Bài giảng Công nghệ Java - Chương 8: Java input/output - Trần Quang Diệu

CÔNG NGHỆ JAVA
CH8. JAVA INPUT/OUTPUT
Quang Dieu Tran PhD
4/7/2018  1
INPUT/OUTPUT STREAMS
1. Stream concepts
2. Input Streams
3. Output Streams
4. Reader
5. Writer
6. Object Serialization
7. Object Input Stream
8. Object Output Stream
4/7/2018  2
Stream concepts
• Data exchange
• Data exchange type: Character, Object, voice, 
picture, audio, video...
Program
File
Memory
Network
Program
information
4/7/2018  3
Stream
• Stream:
– dòng thông tin giữa 2 tác nhân (mức cao)
– một dãy tuần tự các byte (mức thấp)
• Một stream được gắn với một nguồn (source), hay một đích 
(destination)
• Stream operations:
– open stream
– close stream 
– read
– write
– seek
• Input stream: support reading functions
• Output stream: support writing functions
• Filter stream: buffer
4/7/2018  4
This image cannot currently be displayed.
Hình 1: Chương trình xử lý dữ liệu từ 1 input stream
Hình 2: Chương trình ghi dữ liệu ra output stream
Input & Output Stream
4/7/2018  5
Trình tự đọc/ ghi dòng
open input stream
while (more information){
read information
process information
}
close input stream
open output stream
while (more information){
get information from ...
write information
}
close output stream
Đọc thông tin 
từ input stream
Ghi thông tin 
vào output stream
4/7/2018  6
Các loại stream trong package java.io
• các lớp trong gói java.io được thiết kế gồm 2 nhóm chính:
– Nhóm input/output stream, hay nhóm hướng byte
– Nhóm reader/writer, hay nhóm hướng ký tự (unicode)
4/7/2018  7
Nhóm input/output stream
• Được gọi là nhóm hướng byte, vì thao tác đọc/ghi 
áp dụng cho 1 hoặc nhiều byte, chỉ giới hạn xử lý 
các byte 8 bits ISO-Latin-1. Rất thích hợp khi cần 
xử lý dữ liệu nhị phân như ảnh, âm thanh, binary 
files...
• Các input stream được mở rộng từ lớp 
InputStream (Abstract class)
• Các output stream được mở rộng từ lớp 
OutputStream (Abstract class)
4/7/2018  8
Nhóm input stream
4/7/2018  9
Nhóm output stream
4/7/2018  10
Nhóm reader/writer
• Được gọi là nhóm hướng ký tự, vì thao tác đọc/ghi áp dụng cho 1 
hoặc nhiều ký tự Unicode (1 character = 2bytes)
4/7/2018  11
Nhóm Writer
4/7/2018  12
Các loại các stream (tt)
• java IO cũng cung cấp cách thức kết gắn 
stream với các loại tác nhân khác như bộ 
nhớ, file ...
• Các lớp InputStreamReader và 
OutputStreamWriter cung cấp sự chuyển đổi 
giữa stream và reader/writer
• Xem bảng phân loại tóm tắt để biết thêm chi 
tiết
4/7/2018  13
Tổng quát về các Streams
I/O Streams Mô tả khái quát
M
e
m
o
r
y
CharArrayReader
CharArrayWriter
ByteArrayInput-
Stream
ByteArrayOutput-
Stream
Đọc/ghi từ/vào bộ nhớ.
Tạo stream từ một mảng, tiếp theo dùng các
phương thức đọc/ghi để đọc/ghi từ/vào mảng.
Tác vụ đọc sẽ lấy dữ liệu từ mảng
Tác vụ ghi sẽ ghi ra mảng
StringReader
StringWriter
StringBuffer-
InputStream
StringReader để đọc các ký tự từ một
String trong bộ nhớ.
StringWriter để ghi vào String.
StringBufferInputStream tương tự như
StringReader. Sự khác biệt chỉ là
StringBufferInputStream đọc các bytes 
từ một đối tượng StringBuffer
4/7/2018  14
Tổng quát về các Streams
Pipe
PipedReader
PipedWriter
PipedInputStream
PipedOutputStream
Hiện thực các thành phần input và
output của một pipe. Pipes được
dùng như một kênh truyền, nối
output của một thread vào input
của một thread khác.
File
FileReader
FileWriter
FileInputStream
FileOutputStream
Được gọi là các file streams. File
streams dùng để đọc/ghi từ/vào file
trên file system.
Nối
SequenceInput-
Stream
(concatenation)
Nối nhiều input streams thành một
input stream.
4/7/2018  15
Tổng quát về các Streams
Object
Serial-
ization
ObjectInputStream
ObjectOutputStream
Dùng khi cần lưu trữ, khôi phục,
hoặc truyền toàn bộ đối tượng.
Chuyển đổi
dạng dữ
liệu
(Data 
Conver-
sion)
DataInputStream
DataOutputStream
Thuận tiện khi cần đọc/ghi các 
kiểu dữ liệu cơ bản (primitive data 
types) như int, double, ... 
Counting
LineNumberReader
LineNumberInput-
Stream
Theo dõi số hàng trong khi đọc
4/7/2018  16
Tổng quát về các Streams
Printing
PrintWriter
PrintStream
Rất thuận tiện khi cần kết xuất, dễ 
đọc với người. System.out là 
một đối tượng thuộc lớp 
PrintStream.
Đệm 
(Buffer-
ing)
BufferedReader
BufferedWriter
bufferedInput-Stream
BufferedOutput-Stream
Đệm dữ liệu trong các thao tác 
đọc/ghi.
Đệm dữ liệu cải thiện tốc độ đọc 
ghi vì giảm số lần truy xuất thiết 
bị.
Lọc dữ liệu 
(Filtering)
FilterReader
FilterWriter
FilterInputStream
FilterOutputStream
Các lớp abstract này định nghĩa
các giao tiếp cho các filter streams
lọc dữ liệu trong khi đọc/ghi.
4/7/2018  17
Tổng quát về các Streams
C
h
u
yển
đ
ổi
b
yte 
k
ý
tự
(C
on
vertin
g
 b
etw
een
 B
ytes a
n
d
C
h
a
racters)
I
n
p
u
t
S
t
r
e
a
m
R
e
a
d
e
r
O
u
t
p
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t
S
t
r
e
a
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W
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Cặp reader/writer này là cầu nối giữa các byte streams và
character streams.
Một InputStreamReader đọc các bytes từ một
InputStream và chuyển các bytes đó thành các ký tự.
Một OutputStreamWriter chuyển các ký tự sang các
bytes, và ghi các bytes đó vào một OutputStream.
Quá trình chuyển đổi sẽ sử dụng bộ mã mặc định nếu không
được chỉ định rõ.
Gọi System.getProperty("file.encoding") để lấy
về tên bộ mã mặc định.
4/7/2018  18
Các lớp IO 
• InputStream, OutputStream, Reader và Writer là các 
lớp abstract:
– Các lớp input stream được mở rộng từ lớp InputStream
– Các lớp reader được mở rộng từ lớp Reader
– Các lớp output stream được mở rộng từ lớp OutputStream
– Các lớp writer được mở rộng từ lớp Writer
• 2 lớp InputStream và Reader cung cấp những phương thức read
tương đối giống nhau.
• 2 lớp OutputStream và Writer cung cấp những phương thức 
write tương đối giống nhau.
4/7/2018  19
InputSream
• Low-Level Input Stream Purpose of Stream
• ByteArrayInputStream Reads bytes of data from an 
memory array
• FileInputStream Reads bytes of data from a 
file on the local file system
• PipedInputStream Reads bytes of data from a 
thread pipe
• StringBufferInputStream Reads bytes of data from a 
string
• SequenceInputStream Reads bytes of data from 
two or more low-level 
streams, switching from one 
stream to the next when the 
end of the stream is reached
• System.in Reads bytes of data from 
the user console
4/7/2018  20
The java.io.InputStream Class
• int available() throws java.io.IOException— returns the number of bytes
currently available for reading.
• void close() throws java.io.IOException— closes the input stream and
frees any resources (such as file handles or file locks) associated with the
input stream.
• int read() throws java.io.IOException— returns the next byte of data from
the stream. When the end of the stream is reached, a value of –1 is
returned.
• int read(byte[] byteArray) throws java.io.IOException— reads a sequence
of bytes and places them in the specified byte array. This method returns
the number of bytes successfully read, or –1 if the end of the stream has
been reached.
• int read(byte[] byteArray, int offset, int length) throws
java.io.IOException, java.lang.IndexOutOfBoundsException— reads a
sequence of bytes, placing them in the specified array at the specified
offset, and for the specified length, if possible.
4/7/2018  21
The java.io.InputStream Class
• long skip(long amount) throws java.io.IOException— reads, but ignores, 
the specified amount of bytes. These bytes are discarded, and the 
position of the input stream is updated. The skip method returns the 
number of bytes skipped over, which may be less than the requested 
amount.
• The following code fragment reads 10 bytes from the InputStream in and 
stores them in the byte array input. However, if end of stream is 
detected, the loop is terminated early:
byte[] input = new byte[10];
for (int i = 0; i < input.length; i++) {
int b = in.read( );
if (b == -1) break;
input[i] = (byte) b;
}
4/7/2018  22
The java.io.InputStream Class
• For example, you may try to read 1 024 bytes from a network connection, 
when only 512 have actually arrived from the server. The rest are still in 
transit. They'll arrive eventually, but they aren't available now. 
byte[] input = new byte[1024];
int bytesRead = in.read(input);
• It attempts to read 1 024 bytes from the InputStream in into the array 
input. However, if only 512 bytes are available, then bytesRead will be set 
to 512. To guarantee that all the bytes you want are actually read, you 
must place the read in a loop that reads repeatedly until the array is 
filled.
int bytesRead = 0;
int bytesToRead = 1024;
byte[] input = new byte[bytesToRead];
while (bytesRead < bytesToRead) {
bytesRead += in.read(input, bytesRead, bytesToRead - bytesRead);
}
4/7/2018  23
The java.io.File Class
• An abstract representation of file and directory pathnames. 
• For UNIX platforms, the prefix of an absolute pathname is 
always "/". Relative pathnames have no prefix. 
• For Microsoft Windows platforms, the prefix of a pathname 
that contains a drive specifier consists of the drive letter
followed by ":" and possibly followed by "\\" if the pathname 
is absolute (D:\\myfolder\\t.txt). A relative pathname that 
does not specify a drive has no prefix. 
• public File(File parent, String child)
– Creates a new File instance from a parent abstract 
pathname and a child pathname string. 
• public File(String parent, String child)
– Creates a new File instance from a parent pathname string 
and a child pathname string. 
4/7/2018  24
The java.io.File Class
• public File(String pathname)
– Creates a new File instance by converting the given pathname string into 
an abstract pathname. If the given string is the empty string, then the 
result is the empty abstract pathname.
• public String getPath() 
– Converts this abstract pathname into a pathname string.
• public boolean isAbsolute()
– Tests whether this abstract pathname is absolute. 
• public String getAbsolutePath()
– Returns the absolute pathname string of this abstract pathname. 
• public boolean canRead()
– Tests whether the application can read the file denoted by this abstract 
pathname. 
4/7/2018  25
The java.io.File Class
• public boolean canWrite()
– Tests whether the application can modify the file denoted by this 
abstract pathname
• public boolean exists() 
– Tests whether the file or directory denoted by this abstract pathname 
exists. 
• public boolean isDirectory()
– Tests whether the file denoted by this abstract pathname is a directory. 
• public boolean isFile() 
– Tests whether the file denoted by this abstract pathname is a normal 
file.
• public boolean isHidden() 
– Tests whether the file named by this abstract pathname is a hidden file.
4/7/2018  26
The java.io.File Class
• public long length()
Returns the length (the zise in Kbyte) of the file denoted by this abstract 
pathname. The return value is unspecified if this pathname denotes a directory. 
• public boolean delete() 
Deletes the file or directory denoted by this abstract pathname. If this pathname 
denotes a directory, then the directory must be empty in order to be deleted. 
• public String[] list() 
Returns an array of strings naming the files and directories in the directory 
denoted by this abstract pathname. 
• public String[] list(FilenameFilter filter)
Returns an array of strings naming the files and directories in the directory 
denoted by this abstract pathname that satisfy the specified filter. The behavior 
of this method is the same as that of the list() method, except that the strings in 
the returned array must satisfy the filter. If the given filter is null then all names 
are accepted.
4/7/2018  27
The java.io.File Class
• public File[] listFiles()
Returns an array of abstract pathnames denoting the files in the directory 
denoted by this abstract pathname. 
• public File[] listFiles(FilenameFilter filter)
Returns an array of abstract pathnames denoting the files and directories in 
the directory denoted by this abstract pathname that satisfy the specified 
filter.
• public boolean mkdir()
Creates the directory named by this abstract pathname. 
• public boolean mkdirs()
Creates the directory named by this abstract pathname, including any 
necessary but nonexistent parent directories. Note that if this operation fails 
it may have succeeded in creating some of the necessary parent directories. 
4/7/2018  28
The java.io.File Class
//List of all files in D: with extention tgz
import java.io.*;
import java.util.*;
public class DLister {
public static void main(String[] args) {
File path = new File("D:\\");
String[] list;
list = path.list(new DirFilter(".tgz"));
for(int i = 0; i < list.length; i++)
System.out.println(list[i]);
} }
class DirFilter implements FilenameFilter {
String afn;
DirFilter(String afn) { this.afn = afn; }
public boolean accept(File dir, String name) {
String f = new File(name).getName();
return f.indexOf(afn) != -1;
} }
4/7/2018  29
Exercise
• public boolean deleteDir(String path)
• public boolean find(String path, String filePattern) 
• public boolean find(String path, String pattern)
4/7/2018  30
The java.io.FileInputStream Class
• A FileInputStream obtains input bytes from a file in a file system. What files
are available depends on the host environment.
• FileInputStream is meant for reading streams of raw bytes such as image
data. For reading streams of characters, consider using FileReader.
• public FileInputStream(File file) throws FileNotFoundException
Creates a FileInputStream by opening a connection to an actual file, the
file named by the File object file in the file system. A new FileDescriptor
object is created to represent this file connection.
• public FileInputStream(String name) throws FileNotFoundException
Creates a FileInputStream by opening a connection to an actual file, the
file named by the path name name in the file system.
4/7/2018  31
The java.io.FileInputStream Class
• public int read() throws IOException
– Reads a byte of data from this input stream. This method blocks if no 
input is yet available.
– Returns: the next byte of data, or -1 if the end of the file is reached. 
• public int read(byte[] b) throws IOException
• public int read(byte[] b, int off, int len) 
– Reads up to b.length bytes of data from this input stream into an array 
of bytes. This method blocks until some input is available. 
– Parameters:
• b - the buffer into which the data is read. 
• off - the start offset of the data. 
• len - the maximum number of bytes read. 
– Returns: the total number of bytes read into the buffer, or -1 if there is 
no more data because the end of the file has been reached. 
4/7/2018  32
The java.io.FileInputStream Class
• public long skip(long n) throws IOException
– Skips over and discards n bytes of data from the input stream. 
– Parameters: n - the number of bytes to be skipped. 
– Returns: the actual number of bytes skipped. 
– Throws: IOException - if n is negative, or if an I/O error occurs.
• public int available() throws IOException
– Retu ... e 
characters into bytes using a specified encoding and writes the bytes onto 
an underlying output stream.
4/7/2018  80
Writers
• Like OutputStream, the Writer class is never used directly, only 
polymorphically through one of its subclasses. It has five write( ) methods 
as well as a flush( ) and a close( ) method:
• protected Writer( )
• protected Writer(Object lock)
• public abstract void write(char[] text, int offset, int length)
• throws IOException
• public void write(int c) throws IOException
• public void write(char[] text) throws IOException
• public void write(String s) throws IOException
• public void write(String s, int offset, int length) throws
• IOException
• public abstract void flush( ) throws IOException
• public abstract void close( ) throws IOException
4/7/2018  81
Writers
char[] network = {'N', 'e', 't', 'w', 'o', 'r', 'k'};
w.write(network, 0, network.length);
• The same task can be accomplished with these other methods as well:
for (int i = 0; i < network.length; i++) w.write(network[i]);
w.write("Network");
w.write("Network", 0, 7);
• If it's using big-endian Unicode, then it will write these 14 bytes (shown here in 
hexadecimal) in this order:
00 4E 00 65 00 74 00 77 00 6F 00 72 00 6B
• On the other hand, if w uses little-endian Unicode, this sequence of 14 bytes is 
written:
4E 00 65 00 74 00 77 00 6F 00 72 00 6B 00
• If uses Latin-1, UTF-8, or MacRoman, this sequence of seven bytes is written:
4E 65 74 77 6F 72 6B
4/7/2018  82
java.io.FileWriter 
• Convenience class for writing character files. The constructors of this class assume 
that the default character encoding and the default byte-buffer size are acceptable. 
To specify these values yourself, construct an OutputStreamWriter on a 
FileOutputStream. 
• public FileWriter(String fileName) throws IOException 
Constructs a FileWriter object given a file name. 
• public FileWriter(String fileName, boolean append) throws IOException 
Constructs a FileWriter object given a file name with a boolean indicating 
whether or not to append the data written.
• public FileWriter(File file) throws IOException 
Constructs a FileWriter object given a File object. 
• public FileWriter(File file, boolean append) throws IOException 
Constructs a FileWriter object given a File object. If the second argument is true, 
then bytes will be written to the end of the file rather than the beginning. 
• Methods inherited from class java.io.OutputStreamWriter: close, flush, 
getEncoding, write
4/7/2018  83
Text Stream
• Text stream cho phép user nhìn stream dưới dạng “đọc được” 
(readable)
– InputStreamReader, OutputStreamWriter còn cung cấp 
thêm khả năng chuyển đổi stream  reader/writer, khả năng làm 
việc với các bảng mã khác nhau
– BufferedReader cung cấp cách đọc ra từng hàng từ một stream
– BufferedWriter cung cấp cách thức ghi các chuỗi ra stream dưới 
dạng đọc được
– PrintWriter cung cấp cách thức ghi các chuỗi, số nguyên, số thực, 
... ra stream dưới dạng đọc được
4/7/2018  84
P r in tW r it er
P rin tW rite r(arg0 : O utputS tream , auto F lus h : boo lean )
P rin tW rite r(arg0 : O utputS tream )
P rin tW rite r(arg0 : W rite r, au toF lus h : boo lean)
P rin tW rite r(arg0 : W rite r)
flus h() : vo id
c los e() : vo id
c hec k E rror() : boo lean
s etE rror() : vo id
w rite (arg0 : in t ) : vo id
w rite (arg0 : c har[ ] , a rg1 : in t , a rg2 : in t ) : vo id
w rite (arg0 : c har[ ] ) : vo id
w rite (arg0 : S tring , a rg 1 : in t , a rg2 : in t ) : vo id
w rite (arg0 : S tring) : vo id
prin t(a rg0 : bo o lea n) : vo id
prin t(a rg0 : c h ar) : vo id
prin t(a rg0 : in t ) : vo id
prin t(a rg0 : long) : vo id
prin t(a rg0 : floa t) : vo id
prin t(a rg0 : do ub le ) : vo id
prin t(a rg0 : c h ar[ ] ) : vo id
prin t(a rg0 : S tring) : vo id
prin t(a rg0 : O b jec t ) : vo id
prin t ln () : vo id
prin t ln (arg0 : boo lean) : vo id
prin t ln (arg0 : c har) : vo id
prin t ln (arg0 : in t ) : vo id
prin t ln (arg0 : long) : vo id
prin t ln (arg0 : floa t) : vo id
prin t ln (arg0 : doub le) : vo id
prin t ln (arg0 : c har[ ] ) : vo id
prin t ln (arg0 : S tring) : vo id
prin t ln (arg0 : O b jec t) : vo id
(fro m io )
4/7/2018  85
BufferedReader
BufferedReader(arg0 : Reader)
BufferedReader(arg0 : Reader, arg1 : int)
read() : int
read(arg0 : char[], arg1 : int, arg2 : int) : int
readLine(arg0 : boolean) : String
readLine() : String
skip(arg0 : long) : long
ready() : boolean
markSupported() : boolean
mark(arg0 : int) : void
reset() : void
close() : void
(from io)
Reader
(from io)
BufferedReader
4/7/2018  86
BufferedWriter
Writer
(from io)
BufferedWriter
BufferedWriter(arg0 : Writer, arg1 : int)
BufferedWriter(arg0 : Writer)
flushBuffer() : void
write(arg0 : int) : void
write(arg0 : char[], arg1 : int, arg2 : int) : void
write(arg0 : String, arg1 : int, arg2 : int) : void
newLine() : void
flush() : void
close() : void
(from io)
4/7/2018  87
java.io.OutputStreamWriter 
• public OutputStreamWriter(OutputStream out, 
String charsetName) throws UnsupportedEncodingException
Create an OutputStreamWriter that uses the named charset. 
• Charset Description
US-ASCII Seven-bit ASCII, a.k.a. ISO646-US, a.k.a. 
the Basic Latin block of the Unicode 
character set
ISO-8859-1 ISO Latin Alphabet No. 1, a.k.a. ISO-LATIN-1
UTF-8 Eight-bit UCS Transformation Format
UTF-16BE Sixteen-bit UCS Transformation Format, 
big-endian byte order
UTF-16LE Sixteen-bit UCS Transformation Format, 
little-endian byte order
UTF-16 Sixteen-bit UCS Transformation Format, byte order 
identified by an optional byte-order mark
4/7/2018  88
java.io.OutputStreamWriter
• public OutputStreamWriter(OutputStream out)
Create an OutputStreamWriter that uses the default character encoding. 
• public OutputStreamWriter(OutputStream out, CharsetEncoder enc)
Create an OutputStreamWriter that uses the given charset encoder. 
• public String getEncoding()
Return the name of the character encoding being used by this stream. 
• public void write(int c) throws IOException
Write a single character. 
• public void write(char[] cbuf, int off, int len) throws IOException
Write a portion of an array of characters. 
4/7/2018  89
java.io.OutputStreamWriter
• public void write(String str, int off, int len)
throws IOException 
Write a portion of a string. 
• public void flush() throws IOException
Flush the stream. 
• public void close() throws IOException
Close the stream. 
4/7/2018  90
OutputStreamWriter demo
• import java.io.*;
public class OutputStreamToWriterDemo{
public static void main(String args[]){
try{
OutputStream output = new FileOutputStream("utf8.txt");
// Create an OutputStreamWriter
OutputStreamWriter writer = new OutputStreamWriter 
(output,"UTF-8");
// Write to file using a writer
writer.write ("Phạm Văn Tính");
// Flush and close the writer, to ensure it is written
writer.flush();
writer.close();
} catch (IOException ioe){
System.err.println ("I/O error : " + ioe);
}}}
4/7/2018  91
java.io.InputStreamReader 
• An InputStreamReader is a bridge from byte streams to 
character streams: It reads bytes and decodes them into 
characters using a specified charset. The charset that it uses 
may be specified by name or may be given explicitly, or the 
platform's default charset may be accepted.
• public InputStreamReader(InputStream in) 
– Create an InputStreamReader that uses the default 
charset. 
• public InputStreamReader(InputStream in, 
String charsetName) throws UnsupportedEncodingException 
– Create an InputStreamReader that uses the named 
charset. 
• public String getEncoding() 
– Return the name of the character encoding being used by 
this stream. 
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java.io.InputStreamReader 
• public int read() throws IOException 
– Read a single character.
• public int read(char[] cbuf, int offset, int length)
throws IOException 
– Read characters into a portion of an array. 
• public boolean ready() throws IOException 
– Tell whether this stream is ready to be read. An 
InputStreamReader is ready if its input buffer is not empty, 
or if bytes are available to be read from the underlying 
byte stream. 
• public void close() throws IOException
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Charset Translation 
public class InputStreamReaderDemo {
public static void main(String args[]){
try{
OutputStream output = new FileOutputStream("utf8_16.txt");
// Create an OutputStreamWriter
OutputStreamWriter writer = new OutputStreamWriter (output, 
"UTF-16");
InputStream input = new FileInputStream("utf8.txt");
InputStreamReader reader = new InputStreamReader(input, 
"UTF-8");
char[] buff = new char[100];
// Write to file using a writer
int rNumber = reader.read(buff);
System.out.println("Number of char: "+rNumber);
writer.write(buff,0,rNumber);
// Flush and close the writer, to ensure it is written
writer.flush(); writer.close(); 
reader.close(); 
} catch (IOException ioe){
System.err.println ("I/O error : " + ioe);
}}}
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Complete example
• Student List
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Object Streams
• Using a fixed-length record format is a good choice if you need to store data 
of the same type. However, objects that you create in an object-oriented 
program are rarely all of the same type.
• If we want to save files that contain this kind of information, we must first 
save the type of each object and then the data that defines the current 
state of the object. When we read this information back from a file, we 
must:
– Read the object type;
– Create a blank object of that type;
– Fill it with the data that we stored in the file.
• It is entirely possible (if very tedious) to do this by hand. However, Sun 
Microsystems developed a powerful mechanism called object serialization 
to read/write objects from/into the file.
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Storing Objects of Variable Type 
• To save object data, you first need to open an ObjectOutputStream object:
• ObjectOutputStream out = new ObjectOutputStream(
new FileOutputStream( “student.dat"));
• Now, to save an object, you simply use the writeObject method of the 
ObjectOutputStream class as in the following fragment:
• //create objects
Student hoa = new Employee(“Trần Thị Hoa",
1980, “CD02”); 
Student vinh = new Employee(“Lương Thế Vinh",
1981, “DH03”);
• //Storing objects into stream 
out.writeObject(hoa); 
out.writeObject(vinh);
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Reading Objects back
• First get an ObjectInputStream object 
• ObjectInputStream in = new ObjectInputStream(new 
FileInputStream("employee.dat")); 
• Then, retrieve the objects in the same order in which they were written, 
using the readObject method.
• Student st1 = (Student)in.readObject(); 
Student st2 = (Student)in.readObject(); 
.
• When reading back objects, you must carefully keep track of the number of 
objects that were saved, their order, and their types. Each call to 
readObject reads in another object of the type Object. You, therefore, will 
need to cast it to its correct type.
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Serializable interface
• you need to make to any class that you want to save and restore in an 
object stream. The class must implement the Serializable interface:
• class Employee implements Serializable { . . . } 
• The Serializable interface has no methods, so you don't need to change 
your classes in any way.
• To make a class serializable, you do not need to do anything else. 
• Writing an array is done with a single operation:
• Student[] stList = new Student[3];
. . . 
out.writeObject(stList); 
• Similarly, reading in the result is done with a single operation. However, we 
must apply a cast to the return value of the readObject method:
• Student[] newStList = (Student[])in.readObject(); 
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Student List using Object Streams
public class SerialStudent implements Serializable{
private String name;
private int age;
private String cl;
public SerialStudent(String n, int a, String c){
name = n;
age = a;
cl = c;
}
public String getName() { return name; }
public int getAge() { return age; }
public String getCl(){ return cl; }
public String toString() {
return getClass().getName() + "[Name=" + name
+ ",Age=" + age + ",Class=" + cl + "]";
}
public void exportData(PrintWriter out){
out.println(name + "|" + age + "|" + cl);
}}
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Student List using Object Streams 
public class SerialTest {
public static void main(String[] args) {
SerialStudent[] st = new SerialStudent[3];
st[0] = new SerialStudent("Phạm Thị Mỹ Hạnh", 20, "TC02");
st[1] = new SerialStudent("Trần Thị Hoa", 18, "CD02");
st[2] = new SerialStudent("Nguyễn Vãn Vệ", 19, "DH03");
try {
// save all students records to the file studentemployee.dat
ObjectOutputStream out = new ObjectOutputStream(new
FileOutputStream("SerialStudent.dat"));
out.writeObject(st);
out.close();
// retrieve all records into a new array
ObjectInputStream in = new ObjectInputStream(new
FileInputStream("SerialStudent.dat"));
try{
SerialStudent[] newSt = (SerialStudent[])in.readObject();
// print the newly read student records
for (int i = 0; i < newSt.length; i++) System.out.println(newSt[i]);
} catch (ClassNotFoundException e) {};
in.close();
..
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java.io.ObjectOutputStream 
• ObjectOutputStream(OutputStream out)
creates an ObjectOutputStream so that you can 
write objects to the specified OutputStream.
• void writeObject(Object obj)
writes the specified object to the 
ObjectOutputStream. This method saves the class of 
the object, the signature of the class, and the values 
of any non-static, non-transient field of the class and 
its superclasses.
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java.io.ObjectInputStream 
• ObjectInputStream(InputStream is)
creates an ObjectInputStream to read back object 
information from the specified InputStream.
• Object readObject()
reads an object from the ObjectInputStream. In 
particular, this reads back the class of the object, the 
signature of the class, and the values of the 
nontransient and nonstatic fields of the class and all 
of its superclasses. It does deserializing to allow 
multiple object references to be recovered.
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InputStream Summary
FileInputStream 
DataInputStream 
In
p
u
tS
tr
ea
m
 Int 
Float 
Double 
String 
Char 
Object 
R
ea
d
er
InputStreamReader 
FileReader 
char 
BufferedReader line 
CharSet 
ObjectInputStream 
Byte 
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OutputStream Summary
FileOutputStream 
DataOutputStream 
O
u
tp
u
tS
tr
e
a
m
 Int 
Float 
Double 
String 
Char 
String 
line 
W
r
it
e
r
OutputStreamWriter
er 
FileWriter char 
PrintWriter 
CharSet 
String 
line 
byte 
Object ObjectOutputStream 
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Tóm tắt 
• Gói java.io chứa các lớp cho việc xuất nhập dữ liệu.
• Các dòng xuất nhập được chia thành 2 loại: dòng văn bản, 
dòng byte vật lý.
• Dòng văn bản xử lý dữ liệu theo từng ký tự 2 byte
• Dòng byte vật lý xử lý dữ liệu theo từng byte.
• Tác vụ nhập xuất có thể gây lỗi runtime nên cần throws 
IOException.
• Khi lưu trữ dữ liệu vào dòng, cần chọn 1 định dạng lưu trữ 
trước để khi phải đọc ra sẽ đọc được đúng dữ liệu.
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