Io Java Ver2
Transcript of Io Java Ver2
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I/O Basics
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Aside from print( ) and println( ), none of the
I/O methods have been used significantly. The
reason is simple: most real applications of Javaare not text-based, console programs.
they are graphically oriented programs that rely
upon Javas Abstract Window Toolkit (AWT) forinteraction with the user.
Javas support for console I/O is limited andsomewhat awkward to use even in simple
example programs. Text-based console I/O is
just not very important to Java programming.
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Streams Java programs perform I/O through streams.
A stream is an abstraction that eitherproduces or
consumes information.
A stream is linked to a physical device by the Java
I/O system. All streams behave in the same manner, even if the
actual physical devices to which they are linked
differ.
Same I/O classes and methods can be applied to
any type of device.
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Streams(contd.) Input stream can abstract many different kinds of
input: from a disk file, a keyboard, or a networksocket.
Output stream may refer to the console, a disk file,
or a network connection. Streams deal with input/output without having every
part of your code understand the difference
between a keyboard and a network.
Java implements streams within class hierarchiesdefined in thejava.io package.
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Byte Streams and Character Streams Java 2 defines two types of streams: byte and
character. Byte streams providemeans for handling input and
output of bytes and are used for reading or writing
binary data.
Character streams handle input and output ofcharacters. They use Unicode and, therefore, can
be internationalized.
In some cases, character streams are more efficientthan byte streams.
At the lowest level, all I/O is byte-oriented
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The Byte Stream Classes
Byte streams are defined by using two class
hierarchies. At the top are two abstract classes:InputStream and OutputStream.
Each of these abstract classes has several concrete
subclasses, that handle the differences between
various devices, such as disk files, networkconnections, and even memory buffers.
to use the stream classes, you must importjava.io.
The abstract classes InputStream andOutputStream define several key methods that the
other stream classes implement. Two of the most
important are read( ) and write( ),
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which, respectively, read and write bytes of data.
Both methods are declared as abstract inside
InputStream and OutputStream.
They are overridden by derived stream classes.
The Character Stream Classes
Character streams are defined by using two class
hierarchies.
At the top are two abstract classes, Readerand
Writer, which define several key methods that theother stream classes implement.
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These abstract classes handle Unicode character
streams.
Java has several concrete subclasses of each ofthese.
Two of the most important methods are read( ) and
write( ), which read and write characters of data,
respectively.
These methods are overridden by derived stream
classes
First slide is Byte Stream classes Second slide is the character stream I/O classes
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The Predefined Streams java.lang package defines a class called System,
which encapsulates several aspects of the run-timeenvironment.
Using some of its methods, you can do the settings
of various properties associated with the system. System also contains three predefined stream
variables, in, out, and err.
These fields are declared as public, static and final
within System.
They can be used by any other part of the program
and without reference to a specific System object.
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System.out refers to the standard output stream. By
default, this is the console.
System.in refers to standard input, which is the
keyboard by default.
System.errrefers to the standard error stream, which
also is the console by default.
These streams may be redirected to any compatibleI/O device.
System.in is an object of type InputStream
System.out and System.errare objects of typePrintStream.
These are byte streams, used to read and write
characters from and to the console.
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Reading Console Input Console input is accomplished by reading from
System.in. To obtain character-based stream that is attached to
the console, you wrap System.in in a
BufferedReaderobject.
BuffereredReadersupports a buffered input stream.
Constructor is shown here:
BufferedReader(ReaderinputReader)
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System.in refers to an object of type InputStream,
it can be used forinputStream.
The following line of code creates aBufferedReaderthat is connected to the keyboard: BufferedReader br = new BufferedReader(new
InputStreamReader(System.in));
After this statement executes, bris a character-based stream that is linked to the console through
System.in.
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Reading Characters
To read a character from a BufferedReader, use
read( ).
int read( ) throws IOException
It reads a character from the input stream and
returns it as an integer value.
It returns1 when the end of the stream isencountered. It can throw an IOException.
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// Use a BufferedReader to read characters from the console.
import java.io.*;
class BRRead {
public static void main(String args[]) throws IOException {char c;
BufferedReader br = new
BufferedReader(new InputStreamReader(System.in));
System.out.println("Enter characters, 'q' to quit.");
// read charactersdo {
c = (char) br.read();
System.out.println(c);
} while(c != 'q');}
}
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Enter characters, 'q' to quit.
123abcq
1
2
3
a
bc
q
System.in is line buffered, by default. This means that no
input is actually passed to the program until you pressENTER.
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//Read a string from console using a BufferedReader
import java.io.*;
class BRReadLines {
public static void main(String args[]) throws IOException {// create a BufferedReader using System.in
BufferedReader br = new BufferedReader(new
InputStreamReader(System.in));
String str;
System.out.println("Enter lines of text.");
System.out.println("Enter 'stop' to quit.");
do {
str = br.readLine();
System.out.println(str);} while(!str.equals("stop"));
}
}
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import java.io.*; //A tiny Editor
class TinyEdit {
public static void main(String args[]) throws IOException {
// create a BufferedReader using System.inBufferedReader br = new BufferedReader(new
InputStreamReader(System.in));
String str[] = new String[100];
System.out.println("Enter lines of text.");
System.out.println("Enter 'stop' to quit.");
for(int i=0; i
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Here is a sample run:
Enter lines of text.
Enter 'stop' to quit.This is line one.
This is line two.
Just create String objects.stop
Here is your file:
This is line one.
This is line two.
Just create String objects
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Writing Console Output
PrintStream is an output stream derived fromOutputStream, it implements the low-level
method write( ).
write( ) can be used to write to the console:
void write(int byteval)
This method writes to the stream the byte
specified by byteval.
byteval is declared as an integer, only the low-order eight bits are written.
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class WriteDemo {
public static void main(String args[]) {
int b;
b = 'A';
System.out.write(b);
System.out.write('\n');
}
}
Writes A to the console and adds a new line.
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Reading and Writing Files In Java, all files are byte-oriented
FileInputStream and FileOutputStream create bytestreams linked to files
To open a file, create an object of one of these classes,
specifying the name of the file as an argument to the
constructor
FileInputStream (String fileName) throws
FileNotFoundException
FileOutputStream (String fileName) throws
FileNotFoundException
void close( ) throws IOException
int read( ) throws IOException
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/* Display a text file. Specify the name of the file that you want
to see. For example, to see a file called TEST.TXT, use the
following command line.
java ShowFile TEST.TXT */
import java.io.*;
class ShowFile {
public static void main(String args[]) throws IOException {
int i;
FileInputStream fin;
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try {
fin = new FileInputStream(args[0]);
} catch(FileNotFoundException e) {
System.out.println("File Not Found");return;
} catch(ArrayIndexOutOfBoundsException e) {
System.out.println("Usage: ShowFile File");
return;
}
// read characters until EOF is encountered
do {
i = fin.read( );
if(i != -1) System.out.print((char) i);} while(i != -1);
fin.close( );
}
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To write to a file, use write( ) method defined by
FileOutputStream.
void write(int byteval) throws IOException
/* Copy a text file. To use this program, specify the name of the
source file and the destination file.
For example, to copy a file called FIRST.TXT to a file calledSECOND.TXT, use the following command line.
java CopyFile FIRST.TXT SECOND.TXT
*/
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import java.io.*;
class CopyFile {
public static void main(String args[]) throws IOException {
int i;FileInputStream fin;
FileOutputStream fout;
try {
//open input file
try {
fin = new FileInputStream(args[0]);
} catch(FileNotFoundException e) {
System.out.println("Input File Not Found");
return;}
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//open output file
try {
fout = new FileOutputStream(args[1]);
} catch(FileNotFoundException e) {System.out.println("Error Opening Output File");
return;
}
} catch(ArrayIndexOutOfBoundsException e) {
System.out.println("Usage: CopyFile From To");
return;
}
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//Copy File
try {
do {
i = fin.read( );
if(i != -1) fout.write(i);
} while(i != -1);
} catch(IOException e) {
System.out.println("File Error");
}
fin.close();
fout.close();}
}
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