Chapter 16 Simple Input and Output

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Dec 2, 2013 (3 years and 6 months ago)

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Chapter 16 Simple Input and Output


Prerequisites for Part IV


Chapter 8 Inheritance and Polymorphism

Chapter 16 Simple Input and Output

Chapter 15 Exceptions and Assertions

与人玫瑰,手有余香

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Objectives


To discover file properties, delete and rename files using the
File

class
(
§
16.2).


To understand how I/O is processed in Java (
§
16.3).


To distinguish between text I/O and binary I/O (
§
16.3).


To read and write characters using
FileReader

and
FileWriter

(
§
16.4).


To improve the performance of text I/O using
BufferedReader

and
BufferedWriter

(
§
16.4).


To write primitive values, strings, and objects as text using
PrintWriter

and
PrintStream

(
§
16.4).


To read and write bytes using
FileInputStream

and
FileOutputStream
(
§
16.6).


To read and write primitive values and strings using
DataInputStream
/
DataOutputStream

(
§
16.6).


To store and restore objects using
ObjectOutputStream

and
ObjectInputStream
,
and to understand how objects are serialized and what kind of objects can be
serialized (
§
16.9 Optional).


To use the
Serializable

interface to enable objects to be serializable (
§
16.9
Optional).


To use
RandomAccessFile

for both read and write. (
§
16.10 Optional)



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Introduction


Files


Long
-
term storage of large amounts of data


Persistent data exists after termination of program


Files stored on secondary storage devices


Magnetic disks


Optical disks


Magnetic tapes


Sequential and random access files

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The File Class


The
File

class is intended to provide an abstraction that
deals with most of the machine
-
dependent complexities
of files and path names in a machine
-
independent fashion.
The filename is a string.



The
File
class is a wrapper class for the file name and
its directory path.

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java.io.File


+File(pathname: String)

+File(parent: String, child: String)

+File(parent: File, child: String)

+exists(): boolean

+canRead(): boolean

+canWrite(): boolean

+isDirectory(): boolean

+isFile(): boolean

+isAbsolute(): boolean

+isHidden()
: boolean



+getAbsolutePath(): String

+getCanonicalPath(): String


+getName(): String


+getPath(): String


+getParent(): String


+lastModified(): long

+delete(): boolean

+renameTo(dest: File): boolean


Creates a File object for the specified pat
hname. The pathname may be a
directory or a file.

Creates a File object for the child under the directory parent. child may be a
filename or a subdirectory.

Creates a File object for the child under the directory parent. parent is a File
object. In the pre
ceding constructor, the parent is a string.

Returns true if the file or the directory represented by the File object exists.

Returns true if the file represented by the File object exists and can be read.

Returns true if the file represented by the File o
bject exists and can be written.

Returns true if the File object represents a directory.

Returns true if the File object represents a file.

Returns true if the File object is created using an absolute path name.

Returns true if the file represented in the
File object is hidden. The exact
definition of
hidden
is system
-
dependent. On Windows, you can mark a file
hidden in the File Properties dialog box. On Unix systems, a file is hidden if
its name begins with a period character '.'.

Returns the complete abs
olute file or directory name represented by the File
object.

Returns the same as getAbsolutePath() except that it removes redundant
names, such as "." and "..", from the pathname, resolves symbolic links (on
Unix platforms), and converts drive letters to s
tandard uppercase (on Win32
platforms).

Returns the last name of the complete directory and file name represented by
the File object. For example, new File("c:
\
\
book
\
\
test.dat").getName() returns
test.dat.

Returns the complete directory and file name repre
sented by the File object.
For example, new File("c:
\
\
book
\
\
test.dat").getPath() returns c:
\
book
\
test.dat.

Returns the complete parent directory of the current directory or the file
represented by the File object. For example, new
File("c:
\
\
book
\
\
test.dat"
).getParent() returns c:
\
book.

Returns the time that the file was last modified.

Deletes this file. The method returns true if the deletion succeeds.

Renames this file. The method returns true if the operation succeeds.


Obtaining file
properties and
manipulating file

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Example 16.1 Using the File Class

TestFileClass

Run

Objective: Write a program that demonstrates how to
create files in a platform
-
independent way and use the
methods in the File class to obtain their properties. Figure
16.1 shows a sample run of the program on Windows, and
Figure 16.2 a sample run on Unix.

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How is I/O Handled in Java?

A
File

object encapsulates the properties of a file or a path, but does not
contain the methods for reading/writing data from/to a file. In order to
perform I/O, you need to create objects using appropriate Java I/O classes.


Program

Input object

created from an
input class

Output object

created from an
output class

Input stream

Output stream

File

File

FileWriter output = new FileWriter("temp.txt");


output.write("Java 101");


output.close();


FileReader input = new FileReader("temp.txt");

int code = input.read();

System.out.println((char)code);

Return the
unicode of the
char

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Coding Essentials


public static void main(String[] args)


throws IOException
{

FileWriter output =

new FileWriter("temp.txt");

output.write("Java 101");

output.close();


FileReader input =

new FileReader("temp.txt");

int code = input.read();

S
ystem.out.println((char)code);

input.close();

}

Declaring exception in the method

public static void main(String[] args) {


try
{

FileWriter output =

new FileWriter("temp.txt");

output.write("Java 101");

o
utput.close();


FileReader input =

new FileReader("temp.txt");

int code = input.read();

System.out.println((char)code);

input.close();

}


catch
(IOException ex) {

ex.printStackTrace();

}

}

Using try
-
catch block

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Text File vs. Binary File


Data stored in a text file are represented in human
-
readable form.
Data stored in a binary file are represented in binary form. You
cannot read binary files. Binary files are designed to be read by
programs. For example, the Java source programs are stored in
text files and can be read by a text editor, but the Java classes
are stored in binary files and are read by the JVM.
The
advantage of binary files is that they are more efficient to
process than text files.



you can imagine that
a text file consists of a sequence of
characters and a binary file consists of a sequence of bits
.

For
example, the decimal integer 199 is stored as the sequence of
three characters: '1', '9', '9' in a text file and the same integer is
stored as a
byte
-
type value
C7

in a binary file, because decimal
199

equals to hex
C7
.

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Text I/O Classes


Reader

Writer

Object

PrintWriter

BufferedWriter

FileReader

FileWriter

InputStreamReader

BufferedReader

OutputStreamWriter

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Reader

The value is returned as a Unicode.


java.io.Reader


+read(): int


+read(cbuf: char[]): int

+read(cbuf: char[], off:
int, len: int): int


+close(): void

+skip(n: long): long

+markSupported(): boolean

+mark(readlimit: int): void

+reset(): void


Reads the next character from the input
stream. The value returned is an int in
the range from 0 to 65535, which represents a Unicode character. Returns
-
1
at the end of the stream.

Reads characters from the input stream into an array. Returns the actual
number of characters read. Returns
-
1 at
the end of the stream.

Reads characters from the input stream and stores into cbuf[off], cbuf[off+1],
…, cbuf[off+len
-
1]. The actual number of bytes read is returned. Returns
-
1
at the end of the stream.

Closes this input stream and releases any system
resources associated with the
stream.

Skips over and discards n characters of data from this input stream. The actual
number of characters skipped is returned.

Tests if this input stream supports the mark and reset methods.

Marks the current position in t
his input stream.

Repositions this stream to the position at the time the mark method was last
called on this input stream.


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Writer

The Unicode value.


java.io.Writer


+write(int c): void

+write(cbuf:
char
[]): void

+write(cbuf: char[], off:
int, len: int): void

+write(str: String): void

+write(str: String, off: int,
len: int): void

+close(): void

+flush(): void


Writes the specified character to
this output stream. The parameter c is the
Unicode for a character.

Writes all the characters in array cbuf to the output stream.

Writes cbuf[off], cbuf[off+1], …, cbuf[off+len
-
1] into the output stream.

Writes the characters from the string into the ou
tput stream.

Writes a portion of the string characters into the output stream.

Closes this
out
put stream and releases any system resources associated with the
stream.

Flushes this output stream and forces any buffered output characters to be written
out.


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FileReader/FileWriter

FileReader/FileWriter associates an
input/output stream with an external file.
All the methods in FileReader/FileWriter
are inherited from its superclasses.


Reader

Writer

Object

PrintWriter

BufferedWriter

FileReader

FileWriter

InputStreamReader

BufferedReader

OutputStreamWriter

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FileReader

To construct a FileReader, use the following constructors:

public FileReader(String filename)

public FileReader(File file)


A
java.io.FileNotFoundException

would occur if you attempt to
create a
FileReader

with a nonexistent file.

TestFileReader

Run

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FileWriter

To construct a FileWriter, use the following constructors:


public FileWriter(String filename)

public FileWriter(File file)

public FileWriter(String filename, boolean append)

public FileWriter(File file, boolean append)



If the file does not exist, a new file would be created. If the file already
exists, the first two constructors would delete the current contents in
the file. To retain the current content and append new data into the file,
use the last two constructors by passing true to the append parameter.

TestFileWriter

Run

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InputStreamReader/OutputStreamWriter

Optional


Reader

Writer

Object

PrintWriter

BufferedWriter

FileReader

FileWriter

InputStreamReader

BufferedReader

OutputStreamWriter

All the methods in
InputStreamReader
/
OutputStreamWriter

are inherited from
Reader
/
Writer

except
getEncoding()
, which
returns the name of encoding being used by this stream.

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InputStreamReader/OutputStreamWriter

InputStreamReader
/
OutputStreamWriter

are used to convert between
bytes and characters.
Characters written to an
OutputStreamWriter

are encoded into bytes using a specified encoding scheme
.
Bytes read
from an
InputStreamReader

are decoded into characters using a
specified encoding scheme.

You can specify an encoding scheme
using a constructor of
InputStreamReader
/
OutputStreamWriter
. If no
encoding scheme is specified, the system’s default encoding scheme is
used.

Optional


Program

The Unicode of
the character is
returned

A character is converted
into the Unicode

The Unicode of
the character is
sent out

A character stored in
a specified encoding

A character is converted into the
code for the specif
ied encoding

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BufferedReader/BufferedWriter


Reader

Writer

Object

PrintWriter

BufferedWriter

FileReader

FileWriter

InputStreamReader

BufferedReader

OutputStreamWriter

The buffered stream classes inherit methods from their superclasses.
In addition to using the methods from their superclasses,
BufferedReader

has a
readLine()

method to read a line,

and
BufferedWriter

has a
newLine()

method to write a line separator
. If
the end of stream is reached,
readLine()

returns
null
.

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PrintWriter/PrintStream


Reader

Writer

Object

PrintWriter

BufferedWriter

FileReader

FileWriter

InputStreamReader

BufferedReader

OutputStreamWriter

BufferedWriter

is used to output characters and strings
.
PrintWriter

and
PrintStream

can be used to output objects, strings and numeric values as text.

PrintWriter

was introduced in JDK 1.2 to replace
PrintStream
.

Both classes
are almost identical in the sense that they provide the same function and same
methods for outputting strings and numeric values as text.
PrintWriter

is more
efficient than
PrintStream
. So, you use
PrintWriter

rather than
PrintStream
.

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Methods in PrintWriter/PrintStream


public void print(Object o)

public void print(String s)

public void print(char c)

public void print(char[] cArray)

public void print(int i)

public void print(long l)

public void print(float f)

public void print(double d)

public void print(boolean b)


p
ublic void println(Object o)

public void println(String s)

public void println(char c)

public void println(char[] cArray)

public void println(int i)

public void println(long l)

public void println(float f)

public void println(double d)

public void println(
boolean b)


PrintWriter and PrintStream also contain the JDK 1.5 printf
method for printing formatted output, which was introduced
in Section 2.17, “Formatted Output.”

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Constructing PrintWriter

This section introduces PrintWriter, but PrintStream can be used
in the same way. To construct a PrintWriter, use the following
constructors:


public PrintWriter(Writer out)

public PrintWriter(Writer out, boolean autoFlush)



If autoFlush is true, the println methods will cause the buffer to be
flushed.


The constructors and methods in
PrintWriter

and
PrintStream

do
not throw an
IOException
. So you don’t need to invoke them from
a
try
-
catch

block.

TestPrintWriter

Run

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Case Studies: Text Viewer

This case study writes a program that views a text file in a text area.
The user enters a filename in a text field and clicks the View button;
the file is then displayed in a text area.

FileViewer

Run

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Binary I/O


Text I/O requires encoding and decoding.
The JVM converts a
Unicode to a file specific encoding when writing a character

and
coverts a file specific encoding to a Unicode when reading a
character.



Binary I/O does not require conversions.

When you write
a byte to a file, the original byte is copied into the file. When you read
a byte from a file, the exact byte in the file is returned.


Text I/O

The Unicode of
the character

Encoding/

Decoding

The encoding of the character is
stored in the file

Binary I/O

A byte is
read/written

The same byte in the file

Binary I/O is more
efficient

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Binary I/O Classes


InputStream

OutputStream

Object

ObjectOutputSt
ream

FilterOutputStream

FileOutputStream

BufferedInputStream

DataInputStream

BufferedOutputStream

DataOutputStream

PrintStream

ObjectInputStream

FilterInputStream

FileInputStream

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java.io.InputStream


+read(): int


+read(b: byte[]): int

+read(b: byte[], off: int,
len: int): int

+available(): int

+close(): void

+skip(n: long): long

+markSupported(): boolean

+mark(readlimit: int): void

+reset(): void


Reads the next byte of d
ata from the input stream. The value byte is returned as
an int value in the range 0 to 255. If no byte is available because the end of
the stream has been reached, the value

1 is returned.

Reads up to b.length bytes into array b from the input stream and
returns the
actual number of bytes read. Returns
-
1 at the end of the stream.

Reads bytes from the input stream and stores into b[off], b[off+1], …,
b[off+len
-
1]. The actual number of bytes read is returned. Returns
-
1 at the
end of the stream.

Returns
the number of bytes that can be read from the input stream.

Closes this input stream and releases any system resources associated with the
stream.

Skips over and discards n bytes of data from this input stream. The actual
number of bytes skipped is returne
d.

Tests if this input stream supports the mark and reset methods.

Marks the current position in this input stream.

Repositions this stream to the position at the time the mark method was last
called on this input stream.


The value returned is a byte as an int type.

InputStream

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The value is a byte as an int type.

OutputStream


java.io.OutputStream


+write(int b): void

+write(b: byte[]): void

+write(b: byte[], off: int,
len: int): void

+close(): void

+flush(): void


Writes the specified byte to this output stream. The parameter b is an int value.
(byte)b is written to th
e output stream.

Writes all the bytes in array b to the output stream.

Writes b[off], b[off+1], …, b[off+len
-
1] into the output stream.

Closes this input stream and releases any system resources associated with the
stream.

Flushes this output stream and
forces any buffered output bytes to be written out.


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FileInputStream/FileOutputStream

FileInputStream/FileOutputStream
associates a binary input/output stream
with an external file
.

All the methods in
FileInputStream/FileOuptputStream are
inherited from its superclasses.


InputStream

OutputStream

Object

ObjectOutputSt
ream

FilterOutputStream

FileOutputStream

BufferedInputStream

DataInputStream

BufferedOutputStream

DataOutputStream

PrintStream

ObjectInputStream

FilterInputStream

FileInputStream

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FileInputStream

To construct a FileInputStream, use the following
constructors:

public FileInputStream(String filename)

public FileInputStream(File file)


A
java.io.FileNotFoundException

would occur if you attempt to
create a
FileInputStream

with a nonexistent file.

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FileOutputStream

To construct a FileOutputStream, use the following constructors:


public FileOutputStream(String filename)

public FileOutputStream(File file)

public FileOutputStream(String filename, boolean append)

public FileOutputStream(File file, boolean append)



If the file does not exist, a new file would be created. If the file already
exists, the first two constructors would delete the current contents in
the file. To retain the current content and append new data into the file,
use the last two constructors by passing true to the append parameter.

TestFileStream

Run

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FilterInputStream/FilterOutputStream

Filter streams are streams that filter bytes for some purpose.

The basic byte input
stream provides a read method that can only be used for reading bytes. If you want to
read integers, doubles, or strings, you need a filter class to wrap the byte input stream.
Using a filter class enables you to read integers, doubles, and strings instead of bytes
and characters.
FilterInputStream

and
FilterOutputStream

are the base classes for
filtering data.
When you need to process primitive numeric types, use
DatInputStream

and
DataOutputStream

to filter bytes.



InputStream

OutputStream

Object

ObjectOutputSt
ream

FilterOutputStream

FileOutputStream

BufferedInputStream

DataInputStream

BufferedOutputStream

DataOutputStream

PrintStream

ObjectInputStream

FilterInputStream

FileInputStream

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DataInputStream/DataOutputStream

DataInputStream

reads bytes from the stream
and converts them into appropriate primitive
type values or strings.



InputStream

OutputStream

Object

ObjectOutputSt
ream

FilterOutputStream

FileOutputStream

BufferedInputStream

DataInputStream

BufferedOutputStream

DataOutputStream

PrintStream

ObjectInputStream

FilterInputStream

FileInputStream

DataOutputStream

converts primitive type values
or strings into bytes and output the bytes to the
stream
.

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DataInputStream

DataInputStream

extends
FilterInputStream

and implements the
DataInput

interface.


java.io.DataInput


+readBoolean(): boolean

+readByte(): byte

+readChar(): char

+readFloat(): float

+readDouble(): float

+readInt(): int

+readLong(): long

+readShort(): short

+readLine(): String

+readUTF(): String


Reads a
Boolean from the input stream.

Reads a byte from the input stream.

Reads a character from the input stream.

Reads a float from the input stream.

Reads a double from the input stream.

Reads an int from the input stream.

Reads a long from the input stream.

R
eads a short from the input stream.

Reads a line of characters from input.

Reads a string in UTF format.


InputStream


FilterInputStream


DataInputStream

+DataInputStream(

in: InputStream)

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DataOutputStream

DataOutputStream

extends
FilterOutputStream

and implements the
DataOutput

interface.


java.io.DataOutput


+writeBoolean(b: Boolean): void

+writeByte(v: int): void


+writeBytes(s: String): void

+writeChar(c: char): void

+writeChars(s: String): void

+writeFloat(v: float): void

+writeDouble(v: float): void

+wri
teInt(v: int): void

+writeLong(v: long): void

+writeShort(v: short): void

+writeUTF(s: String): void


Writes a Boolean to the output stream.

Writes to the output stream the eight low
-
order bits
of the argument v.

Writes the lower byte of the characters i
n a string to
the output stream.

Writes a character (composed of two bytes) to the
output stream.

Writes every character in the string s, to the output
stream, in order, two bytes per character.

Writes a float value to the output stream.

Writes a double
value to the output stream.

Writes an int value to the output stream.

Writes a long value to the output stream.

Writes a short value to the output stream.

Writes two bytes of length information to the output
stream, followed by the UTF representation of
ev
ery character in the string s.


OutputStream


FilterOutputStream


DataOutputStream

+
DataOutputStream(

in: InputStream)


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Characters and Strings in Binary I/O

A Unicode consists of two bytes. The
writeChar(char c)

method
writes the Unicode of character
c

to the output. The
writeChars(String s)

method writes the Unicode for each character in
the string
s

to the output.
writeBytes(String s)

method writes the
lower byte of the Unicode to the output.

Why UTF? What is UTF?

UTF is a coding scheme that allows systems to operate with both
ASCII and Unicode efficiently. Most operating systems use ASCII.
Java uses Unicode. The ASCII character set is a subset of the
Unicode character set. Since most applications need only the ASCII
character set, it is a waste to represent an 8
-
bit ASCII character as a
16
-
bit Unicode character.
The UTF is an alternative scheme that stores a
character using 1, 2, or 3 bytes. ASCII values (less than 0x7F) are coded in
one byte. Unicode values less than 0x7FF are coded in two bytes. Other
Unicode values are coded in three bytes.

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Using
DataInputStream
/
DataOutputStream


Data streams are used as wrappers on existing input and output
streams to filter data in the original stream. They are created using the
following constructors:

public DataInputStream(InputStream instream)

public DataOutputStream(OutputStream outstream)



The statements given below create data streams. The first statement
creates an input stream for file
in.dat
; the second statement creates an
output stream for file
out.dat
.

DataInputStream infile =


new DataInputStream(new FileInputStream("in.dat"));

DataOutputStream outfile =


new DataOutputStream(new FileOutputStream("out.dat"));

TestDataStream

Run

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Checking End of File

TIP: If you keep reading data at the end of a stream, an
EOFException

would occur. You can use
input.available()

to
check it.
input.available() == 0

indicates that it is the end of a
file.

Order and Format

CAUTION: You have to read the data in the same order and same
format in which they are stored. For example, since names are written
in UTF using
writeUTF
, you must read names using
readUTF
.

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BufferedInputStream/

BufferedOutputStream

Using buffers to speed up I/O


InputStream

OutputStream

Object

ObjectOutputSt
ream

FilterOutputStream

FileOutputStream

BufferedInputStream

DataInputStream

BufferedOutputStream

DataOutputStream

PrintStream

ObjectInputStream

FilterInputStream

FileInputStream

BufferedInputStream
/
BufferedOutputStream

does not contain new
methods. All the methods
BufferedInputStream
/
BufferedOutputStream

are
inherited from the
InputStream
/
OutputStream

classes.


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Constructing
BufferedInputStream
/
BufferedOutputStream


//
Create a BufferedInputStream

public BufferedInputStream(InputStream in)

public BufferedInputStream(InputStream in, int bufferSize)



// Create a BufferedOutputStream

public BufferedOutputStream(OutputStream out)

public BufferedOutputStream(OutputStreamr out, int bufferSize)

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Case Studies: Copy File

This case study develops a program that copies files. The user needs to
provide a source file and a target file as command
-
line arguments using
the following command:



java Copy source target




The program copies a source file to a target file and displays the number
of bytes in the file. If the source does not exist, tell the user the file is
not found. If the target file already exists, tell the user the file already
exists.

Copy

Run

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More on Text Files and Binary Files

Computers do not differentiate between a binary file and a text file. All
files are stored in binary format. So,
all files are essentially binary files.


Text I/O is built upon binary I/O to provide a level of abstraction for
character encoding and decoding
. Encoding and decoding are
automatically performed by text I/O.



In general, you should use text input to read a file created by a text editor
or a text output program, and use binary input to read a file created by a
Java binary output program.
For binary input, you need to know exactly
how data were written in order to read them in correct type and order
.

Binary I/O also contains methods to read/write a character and string.

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Write a byte 199 as a numeric value

import java.io.*;



public class Test {


public static void main(String[] args) throws IOException {


FileOutputStream output = new FileOutputStream("out.dat");


output.write(199); // Output byte 199 to the stream


output.close();




FileInputStream input = new FileInputStream("out.dat");


System.out.println((
char)
input.read()); // Read and display a byte


input.close();


}

}


Binary stream

out.dat

Output 199

0xC7 (199 in decimal)

FileOutputStream

Character stream

out.txt

Output "199"

0x31 0x39 0x39

FileWriter

1 9 9

Here can’t be “199”, otherwise
you use DataOutputStream

Demo Test.java

If no “char”, then the numeric
number will be printed

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Write a byte 199 as characters

import java.io.*;



public class Test {


public static void main(String[] args) throws IOException {


FileWriter output = new FileWriter("out.txt");


output.write("199"); // Output string "199" to the stream


output.close();




// Read and display three characters


FileReader input = new FileReader("out.txt");


System.out.print((char)input.read());


System.out.print((char)input.read());


System.out.println((char)input.read());


input.close();


}

}


Binary stream

out.dat

Output 199

0xC7 (199 in decimal)

FileOutputStream

Character stream

out.txt

Output "199"

0x31 0x39 0x39

FileWriter

1 9 9

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Object I/O

DataInputStream
/
DataOutputStream

enables you to perform I/O for
primitive type values and strings.
ObjectInputStream
/
ObjectOutputStream

enables you to perform I/O
for objects in addition for primitive type values and strings.


InputStream

OutputStream

Object

ObjectOutputSt
ream

FilterOutputStream

FileOutputStream

BufferedInputStream

DataInputStream

BufferedOutputStream

DataOutputStream

PrintStream

ObjectInputStream

FilterInputStream

FileInputStream

Optional

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ObjectInputStream

ObjectInputStream

extends
InputStream

and
implements
ObjectInput

and
ObjectStreamConstants
.


java.io.ObjectInput

+readObject(): Object



Reads an object.


java.io.InputStream


java.io.ObjectInputStream

+ObjectInputStream(in: InputStream)

java.io.DataInput


ObjectStreamConstants

You can replace DataInputStream/DataOutputStream
completely with ObjectInputStream/ObjectOutputStream

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ObjectOutputStream

ObjectOutputStream extends OutputStream and
implements ObjectOutput and ObjectStreamConstants.


java.io.ObjectOutput

+writeObject(o: Object): void



Writes an object.


java.io.OutputStream


java.io.ObjectOutputStream

+ObjectOutputStream(out: OutputStream)

java.io.DataOutput


ObjectStreamConstants

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Using Object Streams

You may wrap an ObjectInputStream/ObjectOutputStream on any
InputStream/OutputStream using the following constructors:


// Create an ObjectInputStream

public ObjectInputStream(InputStream in)



// Create an ObjectOutputStream

public ObjectOutputStream(OutputStream out)

TestObjectOutputStream

Run

TestObjectInputStream

Run

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Using Object Streams Sample

public class Student implements Serializable {


int id;


String name;


int age;


String department;


transient int number;


static int count;

}

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Using Object Streams Sample

public class Objectser {


public static void main(String args[]) {


Student stu=new Student(981036, “Li Ming”, 16, “CSD”);


try {


FileOutputStream fo = new
FileOutputStream(“data.ser”);


ObjectOutputStream so = new ObjectOutputStream(fo);


so.writeObject(stu);


so.close();


}


catch(Exception e) {


System.err.println(e);


}


}

}

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Using Object Streams Sample

public class ObjectRecov {


public static void main(String args[]) {


Student stu;


try {


FileInputStream fi = new FileInputStream(“data.ser”);


ObjectInputStream si = new ObjectInputStream(fi);


stu = (Student)si.readObject();


si.close();


System.out.println(“ID: ”+stu.id+“name:”+


stu.name+“age:”+age+“dept.:”+stu.department);


} catch(Exception e) {


System.out.println(e);


}


}


}

Can this statement
put behind catch?

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The Serializable Interface


Not all objects can be written to an output stream.
Objects that can
be written to an object stream is said to be serializable
. A
serializable object is an instance of the
java.io.Serializable

interface.
So the class of a serializable object must implement
Serializable
.



The
Serializable

interface is a marker interface
.

It has no methods,
so you don't need to add additional code in your class that
implements
Serializable
.



Implementing this interface enables the Java serialization
mechanism to automate the process of storing the objects and
arrays.


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The transient Keyword


The value of the objects’s static variables are not stored



If an object is an instance of
Serializable
, but it contains
non
-
serializable instance data fields, the object can’t be
serialized.



To enable the object to be serialized, you can
use the

transient
keyword to mark these data fields to tell the JVM to
ignore these fields when writing the object to an object stream.

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The transient Keyword, cont.

Consider the following class:



public class Foo implements java.io.Serializable {


private int v1;


private
static

double v2;


private
transient

A v3 = new A();

}

class A { } // A is not serializable



When an object of the Foo class is serialized, only variable v1 is
serialized.
Variable v2 is not serialized because it is a static variable
,
and
variable v3 is not serialized because it is marked transient
. If v3
were not marked transient, a java.io.NotSerializableException would
occur.

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Serializing Arrays

An array is serializable if all its elements are serializable.

So an
entire array can be saved using
writeObject

into a file and
later restored using
readObject
. Listing 16.12 stores an
array of five
int

values an array of three strings, and an
array of two
JButton

objects, and reads them back to
display on the console.

TestObjectStreamForArray

Run

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Random Access Files


All of the streams you have used so far are known as
read
-
only

or

write
-
only

streams.



The external files of these streams are
sequential

files
that cannot be updated without creating a new file.



It is often necessary to modify files or to insert new
records into files.
Java provides the
RandomAccessFile

class to allow a file to be read from and write to at
random locations.

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RandomAccessFile


Creates a RandomAccessFile stream with the specified File object and
mode.

Creates a RandomAccessFile stream with the specified file name
string and mode.

Closes the stream and releases the resource associated with the stream.

Ret
urns the offset, in bytes, from the beginning of the file to where the
next read or write occurs.

Returns the length of this file.

Reads a byte of data from this file and returns

1 an the end of stream.

Reads up to b.length bytes of data from this file in
to an array of bytes.

Reads up to len bytes of data from this file into an array of bytes.

Sets the offset (in bytes specified in pos) from the beginning of the
stream to where the next read or write occurs.

Sets a new length of this file.

Skips over n byt
es of input discarding the skipped bytes.

Writes b.length bytes from the specified byte array to this file, starting
at the current file pointer.

Writes len bytes from the specified byte array starting at offset off to
this file.

DataInput


DataInput


java.io.Ran
domAccessFile


+RandomAccessFile(file: File, mode:
String)

+RandomAccessFile(name: String,
mode: String)

+close(): void

+getFilePointer(): long

+length(): long

+read(): int

+read(b: byte[]): int

+read(b: byte[], off: int, len: int) : int

+seek(long pos)
: void

+setLength(newLength: long): void

+skipBytes(int n): int

+write(b: byte[]): void

+write(byte b[], int off, int len)

+write(b: byte[], off: int, len: int):
void

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File Pointer

A random access file consists of a sequence of bytes. There is a
special marker called
file pointer

that is positioned at one of these
bytes. A read or write operation takes place at the location of the file
pointer. When a file is opened, the file pointer sets at the beginning of
the file. When you read or write data to the file, the file pointer moves
forward to the next data. For example, if you read an
int

value using
readInt()
, the JVM reads four bytes from the file pointer and now the
file pointer is four bytes ahead of the previous location.


byte


file


byte





byte


byte


byte


byte


byte





byte


byte


byte


byte


byte


file pointer


byte


file


byte





byte


byte


byte


byte


byte





byte


byte


byte


byte


byte


file pointer


(
A
) Before readInt()


(
B
) Before readInt()


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RandomAccessFile

Methods

Many methods in
RandomAccessFile

are the same as those
in
DataInputStream

and

DataOutputStream
.


For example,
readInt
()
,
readLong
()
,
writeDouble
()
,
readLine
()
,
writeInt
()
, and
writeLong
()

can be used in data
input stream or data output stream as well as in
RandomAccessFile

streams.

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RandomAccessFile

Methods, cont.


void seek(long pos) throws IOException;


Sets the offset from the beginning of the
RandomAccessFile

stream to where the next read

or write occurs. seek(0) & seek(file.length());


long getFilePointer() IOException;


Returns the current offset, in bytes, from the

beginning of the file to where the next read

or write occurs.

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RandomAccessFile

Methods, cont.


long length()IOException


Returns the length of the file.



final void writeChar(int v) throws IOException


Writes a character to the file as a two
-
byte
Unicode, with the high byte written first.



final void writeChars(String s)

throws IOException


Writes a string to the file as a sequence of

characters.

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RandomAccessFile

Constructor

RandomAccessFile raf =

new RandomAccessFile("test.dat", "rw"); //allows read
and write


RandomAccessFile raf =

new RandomAccessFile("test.dat", "r"); //read only

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A Short Example on
RandomAccessFile

Run

TestRandomAccessFile

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Case Studies: Address Book

Optional

Now let us use
RandomAccessFile

to create a
useful project for storing and viewing and address
book. The user interface of the program is shown in
Figure 16.24. The
Add

button stores a new address
to the end of the file. The
First
,
Next
,
Previous
, and
Last

buttons retrieve the first, next, previous, and
last addresses from the file, respectively.

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Fixed Length String I/O

Random access files are often used to process files of records.

For
convenience, fixed
-
length records are used in random access files
so that a record can be located easily. A record consists of a fixed
number of fields. A field can be a string or a primitive data type. A
string in a fixed
-
length record has a maximum size. If a string is
smaller than the maximum size, the rest of the string is padded with
blanks.


Record 1


Record 2


Record n


Field1 Field 2 … Field k


file


e.g.,


Student 1


Student 2


Student n


name street city state zip

FixedLengthStringIO

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Address Implementation

The rest of the work can be summarized in the following steps:




Create the user interface.


Add a record to the file.




Read a record from the file.



Write the code to implement the button actions.

Run

AddressBook