
• Character streams:
– java.io.Reader/java.io.Writer provide the
interface and a partial implementation for character
input/output streams
– Subclasses of Reader/Writer add/refine the
implementation in Reader/Writer
• Byte streams:
– java.io.InputStream/java.io.OutputStream
provide the interface and a partial implementation for
reading/writing bytes
– All other byte streams are subclasses of
InputStream/OutputStream
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Character Stream Classes (Examples)
©
Reader
shadows indicate
processing streams
StringReader
InputStreamReader
FileReader
FilterReader
PushbackReader
BufferedReader
LineNumberReader
Writer
StringWriter
OutputStreamWriter
FileWriter
FilterWriter
BufferedWriter
PrintWriter
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Byte Stream Classes (Examples)
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FileInputStream
shadows indicate
processing streams
StringBufferInputStream
PipedInputStream
InputStream
ByteArrayInputStream
BufferedInputStream
ObjectInputStream
LineNumberInputStream
FilterInputStream
PushbackInputStream
SequenceInputStream
DataInputStream
FileOutputStream
PipedOutputStream
OutputStream
ByteArrayOutputStream
BufferedOutputStream
FilterOutputStream
PrintStream
ObjectOutputStream
DataOutputStream
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Data Streams: Overview
Type of Device Character Streams
Main Memory
Pipe
Byte Streams
CharArrayReader,
CharArrayWriter
ByteArrayInputStream,
ByteArrayOutputStream
StringReader,
StringWriter
StringBufferInputStream
PipedReader,
PipedWriter
PipedInputStream,
PipedOutputStream
FileReader,
FileInputStream,
FileWriter
FileOutputStream
• CharArrayReader/CharArrayWriter
ByteArrayInputStream/ByteArrayOutputStream
– read from or write to an existing array in main memory
• StringReader / StringWriter, StringBufferInputStream
– read from or write to Strings in main memory
• FileReader/FileWriter
FileInputStream/FileOutputStream
– read from or write to a file on disc
File
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Common patterns of I/O-streams
• Working with streams follows a pattern, independent of the
data type, source, or sink
• Reading
• Open stream: implicit by creation
• Read data as long as data is needed and available
• Close stream
• After reading/writing, you should close() the stream!
• Writing
• Open stream
• Write data as long as required
• Close stream
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Common interfaces of I/O-streams
• Use I/O Streams only via common Interface
• Information hiding via subtype polymorphism
InputStream
public int read()
public int read(byte[] bbuf)
public int read(byte[] bbuf, int offset, int len)
Reader
public int read()
public int read(char[] cbuf)
public int read(char[] cbuf, int offset, int len)
OutputStream
public int write(int b)
public int write(byte[] bbuf)
public int write(byte[] bbuf, int offset, int len)
Writer
public int write(int c)
public int write(char[] cbuf)
public int write(char[] cbuf, int offset, int len)
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The java.io.Reader class
public
public
public
public
...
int read(char[] c) throws IOException
long skip(long n) throws IOException
void reset() throws IOException
int close() throws IOException
• public abstract int read(char[] c) throws
IOException
– reads the next Unicode characters into an array (c)
– Returns the number of characters read, or -1 if the end of the
stream has been reached ("EOF“, End of File)
– Any other problem causes an IOException,
• e.g., stream already closed, network connection lost, ...
For more details, see the API specification
http://java.sun.com/javase/6/docs/api/
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Java Documentation
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Prof. Dr. M. Mühlhäuser
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Java Documentation
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File Streams
• File streams represent I/O streams from and to files in the
file system:
– FileReader for reading and FileWriter for writing
character-wise from/to a file
– Similarly FileInputStream, FileOutputStream for
byte-wise reading/writing
• File streams are created by providing the source /
destination file by means of:
–
A file name (String)
–
A file object (java.io.File)
–
A file descriptor (java.io.FileDescriptor)
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Class java.io.FileReader
Example: print file contents to screen (as int) and write 'a' to 'z' to a file
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;
public class ReadWriteFile {
// constructor etc.
public void readFrom(String fileName) throws IOException {
FileReader in = new FileReader(fileName);
int b;
while ((b = in.read()) != -1) System.out.print(b);
in.close();
}
}
public void writeAToZ(String filename) throws IOException {
FileWriter out = new FileWriter(filename);
for (char c = 'a'; c <= 'z'; c++) out.write(c);
out.close();
}
// main() etc.
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Prof. Dr. M. Mühlhäuser
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©
Outline
• Introduction to Input/Output Streams and Java
Input/Output (I/O)
• Overview of Processing Streams
• Wrapping Streams and the Decorator Pattern
• User Defined Streams, Stream Tokenizer and
Random Access
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Processing Streams
• A processing stream contains another (data or processing) stream
– The latter is used as a data source or data sink, respectively
• Data might be transformed or functionality added
–
–
–
–
–
data buffering
counting lines
converting bytes and chars
compression
…
processing stream
device data stream
data drain
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data source
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Processing Streams: Overview
process
character streams
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byte streams
filtering
FilterReader,
FilterWriter
FilterInputStream,
FilterOutputStream
buffering
BufferedReader,
BufferedWriter
BufferedInputStream,
BufferedInputStream
byte / char
conversion
InputStreamReader,
OutputStreamWriter
counting
LineNumberReader
datatype
handling
©
byte  char
char  byte
LineNumberInputStream
DataInputStream,
DataOutputStream
undoing
PushbackReader
PushbackInputStream
printing
PrintWriter
PrintStream
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Roots of processing streams
• FilterReader/-Writer and
FilterInputStream/FilterOutputStream are general
super-classes for every processing stream
• They encapsulate an internal stream in
• Default implementations of an operation op:
pass on op() to the underlying stream
• concrete filter (sub-)classes refine this functionality
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Java Documentation
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Buffered Streams
• A buffered stream temporarily stores data from
another stream in an internal buffer
• Reading from a buffered stream fills the buffer
when it is empty
– Further "read"-operations will access the buffer without
reading from the attached stream
– When the buffer is empty, additional data will be read
from the underlying stream
• Writing to a buffered stream fills the buffer
before the attached data sink is written to
– The buffer is also emptied if the stream is flushed
explicitly (flush()) or closed (close())
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Buffered Streams
• BufferedReader and BufferedWriter
– Subclasses of Reader/Writer, respectively
• BufferedInputStream and
BufferedOutputStream
– Subclasses of FilterInputStream /
FilterOutputStream
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Buffered Streams
Data Sink
BufferedWriter
Program
write
write
b
bbuf
write bbuf
write bbuf
flush
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Buffered Streams: Performance
Reading a file of 2.5MB (x=buffer size, y=time [ms])
Buffer sizeTime[ms]
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6433
2
3453
4
1935
8
988
16
471
32
365
64
363
128
87
256
327
512
65
1024
162
2048
214
4096
260
8192
61
16384
166
32768
159
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Data Streams
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• A data stream lets an application read/write primitive Java data types
from/to an underlying I/O stream in a machine-independent way.
• An application uses a DataOutputStream to write data that can
later be read by a DataInputStream.
FilterInputStream
FilterOutputStream
<<interface>>
DataInput
<<interface>>
DataOutput
DataOutputStream
DataInputStream
readBoolean(): boolean
readByte(): byte
readShort() : short
readChar() : char
readInt() : int
readFloat() : float
writeBoolean(boolean) : void
writeByte(byte) : void
writeShort(short) : void
writeChar(char) : void
writeInt(int) : void
writeFloat(float) : void
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Data Streams: Example
import java.io.FileOutputStream;
import java.io.DataOutputStream;
import java.io.IOException;
public class DataStreamExample {
String fileName = // some name;
// some methods
public void writeData() throws IOException {
FileOutputStream fos = new FileOutputStream(filename);
DataOutputStream out = new DataOutputStream(fos);
out.writeInt(9);
out.writeDouble(Math.PI);
out.writeBoolean(true);
out.close();
}
// other methods
public void readData() throws IOException {
FileInputStream fis = new FileInputStream(fileName);
DataInputStream in = new DataInputStream(fis);
int i = in.readInt();
double d = in.readDouble();
boolean b = in.readBoolean();
in.close();
System.out.println("Read "+ i + ", " + d + ", and " + b+ ".");
}
}
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Standard-I/O
• Package java.lang contains the class System with the
following class variables:
static in
standard input
(keyboard)
static out
standard output
(screen)
static err
standard error messages
(screen)
• System.in yields an object of type InputStream
– enables to read() characters (bytes) from the keyboard
• System.out, System.err are of type PrintStream
– offer print(...) and println(...) to output data to
the screen
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PrintStream
• Adds the ability to conveniently
print user-readable representations
of various data values
FilterOutputStream
• Never throws an IOException
– Exceptional situations set an
internal flag that can be tested
via the checkError method
• All characters printed by a
PrintStream are converted into
bytes using the platform's default
character encoding
– PrintWriter should be used
in situations that require
writing characters rather than
bytes.
PrintStream
print(boolean) : void
print(double) : void
print(char) : void
print(double) : void
print(float) : void
...
println(Object): void
println(String): void
...
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Outline
• Introduction to Input/Output Streams and Java
Input/Output (I/O)
• Overview of Processing Streams
• Wrapping Streams and the Decorator Pattern
• User Defined Streams, StreamTokenizer and
Random Access
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The Architecture of Java Streams
©
• Streams can be wrapped around each other
–
Abstraction layers, where underlying “primitive” streams are used by the
enclosing (“higher”, more comfortable) streams
BufferedOutputStream
FileOutputStream
ZipOutputStream
Data sink,
e.g. file
// create a buffered compressed output stream to a file
OutputStream out = new FileOutputStream(filename);
out = new BufferedOutputStream(out);
out = new ZipOutputStream(out);
// ... more features can be dynamically added
// the stream features are not visible for clients
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The Architecture of Java Streams
• The technique that lets us combine streams
at runtime is of more general interest.
– A general way to extend objects with
new features dynamically
• In software technology, such kind of
techniques are documented as design
patterns.
– Reusable, documented design ideas
– More in the lecture „SE Design“
• The technique underlying the architecture of
Java I/O streams is known as the Decorator
Pattern.
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Outline
• Introduction to Input/Output Streams and Java
Input/Output (I/O)
• Overview of Processing Streams
• Wrapping Streams and the Decorator Pattern
• User Defined Streams, Stream Tokenizer and
Random Access
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User-Defined Processing Streams
• Sometimes it is useful to define streams that
process (e.g., filtering, statistics, production of
information chunks) data from other streams
• Best implemented by inheriting from
FilterReader/FilterInputStream or
FilterWriter/FilterOutputStream,
respectively
• In the following example, a process stream filters
out all lines which do not contain a certain
substring  Unix "grep" command
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User-Defined Processing Streams
import java.io.BufferedReader;
import java.io.FilterReader;
import java.io.IOException;
class GrepReader extends FilterReader {
String substring;
BufferedReader in;
GrepReader(BufferedReader reader, String pattern) {
super(reader);
in = reader;
substring = pattern;
}
// return the next line containing the search pattern
String readLine() throws IOException {
String line;
while (((line = in.readLine()) != null) &&
(line.indexOf(substring) == -1)) ;
return line;
}
}
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User-Defined Processing Streams
import java.io.*;
public class Grep {
public static void main(String[] args) {
if ((args.length == 0) || (args.length > 2)) {
System.out.println("Usage: java Grep <substring> [<filename>]");
System.exit(0);
}
try {
Reader d;
if (args.length == 2)
d = new FileReader(args[1]);
else
d = new InputStreamReader(System.in);
GrepReader g = new GrepReader(new BufferedReader(d), args[0]);
String line;
while ((line = g.readLine()) != null) System.out.println(line);
g.close();
}
catch (IOException e) {
System.err.println(e);
}
}
}
stream being
searched
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Random Access
• Until now, we have only considered sequential
streams that could only be read or written
sequentially
• Sequential streams are a good fit for sequential
storage media, e.g. magnetic tapes
• Random Access Files allow for non-sequential
access (random access) to the contents of a file
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Motivating Example for Random Access
• Let us regard the file format of a ZIP archive:
– ZIP archives contain files and are usually compressed to
save disk space
– Apart from the files, there is an additional entry at the
end of the file, the so-called dir-entry (for directory)
– The dir-entry is used to keep track of which files are
contained and where – within the archive – they begin
• How to extract one file?
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Motivating Example for Random Access
• Extracting a file using a sequential stream:
– Search the whole archive until the file is found
– Extract the file
• On average: half of the entries are searched…
• Extracting a file using a random access stream:
– Jump to the directory & read the entry of the file
– Jump to the recorded position of the file
– Extract the file
• Only two entities (directory & file) need to be read
• This approach is far more efficient
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Random Access in Java
• Realized by the class RandomAccessFile
• Can be used for reading and writing—implements the
interfaces DataInput and DataOutput
• Similar to FileInputStream and FileOutputStream,
you open a RandomAccessFile on a file and pass a file
name or a File object as a parameter
• Additionally, you have to pass a parameter to specify
whether the file is opened for read-only or also for
writing
– You have to be able to read a file to be able to write to it
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The Class RandomAccessFile
• Creation a RandomAccessFile for reading file
“random.txt”:
– new RandomAccessFile("random.txt", "r");
• Creating a RandomAccessFile for reading and
writing on “random.txt”:
– new RandomAccessFile("random.txt", "rw");
• Afterwards, read and write operations may be used
to read and write data from/to the file
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The Class RandomAccessFile
• RandomAccessFile supports a file pointer: points to the
current position in a file
• Upon creation, the pointer is 0, i.e., beginning of the file
• Calls to read and write operations automatically move the file
pointer by the number of read, respectively written bytes
• RandomAccessFile also supports positioning operations
• int skipBytes(int n) throws IOException
moves read/write position by n bytes (relative positioning)
• native void seek(long pos) throws IOException
positions read/write position right before pos (absolute positioning)
• native long getFilePointer() throws IOException
returns the current read/write position
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Stream Tokenizer
• java.io.StreamTokenizer supports the
generation of tokens out of character sequences
• A token is obtained by calling nextToken()
– Ignores Whitespace
• User can define what is to be regarded as whitespace
– The type of the token read in is stored in the attribute
ttype
• The token types are explained on the next slide
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StreamTokenizer: Token Types
• Basic token types:
– StreamTokenizer.TT_NUMBER – number scanned; the number
is stored in nval as a double
– StreamTokenizer.TT_WORD – a composed word is recognized
which is stored in sval
– StreamTokenizer.TT_EOL – end of line, if the tokenizer is
configured to recognize EOL as a token
(eolIsSignificant(true))
– StreamTokenizer.TT_EOF – end of file
– Any other value is an encoding of the character read in
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Customizing StreamTokenizer
• public void wordChars(int low, int hi)
– Defines what is to be regarded as a word
– All characters in the interval [low, hi] are "word parts“
• public void whitespaceChars(int low, int
hi)
– Defines whitespace
– All characters in the interval [low, hi] are "space" characters
• public void eolIsSignificant(boolean flag)
– Defines whether the end of line character is of interest
• public void quoteChar(char c)
– Defines quote character
• See the API documentation for more features…
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StreamTokenizer: Example
• Read a list of tea data with the
following format:
– Each line contains data for one
type of tea, i.e., the end of line
token is of relevance
– Data items per tea type
(separated by spaces)
• Tea name in quotes
• Number of seconds required
• Number of recommended tea
spoons per liter
"Ali Baba's 40 scents" 180 5
"Asatsuyu" 90 7
"Generic Black Tea" 180 5
"Caramel" 120 6
"Ceylon Pekoe" 120 6
"China Jasmin" 120 6
"Chinese Love Dream" 150 5
"One For All" 540 1
"Cherry Cream" 120 6
• What would be a complex program
without further support, is easily
written with the help of
StreamTokenizer
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StreamTokenizer Example
void parseTeas(InputStream is) throws IOException {
StreamTokenizer stok = new StreamTokenizer(is);
stok.eolIsSignificant(true); // EOL is important!
stok.quoteChar('\"'); // quote char
int token = 0;
String teaName = null;
int nrSecs, nrSpoons;
while ((token = stok.nextToken()) != StreamTokenizer.TT_EOF) {
teaName = stok.sval;
// 1. token (name)
token = stok.nextToken();
nrSecs = (int)stok.nval;
// 2. (secs)
token = stok.nextToken();
nrSpoons = (int)stok.nval; // 3. (spoons)
token = stok.nextToken(); // consume EOL
System.out.println(teaName + "=>" + nrSpoons +
" spoons, seconds: " + nrSecs);
}
}
Normally, you should check the type actually read in before assigning it!
Introduction to Computer Science I: T20
50
Dr. G. Rößling
Prof. Dr. M. Mühlhäuser
RBG / Telekooperation
©
Scanner
• Since Java 1.5, the Scanner class can be used for parsing
• A Scanner can be opened on a Stream, file, …
– Typical use: Scanner sc = new Scanner(System.in);
• The class offers many helpful methods, such as…:
– String next(); // returns the next token
– x nextX(); // for x = byte, double, float, int, long, short, Line
• Throws InputMismatchException if the type is not found
– boolean hasNextX(); // same x as above
• Checks if the next element would match type X
– You can redefine the delimiter, even to words
– Additionally, regular expressions can be used for matching
• Check the API documentation for more information
Introduction to Computer Science I: T20
51
Dr. G. Rößling
Prof. Dr. M. Mühlhäuser
RBG / Telekooperation
©
What else?
• Encrypted files, compressed files, files sent over
internet connections, ...
• Exceptions! All I/O involves exceptions!
– See part T16 for information on handling exceptions
try {
statements involving I/O
}
catch (IOException e) {
e.printStackTrace();
}
Introduction to Computer Science I: T20
52