Files and Streams
Dr. Jürgen Eckerle
Files and Streams
• System input and output through data streams,
serialization and the file system is provided by
the package java.io.
• It must be imported by
import java.io.*;
• Streams represent an abstract input or output
device.
• There are super classes InputStream and
OutputStream, Reader and Writer.
• Concrete subclasses link the access routines to
real input or output devices like files, strings or
communication channels.
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Files and Streams
• Streams can be concatenated and nested.
• Nested streams allow to realize filters with
extended functionalities.
• There are two categories of stream classes:
– Byte streams (based on 8 Bit units)
– Character streams (based on 16 Bit Unicode
characters and realized with JDK 1.1).
Files and Streams
• Java uses the 16 Bit Unicode for internal
representation of characters, and
• the 8 Bit characters for external communication
with files.
• There are a set of bridge classes converts
Character streams into Byte streams and vice
versa.
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Character Streams
The abstract class Writer contains the following
methods:
protected Writer()
public void close()
public void flush()
public void write(int c)
public void write(char[] cbuf)
abstract public void write(char[] cbuf, int off, int len)
public void write(String str)
public void write(String str, int off, int len)
Character Streams
• The constructor Writer() allows to open the
output stream and to prepare it for the following
write(...) commands.
• close() closes the output stream.
• The write(..) command is overloaded and is
used to write the given input value to the output
stream.
• The write(int) command allows to write an int
value as a byte to the output stream. All the
other write(...) commands are based on it.
• flush() forces to write characters possibly
currently stored in a buffer to the output device.
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Character Streams
There are a set of concrete subclasses of the
abstract class Writer, for example:
• OutputStreamWriter a superclass for all Writer
classes which convert a character stream into a
byte stream.
• FileWriter allows to print the output into a file.
• FilterWriter
• PrintWriter
• BufferedWriter
Character Streams
BufferedWriter
• is used to store the characters sent to the output
stream in an internal buffer.
• The internal stored characters are written to the
buffer (file) if either
– the buffer is full, or
– the flush() command has been explicitely
called.
• Useful if the output is written into a file, since the
number of external accesses will be reduced.
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Character Streams
PrintWriter
• allows to print both as the primitive types as the
objects in text form.
FilterWriter
• an abstract superclass which is used to
implement self-defined Filter classes as a
subclass of it.
BufferedWriter, PrintWriter and FilterWriter can be
used to realize nested streams.
Exercise 1
• Write a small programm to write the string “Hallo
Java“ into a text file. End of line character is
“\r\n“.
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Character Streams
Nested streams.
• BufferedWriter, PrintWriter and FilterWriter can
be used to realize nested streams.
• For that reason, these classes have a member
variable of type Writer which is initialized with
the constructor.
• For example in the class BufferedWriter:
– public BufferedWriter(Writer out)
– public BufferedWriter(Writer out, int size)
Character Streams
Nested streams.
• Calling the write method means that
– first, apply the filter functions realized in the own class
and
– then, apply the write(...) method of the superclass.
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Character Streams – Examples
Writer f1;
BufferedWriter f2;
String s;
try {
f1 = new FileWriter("buffer.txt");
f2 = new BufferedWriter(f1);
for (int i = 1; i <= 10000; ++i) {
s = "Dies ist die " + i + ". Zeile";
f2.write(s);
f2.newLine();
}
f2.close();
f1.close();
} catch (IOException e) {
System.out.println("Fehler beim Erstellen der Datei");
}
PrintWriter f;
double sum = 0.0;
int nenner;
try {
f = new PrintWriter(
new BufferedWriter(
new FileWriter("zwei.txt")));
for (nenner = 1; nenner <= 1024; nenner *= 2) {
sum += 1.0 / nenner;
f.print("Summand: 1/");
f.print(nenner);
f.print(" Summe: ");
f.println(sum);
}
f.close();
} catch (IOException e) {
System.out.println("Fehler beim Erstellen der Datei");
}
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Exercise 2
• Write a small programm to write a sequence of
values of type int into a text file.
Exercise 3
• UpCaseWriter.java print a text in uppercase into
text file. Study the listing of it. Explain why it
works.
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import java.io.*;
class UpCaseWriter extends FilterWriter
{
public UpCaseWriter(Writer out) {
super(out);
}
public void write(int c) throws IOException {
super.write(Character.toUpperCase((char)c));
}
public void write(char[] cbuf, int off, int len) throws IOException {
for (int i = 0; i < len; ++i) {
write(cbuf[off + i]);
}
}
public void write(String str, int off, int len) throws IOException {
write(str.toCharArray(), off, len);
}
}
public class Exercise3
{
public static void main(String[] args) {
PrintWriter f;
String s = "und dieser String auch";
try {
f = new PrintWriter( new UpCaseWriter( new FileWriter("upcase.txt")));
//Aufruf von außen
f.println("Diese Zeile wird schön groß geschrieben");
//Test von write(int)
f.write('a'); f.println();
//Test von write(String)
f.write(s); f.println();
//Test von write(String, int, int)
f.write(s,0,17); f.println();
//Test von write(char[], int, int)
f.write(s.toCharArray(),0,10); f.println();
//--f.close();
} catch (IOException e) {
System.out.println("Fehler beim Erstellen der Datei");
}
}
}
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Character Streams
The abstract class Reader contains the following
methods:
public Reader()
public void close()
public void mark(int readAheadlimit)
public boolean markSupported()
public int read()
public int read(char[] cbuf)
public int read(char[] cbuf, int off, int len)
public long skip(long n)
public boolean ready()
public void reset()
Character Streams
• The constructor Reader() allows to open the
input stream and to prepare it for the following
read(...) commands.
• close() closes the input stream.
• The read(...) method is overloaded and is used
to read a character from a given input stream.
• The read() method reads the next character and
returns as an int value in the range [0, 65535]. A
return value -1 indicates that the end of the input
stream is reached.
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Character Streams
• The read(char[ ], int, int) method reads a set of
characters and fill the given input array char.
The return value yield the number of input
values readed from the input stream. A return
value -1 indicates that the end of the input
stream is reached.
• The method boolean ready() returns the value
true, if the next character can be read without
being blocked.
Character Streams
• The method markSupported() checks whether
marking a position is supported.
• If marking is supported a given position can be
marked by calling the method mark(int). The
input value readAheadLine specifies the number
of characters which can be read until the mark is
invalid.
• If a position is marked, the position index is set
to the marked position by calling reset().
• The method skip(int) allows to skip a given
number of input value. If it is not possible to skip
the specified number of characters, the return
values yields the actual number.
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Character Streams
Subclasses of the abstract class Reader:
• InputStreamReader a superclass of all Reader
classes which converts a byte stream into a
character stream.
• FileReader allows to read the input stream from
a file.
• FilterReader: An abstract class for self-defining a
Flterclass.
• BufferedReader
• PushbackReader
Character Streams
• StringReader and CharArrayReader allows to
read from a string object resp. a char array.
Reader f;
int c;
String s;
s = "Das folgende Programm zeigt die Verwendung\r\n";
try {
f = new StringReader(s);
while ((c = f.read()) != -1) {
System.out.print((char)c);
}
f.close();
} catch (IOException e) {
System.out.println("Fehler beim Lesen des Strings");
}
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Exercise 4
• Write a program which corresponds with the
Exercise 1, that is, a program which reads a
string from a text file and print it with the method
System.out.print() on the command line.
• Write a program which corresponds with
Exercise 2, that is, a program which reads read
a sequence of integers and print them with the
method System.out.println().
Character Streams
Nested Readers.
• The Reader classes
–
–
–
–
BufferedReader,
LineNumberReader,
FilterReader, and
PushbackReader
• can be used, since they have a member variable
Reader which is initialized by the constructor.
• In contrast to the Writer reading means
– first, apply the read method from the inner Reader,
– then, apply the read method of the Filter Reader.
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Character Streams
• LineNumberReader extends the functionality of the
BufferedReader by ability to count the number of input
lines during the read process. There are two additional
methods
– int getLineNumber()
– void setLineNumber(int)
• FilterReader is used as a superclass which a selfdefined FilterReader class has to extend.
• PushbackReader allows to give a character back to the
input stream (by unread(...)) after a call of the read()
method. This class is used, if the next character must be
known to make the right decision, without processing it.
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