Files in Java

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Dec 2, 2013 (3 years and 11 months ago)

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Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
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Files in Java

1

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
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The File Class

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.

2

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Obtaining
file
properties
and
manipulating
file

3


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.


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Problem: Explore File Properties

4

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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Text I/O


File

object



encapsulates properties of file or path


does not contain methods for reading/writing data
from/to a file.


To perform I/O, create objects using Java I/O classes.


Scanner

class
reads

input from a text file


PrintWriter

class
writes
output to a text file

5

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Writing Data Using
PrintWriter


6

WriteData

Run


java.io.PrintWriter

+PrintWriter(filename: String)

+print(s: String): void

+print(c: char): void

+print(cArray: char[]): void

+print(i: int): void

+print(l: long): void

+print(f: float): void

+print(d: double): void

+print(b: boolean): void

Also cont
ains the overloaded
println methods.

Also contains the overloaded
printf methods.


.

Creates a PrintWriter for the specified file.

Writes a string.

Writes a character.

Writes an array of character.

Writes an int value.

Writes a long value.

Writes a float
value.

Writes a double value.

Writes a boolean value.

A println method acts like a print method; additionally it
prints a line separator.

The line separator string is defined
by the system. It is
\
r
\
n on Windows and
\
n on Unix.

The
print
f
method
was intro
duced in
§3.6, “Formatting
Console Output and Strings.”


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Problem: Replacing Text

Write a class named
ReplaceText

that replaces a string in a text
file with a new string. The filename and strings are passed as
command
-
line arguments as follows:

java ReplaceText sourceFile targetFile oldString newString

For example, invoking

java ReplaceText FormatString.java t.txt StringBuilder StringBuffer

replaces all the occurrences of
StringBuilder

by
StringBuffer

in
FormatString.java and saves the new file in t.txt.

7

ReplaceText

Run

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(GUI) File Dialogs

8

ReadFileUsingJFileChooser

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Chapter 19 Text and Binary I/O

9

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Files


Text File


Data format is easily read by humans.


Binary File


designed
to be read by
programs


Can be processed more efficiently



Example
,



Text File: Java
source
(can
be read by a text
editor)


Binary File:
Java
class (read
by the
JVM)

10







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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.

11

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

output.format("%s", "Java 101");

output.close();

Scanner input = new Scanner(new File("temp.txt"));

System.out.println(input.nextLine());


Program

Input object

created from an
input class

Output object

created from an
output class

Input stream

Output stream

File

File

01011…1001

11001…1011

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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 program

The Unicode of
the character

Encoding/

Decoding


Binary I/O program

A byte is read/written


(b)

(a)

e.g.
,

"199"

The encoding of the character
is stored in the file


0x31

e.g.
,

199

0
0110111

0
0110001 00111001 00111001

0x39

0x39

0xC7


The same byte in the file


12

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Binary I/O Classes

13


InputStream

OutputStream

Object

ObjectOutputSt
ream

FilterOutputStream

FileOutputStream

BufferedInputStream

DataInputStream

BufferedOutputStream

DataOutputStream

PrintStream

ObjectInputStream

FilterInputStream

FileInputStream

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InputStream

The value returned is a byte as an int type.

14


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.


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OutputStream

The value is a byte as an int type.

15


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.

16


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.

17

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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.

18

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.

19


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.

20


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

21

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

22

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(

out: OutputStream)


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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.

23

Why UTF
-
8? What is UTF
-
8?


UTF
-
8 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
-
8 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"));

24

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. So how do you check the end of a
file? You can use
input.available()

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

indicates that it is the end of a file.

25

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
-
8 using
writeUTF
, you must read names using
readUTF
.

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BufferedInputStream/

BufferedOutputStream

Using buffers to speed up I/O

26


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)

27

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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.

28

Copy

Run

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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.

29


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
.


30


java.io.ObjectInput

+readObject(): Object



Reads an object.


java.io.InputStream


java.io.ObjectInputStream

+ObjectInputStream(in: InputStream)

java.io.DataInput


ObjectStreamConstants

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ObjectOutputStream

ObjectOutputStream extends OutputStream and
implements ObjectOutput and ObjectStreamConstants.

31


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)

32

TestObjectOutputStream

Run

TestObjectInputStream

Run

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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.

33

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The
transient

Keyword

If an object is an instance of
Serializable
, but it
contains non
-
serializable instance data fields, can the
object be serialized? The answer is no. 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.

34

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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.

35

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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.

36

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.

37

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RandomAccessFile

38


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.

39


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.


long getFilePointer() IOException;


Returns the current offset, in bytes, from the

beginning of the file to where the next read

or write occurs.

41

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671

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.

42

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671

RandomAccessFile

Constructor

RandomAccessFile raf =

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


RandomAccessFile raf =

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

43

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671

A Short Example on
RandomAccessFile

44

Run

TestRandomAccessFile

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671

Case Studies: Address Book

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.

45

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671

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.

46


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

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671

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.

47

Run

AddressBook