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Công nghệ Java - Chương 8: Java input / output

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  1. CÔNG NGHỆ JAVA CH8. JAVA INPUT/OUTPUT Quang Dieu Tran PhD 7/5/2021 1
  2. 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 7/5/2021 2
  3. Stream concepts File information Program Memory Network • Data exchange Program • Data exchange type: Character, Object, voice, picture, audio, video 7/5/2021 3
  4. 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 7/5/2021 4
  5. Input & Output Stream Hình 1: Chương trình xử lý dữ liệu từ 1 input stream Hình 2: Chương trình ghi dữ liệu ra output stream 7/5/2021 5
  6. Trình tự đọc/ ghi dòng open input stream while (more information){ Đọc thông tin read information process information từ input stream } close input stream open output stream while (more information){ get information from Ghi thông tin write information vào output stream } close output stream 7/5/2021 6
  7. 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) 7/5/2021 7
  8. 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) 7/5/2021 8
  9. Nhóm input stream 7/5/2021 9
  10. Nhóm output stream 7/5/2021 10
  11. 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) 7/5/2021 11
  12. Nhóm Writer 7/5/2021 12
  13. 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 7/5/2021 13
  14. Tổng quát về các Streams I/O Streams Mô tả khái quát CharArrayReader Đọc/ghi từ/vào bộ nhớ. CharArrayWriter Tạo stream từ một mảng, tiếp theo dùng các ByteArrayInput- phương thức đọc/ghi để đọc/ghi từ/vào mảng. Stream Tác vụ đọc sẽ lấy dữ liệu từ mảng ByteArrayOutput- Tác vụ ghi sẽ ghi ra mảng Memory Stream StringReader StringReader để đọc các ký tự từ một StringWriter String trong bộ nhớ. StringBuffer- StringWriter để ghi vào String. InputStream StringBufferInputStream tương tự như StringReader. Sự khác biệt chỉ là StringBufferInputStream đọc các bytes từ một đối tượng StringBuffer 7/5/2021 14
  15. Tổng quát về các Streams PipedReader Hiện thực các thành phần input và PipedWriter output của một pipe. Pipes được Pipe PipedInputStream dùng như một kênh truyền, nối PipedOutputStream output của một thread vào input của một thread khác. FileReader Được gọi là các file streams. File FileWriter streams dùng để đọc/ghi từ/vào file File FileInputStream trên file system. FileOutputStream SequenceInput- Nối nhiều input streams thành một Stream input stream. Nối (concatenation) 7/5/2021 15
  16. Tổng quát về các Streams Object ObjectInputStream Dùng khi cần lưu trữ, khôi phục, Serial- ObjectOutputStream hoặc truyền toàn bộ đối tượng. ization Chuyển đổi dạng dữ Thuận tiện khi cần đọc/ghi các liệu DataInputStream kiểu dữ liệu cơ bản (primitive data (Data DataOutputStream types) như int, double, Conver- sion) LineNumberReader Counting LineNumberInput- Theo dõi số hàng trong khi đọc Stream 7/5/2021 16
  17. Tổng quát về các Streams Rất thuận tiện khi cần kết xuất, dễ PrintWriter đọc với người. System.out là Printing PrintStream một đối tượng thuộc lớp PrintStream. BufferedReader Đệm dữ liệu trong các thao tác Đệm BufferedWriter đọc/ghi. (Buffer- bufferedInput-Stream Đệm dữ liệu cải thiện tốc độ đọc ing) BufferedOutput-Stream ghi vì giảm số lần truy xuất thiết bị. FilterReader Các lớp abstract này định nghĩa Lọc dữ liệu FilterWriter các giao tiếp cho các filter streams (Filtering) FilterInputStream FilterOutputStream lọc dữ liệu trong khi đọc/ghi. 7/5/2021 17
  18. Tổng quát về các Streams ( Converting between Bytes and Bytes between Converting Chuyển OutputStreamWriter InputStreamReader Cặp reader/writer này là cầu nối giữa các byte streams và character streams. Một InputStreamReader đọc các bytes từ một Characters đổi InputStream và chuyển các bytes đó thành các ký tự. Một OutputStreamWriter chuyển các ký tự sang các byte byte bytes, và ghi các bytes đó vào một OutputStream. )  Quá trình chuyển đổi sẽ sử dụng bộ mã mặc định nếu không ký được chỉ định rõ. Gọi System.getProperty("file.encoding") để lấy tự về tên bộ mã mặc định. 7/5/2021 18
  19. 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. 7/5/2021 19
  20. 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 7/5/2021 20
  21. 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. 7/5/2021 21
  22. 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; } 7/5/2021 22
  23. 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); } 7/5/2021 23
  24. 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. 7/5/2021 24
  25. 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. 7/5/2021 25
  26. 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. 7/5/2021 26
  27. 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. 7/5/2021 27
  28. 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. 7/5/2021 28
  29. 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; } } 7/5/2021 29
  30. Exercise • public boolean deleteDir(String path) • public boolean find(String path, String filePattern) • public boolean find(String path, String pattern) 7/5/2021 30
  31. 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. 7/5/2021 31
  32. 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. 7/5/2021 32
  33. 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 –Returns the number of bytes that can be read from this file input stream without blocking. • public void close() throws IOException –Closes this file input stream and releases any system resources associated with the stream. 7/5/2021 33
  34. FileInputStream Demo • Below we examine a practical application of using a low-level input stream to display the contents of a file. A byte at a time is read from the file and displayed to the screen. // Create an input stream, reading from the specified file InputStream fileInput = new FileInputStream (fName); // Read the first byte of data int data = fileInput.read(); // Repeat : until end of file (EOF) reached while (data != -1){ // Send byte to standard output System.out.write ( data ); // Read next byte data = fileInput.read(); } 7/5/2021 34
  35. FileInputStreamDemo import java.io.*; public class FileInputStreamDemo{ public static void main(String args[]){ if (args.length != 1){ System.err.println ("Syntax - FileInputStreamDemo file"); return; } try{ InputStream fileInput = new FileInputStream( args[0] ); int data = fileInput.read(); while (data != -1){ System.out.write ( data ); data = fileInput.read(); } // Close the file fileInput.close(); } catch (IOException ioe){ System.err.println ("I/O error - " + ioe); }}} 7/5/2021 35
  36. Ouput Streams • Low-Level Output Stream Purpose of Stream • ByteArrayOutputStream Writes bytes of data to an array of bytes. • FileOutputStream Writes bytes of data to a local file. • PipedOutputStream Writes bytes of data to a communications pipe, which will be connected to a PipedInputStream. • StringBufferOutputStream Writes bytes to a string buffer (a substitute data structure for the fixed- length string). • System.err Writes bytes of data to the error stream of the user console, also known as standard error. In addition, this stream is cast to a PrintStream. • System.out Writes bytes of data to the user console, also known as standard output. In addition, this stream is cast to a PrintStream. 7/5/2021 36
  37. The java.io.OutputStream Class • void close() throws java.io.IOException— closes the output stream, notifying the other side that the stream has ended. Pending data that has not yet been sent will be sent, but no more data will be delivered. • void flush() throws java.io.IOException— performs a "flush" of any unsent data and sends it to the recipient of the output stream. To improve performance, streams will often be buffered, so data remains unsent. This is useful at times, but obstructive at others. The method is particularly important for OutputStream subclasses that represent network operations, as flushing should always occur after a request or response is sent so that the remote side isn't left waiting for data. 7/5/2021 37
  38. The java.io.OutputStream Class • void write(int byte) throws java.io.IOException writes the specified byte. This is an abstract method, overridden by OutputStream subclasses. • void write(byte[] byteArray) throws java.io.IOException writes the contents of the byte array to the output stream. The entire contents of the array (barring any error) will be written. • void write(byte[] byteArray, int offset, int length) throws java.io.IOException writes the contents of a subset of the byte array to the output stream. This method allows developers to specify just how much of an array is sent, and which part, as opposed to the OutputStream.write(byte[] byteArray) method, which sends the entire contents of an array. 7/5/2021 38
  39. Java.io.FileOutputStream • public FileOutputStream(String name) throws FileNotFoundException –Creates an output file stream to write to the file with the specified name. –Throws: FileNotFoundException - if the file exists but is a directory rather than a regular file, does not exist but cannot be created, or cannot be opened for any other reason • public FileOutputStream(String name, boolean append) throws FileNotFoundException –Creates an output file stream to write to the file with the specified name. If the second argument is true, then bytes will be written to the end of the file rather than the beginning. –If the file exists but is a directory rather than a regular file, does not exist but cannot be created, or cannot be opened for any other reason then a FileNotFoundException is thrown. 7/5/2021 39
  40. Java.io.FileOutputStream • public FileOutputStream(File file) throws FileNotFoundException –Creates a file output stream to write to the file represented by the specified File object • public FileOutputStream(File file, boolean append) throws FileNotFoundException • public void write(int b) throws IOException –Writes the specified byte to this file output stream. Implements the write method of OutputStream. • public void write(byte[] b) throws IOException –Writes b.length bytes from the specified byte array to this file output stream. • public void write(byte[] b, int off, int len) throws IOException –Writes len bytes from the specified byte array starting at offset off to this file output stream. • public void close() throws IOException –Closes this file output stream and releases any system resources associated with this stream. This file output stream may no longer be used for writing bytes. 7/5/2021 40
  41. FileOutputStream Demo (FileCopy) • The program copies a file by reading the contents of the file and writing it, one byte at a time, to a new file. • To open a file for writing, a FileOutputStream is used. This class will create a file, if one does not already exist, or override the contents of the file (unless opened in append mode). // Output file for output OutputStream output = new FileOutputStream(destination); • Once opened, it can be written to by invoking the OutputStream.write() method. This method is called repeatedly by the application, to write the contents of a file that it is reading. while ( data != -1){ // Write byte of data to our file output.write (data); // Read next byte data=input.read(); } 7/5/2021 41
  42. FileOutputStream Demo (FileCopy) import java.io.*; public class FileOutputStreamDemo{ public static void main(String args[]){ // Two parameters are required, the source and destination if (args.length != 2){ System.err.println("Syntax - FileOutputStreamDemo src dest"); return; } String source = args[0]; String destination = args[1]; try{ // Open source file for input InputStream input = new FileInputStream( source ); System.out.println ("Opened " +source + " for reading."); // Output file for output OutputStream output = new FileOutputStream(destination); System.out.println ("Opened " +destination + " for writing."); 7/5/2021 42
  43. FileOutputStream Demo (FileCopy) int data = input.read(); while ( data != -1){ // Write byte of data to our file output.write (data); // Read next byte data=input.read(); } // Close both streams input.close(); output.close(); System.out.println ("I/O streams closed"); } catch (IOException ioe){ System.err.println ("I/O error - " + ioe); }}} 7/5/2021 43
  44. Filter Streams • Filter streams add additional functionality to an existing stream, by processing data in some form (such as buffering for performance) or offering additional methods that allow data to be accessed in a different manner (for example, reading a line of text rather than a sequence of bytes). • Filters make life easier for programmers, as they can work with familiar constructs such as strings, lines of text, and numbers, rather than individual bytes. • Filter streams can be connected to any other stream, to a low- level stream or even another filter stream. Filter streams are extended from the java.io.FilterInputStream and java.io.FilterOutputStream classes. 7/5/2021 44
  45. Filter Streams • For example, suppose you wanted to connect a PrintStream (used to print text to an OutputStream subclass) to a stream that wrote to a file. The following code may be used to connect the filter stream and write a message using the new filter. • FileOutputStream fout = new FileOutputStream ( somefile ); PrintStream pout = new PrintStream (fout); pout.println ("hello world"); • This process is fairly simple as long as the programmer remembers two things: 1. Read and write operations must take place on the new filter stream. 2. Read and write operations on the underlying stream can still take place, but not at the same time as an operation on the filter stream. 7/5/2021 45
  46. Filter Input Streams •Filter Input Stream Purpose of Stream •BufferedInputStream Buffers access to data, to improve efficiency. •DataInputStream Reads primitive data types, such as an int, a float, a double, or even a line of text, from an input stream. •PushBackInputStream Allows a byte of data to be pushed into the head of the stream. 7/5/2021 46
  47. BufferedInputStream • BufferedInputStream (InputStream input) creates a buffered stream that will read from the specified InputStream object. • BufferedInputStream (InputStream input, int bufferSize) throws java.lang.IllegalArgumentException— creates a buffered stream, of the specified size, which reads from the InputStream object passed as a parameter. • BufferedInputStream does not declare any new methods of its own. It only overrides methods from InputStream. • Chaining Filters Together DataOutputStream dout = new DataOutputStream( new BufferedOutputStream( new FileOutputStream("data.txt"))); DataInputStream din = new DataInputStream( new BufferedInputStream( new FileInputStream("data.txt"))); 7/5/2021 47
  48. FileCopy using BufferedInputStream import java.io.*; public class BufferCopyFile{ public static void copy(String sfile, String destfile) throws IOException{ int bufferSize = 10240; BufferedInputStream fis = new BufferedInputStream(new FileInputStream(sfile),bufferSize); BufferedOutputStream fos = new BufferedOutputStream(new FileOutputStream(destfile),bufferSize); int c; long beginTime = System.currentTimeMillis(); while((c=fis.read())>-1) fos.write(c); long endTime = System.currentTimeMillis(); System.out.println("Copy time: "+(endTime-beginTime)+ "ms"); fis.close(); fos.close(); } 7/5/2021 48
  49. A Speed of new FileCopy • Example runs of the following BufferCopyFile, with text files of various sizes, shows gains of ~3x. (In Java Platform Performance by Wilson and Kesselman, an example using a 370K JPEG file has a gain in execution speed of 83x!) • Size - 624 bytes : With buffering: 10 ms Without buffering: 30 ms • Size - 10,610 bytes : With buffering: 30 ms Without buffering: 80 ms • Size - 742,702 bytes : With buffering: 180 ms Without buffering: 741 ms 7/5/2021 49
  50. DataInputStream Class • The DataInputStream and DataOutputStream classes provide methods for reading and writing Java's primitive data types and strings in a binary format. The binary formats used are primarily intended for exchanging data between two different Java programs whether through a network connection, a data file, a pipe, or some other intermediary. What a data output stream writes, a data input stream can read. • Constructors DataInputStream (InputStream in) creates a data input stream, reading from the specified input stream. 7/5/2021 50
  51. DataInputStream Class • public final int read(byte[] b) throws IOException – Reads some number of bytes from the contained input stream and stores them into the buffer array b. This method blocks until input data is available, end of file is detected, or an exception is thrown. • public final int read(byte[] b, int off, int len) throws IOException – Reads up to len bytes of data from the contained input stream into an array of bytes. This method blocks until input data is available, end of file is detected, or an exception is thrown. ▪ public final int skipBytes(int n) ▪ public final boolean readBoolean() ▪ public final byte readByte() : signed 8-bit byte ▪ public final int readUnsignedByte() : an unsigned 8-bit number 7/5/2021 51
  52. DataInputStream Class • public final short readShort() : a signed 16-bit number • public final int readUnsignedShort() : an unsigned 16-bit integer • public final char readChar() : 2 bytes of this input stream as a Unicode character • public final int readInt() : 4 bytes of this input stream, interpreted as an int • public final long readLong() : eight bytes of this input stream, interpreted as a long • public final float readFloat() : 4 bytes of this input stream, interpreted as a float. • public final double readDouble() : 8 bytes of this input stream, interpreted as a double 7/5/2021 52
  53. PrintStream Class • The PrintStream class is the first filter output stream most programmers encounter because System.out is a PrintStream. However, other output streams can also be chained to print streams, using these two constructors: • public PrintStream(OutputStream out) public PrintStream(OutputStream out, boolean autoFlush) • By default, print streams should be explicitly flushed. However, if the autoFlush argument is true, then the stream will be flushed every time a byte array or linefeed is written or a println( ) method is invoked. 7/5/2021 53
  54. PrintStream Class • void print(boolean value)— prints a boolean value. • void print(char character)— prints a character value. • void print(char[] charArray)— prints an array of characters. • void print(double doubleValue)— prints a double value. • void print(float floatValue)— prints a float value. • void print(int intValue)— prints an int value. • void print(long longValue)— prints a long value. • void print(Object obj)— prints the value of the specified object's toString() method. • void print(String string)— prints a string's contents. • void println()— sends a line separator (such as '\n'). This value is system dependent and determined by the value of the system property "line.separator." 7/5/2021 54
  55. Random Access File Stream • Lớp RandomAccessFile: – Cung cấp cách thức đọc/ghi dữ liệu từ/ra file – cung cấp thêm thao tác seek → vị trí đọc/ghi là bất kỳ (random access) • một random access file chứa 1 file pointer chỉ đến vị trí sẽ được truy xuất: – phương thức seek di chuyển file pointer đến vị trí bất kỳ – phương thức getFilePointer trả về vị trí hiện tại của file pointer 7/5/2021 55
  56. Random Access File Stream 7/5/2021 56
  57. Java.io.RandomAccessFile • A random access file behaves like a large array of bytes stored in the file system. There is a kind of cursor, or index into the implied array, called the file pointer; input operations read bytes starting at the file pointer and advance the file pointer past the bytes read • The file pointer can be read by the getFilePointer method and set by the seek method • public RandomAccessFile(String name, String mode) public RandomAccessFile(File file, String mode) – Creates a random access file stream to read from, and optionally to write to, a file with the specified name. – "r“ Open for reading only. – "rw“ Open for reading and writing. If the file does not already exist then an attempt will be made to create it. 7/5/2021 57
  58. Java.io.RandomAccessFile • public int read() throws IOException – Reads a byte of data from this file. The byte is returned as an integer in the range 0 to 255 (0x00-0x0ff). This method blocks if no input is yet available. • public int read(byte[] b, int off, int len) throws IOException – Reads up to len bytes of data from this file into an array of bytes. This method blocks until at least one byte of input is available. • public int read(byte[] b) throws IOException – Reads up to b.length bytes of data from this file into an array of bytes. This method blocks until at least one byte of input is available. • public long getFilePointer() throws IOException – Returns the current offset in this file. 7/5/2021 58
  59. Java.io.RandomAccessFile • public void write(int b) throws IOException – Writes the specified byte to this file. The write starts at the current file pointer. • public void write(byte[] b) throws IOException – Writes b.length bytes from the specified byte array to this file, starting at the current file pointer. • public void write(byte[] b, int off, int len) throws – Writes len bytes from the specified byte array starting at offset off to this file • public void seek(long pos) throws IOException – Sets the file-pointer offset, measured from the beginning of this file, at which the next read or write occurs. The offset may be set beyond the end of the file. Setting the offset beyond the end of the file does not change the file length. The file length will change only by writing after the offset has been set beyond the end of the file. 7/5/2021 59
  60. Java.io.RandomAccessFile • public long length() throws IOException – Returns the length of this file. • public void setLength(long newLength) throws . – Sets the length of this file. If the present length of the file is greater than the newLength argument then the file will be truncated. In this case, if the file offset as returned by the getFilePointer method is greater than newLength then after this method returns the offset will be equal to newLength. – If the present length of the file as returned by the length method is smaller than the newLength argument then the file will be extended. In this case, the contents of the extended portion of the file are not defined. • public void close() throws IOException – Closes this random access file stream and releases any system resources associated with the stream 7/5/2021 60
  61. Java.io.RandomAccessFile • public final boolean readBoolean() • public final byte readByte() • public final int readUnsignedByte() • public final short readShort() • public final int readUnsignedShort() • public final char readChar() • public final int readInt() • public final long readLong() • public final float readFloat() • public final double readDouble() • public final String readLine() • public final void writeBoolean(boolean v) • public final void writeByte(int v) • public final void writeShort(int v) 7/5/2021 61
  62. Java.io.RandomAccessFile • public final void writeChar(int v) • public final void writeInt(int v) • public final void writeLong(long v) • public final void writeFloat(float v) • public final void writeDouble(double v) • public final void writeBytes(String s) throws IOException Writes the string to the file as a sequence of bytes. Each character in the string is written out, in sequence, by discarding its high eight bits. The write starts at the current position of the file pointer. • public final void writeChars(String s) throws IOException Writes a string to the file as a sequence of characters. Each character is written to the data output stream as if by the writeChar method. The write starts at the current position of the file pointer. 7/5/2021 62
  63. New I/O • The Java “new” I/O library, introduced in JDK 1.4 in the java.nio.* packages, has one goal: speed. • The speed comes by using structures which are closer to the operating system’s way of performing I/O: channels and buffers. • The NIO APIs include the following features: – Buffers for data of primitive types – Character-set encoders and decoders – Channels, a new primitive I/O abstraction – A file interface that supports locks and memory mapping – A multiplexed, non-blocking I/O facility for writing scalable servers 7/5/2021 63
  64. java.nio.ByteBuffer • public static ByteBuffer allocate(int capacity) Allocates a new byte buffer. The new buffer's position will be zero, its limit will be its capacity • public abstract byte get() Relative get method. Reads the byte at this buffer's current position, and then increments the position. • public abstract ByteBuffer put(byte b) Relative put method (optional operation). Writes the given byte into this buffer at the current position, and then increments the position. • public abstract byte get(int index) Absolute get method. Reads the byte at the given index. • public abstract ByteBuffer put(int index, byte b) Absolute put method (optional operation). Writes the given byte into this buffer at the given index. • public ByteBuffer get(byte[] dst, int offset, int length) Relative bulk get method. Otherwise, this method copies length bytes from this buffer into the given array, starting at the current position of this buffer and at the given offset in the array. The position of this buffer is then incremented by length. 7/5/2021 64
  65. java.nio.ByteBuffer •public ByteBuffer get(byte[] dst) Relative bulk get method. This method transfers bytes from this buffer into the given destination array. •public ByteBuffer put(ByteBuffer src) Relative bulk put method (optional operation). This method transfers the bytes remaining in the given source buffer into this buffer. If there are more bytes remaining in the source buffer than in this buffer, that is, if src.remaining() > remaining(), then no bytes are transferred and a BufferOverflowException is thrown. •public ByteBuffer put(byte[] src, int offset, int length) Relative bulk put method (optional operation). this method copies length bytes from the given array into this buffer, starting at the given offset in the array and at the current position of this buffer. The position of this buffer is then incremented by length. •public final ByteBuffer put(byte[] src) Relative bulk put method (optional operation). This method transfers the entire content of the given source byte array into this buffer. •public final byte[] array() Returns the byte array that backs this buffer (optional operation). 7/5/2021 65
  66. java.nio.ByteBuffer • public abstract char getChar() Relative get method for reading a char value. Reads the next two bytes at this buffer's current position, composing them into a char value according to the current byte order, and then increments the position by two. • public abstract ByteBuffer putChar(char value) Relative put method for writing a char value (optional operation). Writes two bytes containing the given char value, in the current byte order, into this buffer at the current position, and then increments the position by two. • public abstract char getChar(int index) Absolute get method for reading a char value. Reads two bytes at the given index, composing them into a char value according to the current byte order. • public abstract ByteBuffer putChar(int index, char value) Absolute put method for writing a char value (optional operation). Writes two bytes containing the given char value, in the current byte order, into this buffer at the given index. • public abstract CharBuffer asCharBuffer() Creates a view of this byte buffer as a char buffer. 7/5/2021 66
  67. java.nio.ByteBuffer • public abstract short getShort() • public abstract ByteBuffer putShort(short value) • public abstract short getShort(int index) • public abstract ByteBuffer putShort(int index, short value) • public abstract int getInt() • public abstract ByteBuffer putInt(int value) • public abstract int getInt(int index) • public abstract ByteBuffer putInt(int index, int value) • public abstract IntBuffer asIntBuffer() • Long, Float, Double 7/5/2021 67
  68. java.nio.Buffer • A container for data of a specific primitive type: • A buffer is a linear, finite sequence of elements of a specific primitive type. Aside from its content, the essential properties of a buffer are its capacity, limit, and position: • A buffer's capacity is the number of elements it contains. The capacity of a buffer is never negative and never changes. • A buffer's limit is the index of the first element that should not be read or written. A buffer's limit is never negative and is never greater than its capacity. • A buffer's position is the index of the next element to be read or written. A buffer's position is never negative and is never greater than its limit. 7/5/2021 68
  69. java.nio.Buffer • public final int capacity() Returns this buffer's capacity. • public final int position() Returns this buffer's position. • public final Buffer position(int newPosition) Sets this buffer's position. If the mark is defined and larger than the new position then it is discarded. • public final int limit() Returns this buffer's limit. • public final Buffer limit(int newLimit) Sets this buffer's limit. If the position is larger than the new limit then it is set to the new limit. If the mark is defined and larger than the new limit then it is discarded. 7/5/2021 69
  70. java.nio.Buffer • public final Buffer flip() Flips this buffer. The limit is set to the current position and then the position is set to zero. If the mark is defined then it is discarded. After a sequence of channel-read or put operations, invoke this method to prepare for a sequence of channel-write or relative get operations. For example: buf.put(magic); // Prepend header in.read(buf); // Read data into rest of buffer buf.flip(); // Flip buffer out.write(buf); // Write header + data to channel • public final Buffer rewind() Rewinds this buffer. The position is set to zero and the mark is discarded. Invoke this method before a sequence of channel-write or get operations, assuming that the limit has already been set appropriately. For example: out.write(buf); // Write remaining data buf.rewind(); // Rewind buffer buf.get(array); // Copy data into array 7/5/2021 70
  71. java.nio.Buffer • public final Buffer clear() Clears this buffer. The position is set to zero, the limit is set to the capacity, and the mark is discarded. Invoke this method before using a sequence of channel-read or put operations to fill this buffer. For example: buf.clear(); // Prepare buffer for reading in.read(buf); // Read data • public final int remaining() Returns the number of elements between the current position and the limit. • public final boolean hasRemaining() Tells whether there are any elements between the current position and the limit. 7/5/2021 71
  72. java.nio.channels.FileChannel • This class does not define methods for opening existing files or for creating new ones; such methods may be added in a future release. In this release a file channel can be obtained from an existing FileInputStream, FileOutputStream, or RandomAccessFile object by invoking that object's getChannel method, which returns a file channel that is connected to the same underlying file. • public abstract int read(ByteBuffer dst) • public abstract long read(ByteBuffer[] dsts, int offset, int length) • public abstract int write(ByteBuffer src) • public abstract long write(ByteBuffer[] srcs, int offset, int length) • public abstract long position() • public abstract FileChannel position(long newPosition) 7/5/2021 72
  73. java.nio.channels.FileChannel • public abstract long size() throws IOException Returns the current size of this channel's file, measured in bytes • public abstract FileChannel truncate(long size) throws IOException Truncates this channel's file to the given size. If the given size is less than the file's current size then the file is truncated, discarding any bytes beyond the new end of the file. If the given size is greater than or equal to the file's current size then the file is not modified. In either case, if this channel's file position is greater than the given size then it is set to that size. • public abstract void force(boolean metaData) throws IOException Forces any updates to this channel's file to be written to the storage device that contains it. • public abstract long transferTo(long position, long count, WritableByteChannel target) throws IOException Transfers bytes from this channel's file to the given writable byte channel. 7/5/2021 73
  74. java.nio.channels.FileChannel • public abstract long transferFrom(ReadableByteChannel src, long position, long count) throws IOException Transfers bytes into this channel's file from the given readable byte channel. • public abstract int read(ByteBuffer dst, long position) • public abstract int write(ByteBuffer src, long position) • public abstract FileLock lock(long position, long size, boolean shared) throws IOException • public abstract FileLock tryLock(long position, long size, boolean shared) throws IOException Acquires a lock on the given region of this channel's file. position : The position at which the locked region is to start; must be non- negative size : The size of the locked region; must be non-negative, and the sum position + size must be non-negative shared : true to request a shared lock, in which case this channel must be open for reading (and possibly writing); false to request an exclusive lock, in which case this channel must be open for writing (and possibly reading) 7/5/2021 74
  75. java.nio.channels.FileChannel • public abstract MappedByteBuffer map(FileChannel.MapMode mode, long position, long size) throws IOException – Maps a region of this channel's file directly into memory. A region of a file may be mapped into memory in one of three modes: – Read-only: Any attempt to modify the resulting buffer will cause a ReadOnlyBufferException to be thrown. (MapMode.READ_ONLY) – Read/write: Changes made to the resulting buffer will eventually be propagated to the file; they may or may not be made visible to other programs that have mapped the same file. (MapMode.READ_WRITE) – Private: Changes made to the resulting buffer will not be propagated to the file and will not be visible to other programs that have mapped the same file; instead, they will cause private copies of the modified portions of the buffer to be created. (MapMode.PRIVATE) – For a read-only mapping, this channel must have been opened for reading; for a read/write or private mapping, this channel must have been opened for both reading and writing. 7/5/2021 75
  76. java.nio.channels.FileChannel import java.io.*; import java.nio.*; import java.nio.channels.*; public class ChannelCopy { private static final int BSIZE = 1024; public static void main(String[] args) throws Exception { FileChannel in = new FileInputStream("data.pdf").getChannel(), out = new FileOutputStream("data1.pdf").getChannel(); ByteBuffer buffer = ByteBuffer.allocate(BSIZE); while(in.read(buffer) != -1) { buffer.flip(); // Prepare for writing out.write(buffer); buffer.clear(); // Prepare for reading } System.out.println(“Done "); in.close(); out.close(); }} 7/5/2021 76
  77. java.nio.channels.FileChannel import java.io.*; import java.nio.*; import java.nio.channels.*; public class TransferTo { public static void main(String[] args) throws Exception { FileChannel in = new FileInputStream("data.pdf").getChannel(), out = new FileOutputStream("data1.pdf").getChannel(); in.transferTo(0, in.size(), out); System.out.println(“Done "); in.close(); out.close(); } } 7/5/2021 77
  78. CRC without Memory-Mapped file import java.io.*; import java.util.zip.*; // This program computes the CRC checksum of a file, using an // input stream. public class CRC { public static void main(String[] args) throws IOException { InputStream in = new FileInputStream("data.pdf"); CRC32 crc = new CRC32(); int c; long start = System.currentTimeMillis(); while((c = in.read()) != -1) crc.update(c); long end = System.currentTimeMillis(); System.out.println(Long.toHexString(crc.getValue())); System.out.println((end - start) + " milliseconds"); } } 7/5/2021 78
  79. CRC with Memory-Mapped file import java.io.*; import java.nio.*; import java.nio.channels.*; import java.util.zip.*; //compute the CRC checksum of a file, using a memory-mapped file. public class NIOCRC { public static void main(String[] args) throws Exception { FileInputStream in = new FileInputStream("data.pdf"); FileChannel channel = in.getChannel(); CRC32 crc = new CRC32(); long start = System.currentTimeMillis(); MappedByteBuffer buffer = channel.map( FileChannel.MapMode.READ_ONLY, 0, (int)channel.size()); while (buffer.hasRemaining()) crc.update(buffer.get()); long end = System.currentTimeMillis(); System.out.println(Long.toHexString(crc.getValue())); System.out.println((end - start) + " milliseconds"); }} 7/5/2021 79
  80. Readers and Writers • While input streams and output streams may be used to read and write text as well as bytes of information and primitive data types, a better alternative is to use readers and writers. Readers and writers were introduced in JDK1.1 to better support Unicode character streams. • The most important concrete subclasses of Reader and Writer are the InputStreamReader and the OutputStreamWriter classes. An InputStreamReader contains an underlying input stream from which it reads raw bytes. It translates these bytes into Unicode characters according to a specified encoding. An OutputStreamWriter receives Unicode characters from a running program. It then translates those characters into bytes using a specified encoding and writes the bytes onto an underlying output stream. 7/5/2021 80
  81. 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 7/5/2021 81
  82. 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 7/5/2021 82
  83. 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 7/5/2021 83
  84. 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 7/5/2021 84
  85. 7/5/2021 85
  86. BufferedReader 7/5/2021 86
  87. BufferedWriter 7/5/2021 87
  88. 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 7/5/2021 88
  89. 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. 7/5/2021 89
  90. 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. 7/5/2021 90
  91. 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); }}} 7/5/2021 91
  92. 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. 7/5/2021 92
  93. 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 7/5/2021 93
  94. 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); }}} 7/5/2021 94
  95. Complete example • Student List 7/5/2021 95
  96. 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. 7/5/2021 96
  97. 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); 7/5/2021 97
  98. 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. 7/5/2021 98
  99. 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(); 7/5/2021 99
  100. 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); }} 7/5/2021 100
  101. 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(); 7/5/2021 101
  102. 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. 7/5/2021 102
  103. 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. 7/5/2021 103
  104. InputStream Summary Int Float Net Double DataInputStream String Char FileInputStream ObjectInputStream File Object InputStream Byte CharSet InputStreamReader char line BufferedReader Reader FileReader 7/5/2021 104
  105. OutputStream Summary byte Object ObjectOutputStream Int Float Net Double DataOutputStream String Char putStream FileOutputStream File String Out line CharSet OutputStreamWriter er String line PrintWriter char Writer FileWriter 7/5/2021 105
  106. 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. 7/5/2021 106