1. Liang,Introduction to Java Programming,revised by Dai-kaiyu 1 Chapter 16 Simple Input and Output 与人玫瑰，手有余香

2. Liang,Introduction to Java Programming,revised by Dai-kaiyu 2 Objectives F To discover file properties, delete and rename files using the File class (§16.2). F To understand how I/O is processed in Java (§16.3). F To distinguish between text I/O and binary I/O (§16.3). F To read and write characters using FileReader and FileWriter (§16.4). F To improve the performance of text I/O using BufferedReader and BufferedWriter (§16.4). F To write primitive values, strings, and objects as text using PrintWriter and PrintStream (§16.4). F To read and write bytes using FileInputStream and FileOutputStream (§16.6). F To read and write primitive values and strings using DataInputStream/DataOutputStream (§16.6). F To store and restore objects using ObjectOutputStream and ObjectInputStream, and to understand how objects are serialized and what kind of objects can be serialized (§16.9 Optional). F To use the Serializable interface to enable objects to be serializable (§16.9 Optional). F To use RandomAccessFile for both read and write. (§16.10 Optional)

3. Liang,Introduction to Java Programming,revised by Dai-kaiyu 3 Introduction Files  Long-term storage of large amounts of data  Persistent data exists after termination of program  Files stored on secondary storage devices Magnetic disks Optical disks Magnetic tapes  Sequential and random access files

4. Liang,Introduction to Java Programming,revised by Dai-kaiyu 4 The File Class The File class is intended to provide an abstraction that deals with most of the machine-dependent complexities of files and path names in a machine-independent fashion. The filename is a string. The File class is a wrapper class for the file name and its directory path.

5. Liang,Introduction to Java Programming,revised by Dai-kaiyu 5 Obtaining file properties and manipulating file

6. Liang,Introduction to Java Programming,revised by Dai-kaiyu 6 Example 16.1 Using the File Class TestFileClass Run Objective: Write a program that demonstrates how to create files in a platform-independent way and use the methods in the File class to obtain their properties. Figure 16.1 shows a sample run of the program on Windows, and Figure 16.2 a sample run on Unix.

7. Liang,Introduction to Java Programming,revised by Dai-kaiyu 7 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. FileWriter output = new FileWriter("temp.txt"); output.write("Java 101"); output.close(); FileReader input = new FileReader("temp.txt"); int code = input.read(); System.out.println((char)code); Return the unicode of the char

8. Liang,Introduction to Java Programming,revised by Dai-kaiyu 8 Coding Essentials

9. Liang,Introduction to Java Programming,revised by Dai-kaiyu 9 Text File vs. Binary File Data stored in a text file are represented in human-readable form. Data stored in a binary file are represented in binary form. You cannot read binary files. Binary files are designed to be read by programs. For example, the Java source programs are stored in text files and can be read by a text editor, but the Java classes are stored in binary files and are read by the JVM. The advantage of binary files is that they are more efficient to process than text files. you can imagine that a text file consists of a sequence of characters and a binary file consists of a sequence of bits. For example, the decimal integer 199 is stored as the sequence of three characters: '1', '9', '9' in a text file and the same integer is stored as a byte-type value C7 in a binary file, because decimal 199 equals to hex C7.

10. Liang,Introduction to Java Programming,revised by Dai-kaiyu 10 Text I/O Classes

11. Liang,Introduction to Java Programming,revised by Dai-kaiyu 11 Reader The value is returned as a Unicode.

12. Liang,Introduction to Java Programming,revised by Dai-kaiyu 12 Writer The Unicode value.

13. Liang,Introduction to Java Programming,revised by Dai-kaiyu 13 FileReader/FileWriter FileReader/FileWriter associates an input/output stream with an external file. All the methods in FileReader/FileWriter are inherited from its superclasses.

14. Liang,Introduction to Java Programming,revised by Dai-kaiyu 14 FileReader To construct a FileReader, use the following constructors: public FileReader(String filename) public FileReader(File file) A java.io.FileNotFoundException would occur if you attempt to create a FileReader with a nonexistent file. TestFileReader Run

15. Liang,Introduction to Java Programming,revised by Dai-kaiyu 15 FileWriter To construct a FileWriter, use the following constructors: public FileWriter(String filename) public FileWriter(File file) public FileWriter(String filename, boolean append) public FileWriter(File file, boolean append) If the file does not exist, a new file would be created. If the file already exists, the first two constructors would delete the current contents in the file. To retain the current content and append new data into the file, use the last two constructors by passing true to the append parameter. TestFileWriter Run

16. Liang,Introduction to Java Programming,revised by Dai-kaiyu 16 InputStreamReader/OutputStreamWriter Optional All the methods in InputStreamReader/OutputStreamWriter are inherited from Reader/Writer except getEncoding(), which returns the name of encoding being used by this stream.

17. Liang,Introduction to Java Programming,revised by Dai-kaiyu 17 InputStreamReader/OutputStreamWriter InputStreamReader/OutputStreamWriter are used to convert between bytes and characters. Characters written to an OutputStreamWriter are encoded into bytes using a specified encoding scheme. Bytes read from an InputStreamReader are decoded into characters using a specified encoding scheme. You can specify an encoding scheme using a constructor of InputStreamReader/OutputStreamWriter. If no encoding scheme is specified, the system’s default encoding scheme is used. Optional

18. Liang,Introduction to Java Programming,revised by Dai-kaiyu 18 BufferedReader/BufferedWriter The buffered stream classes inherit methods from their superclasses. In addition to using the methods from their superclasses, BufferedReader has a readLine() method to read a line, and BufferedWriter has a newLine() method to write a line separator. If the end of stream is reached, readLine() returns null.

19. Liang,Introduction to Java Programming,revised by Dai-kaiyu 19 PrintWriter/PrintStream BufferedWriter is used to output characters and strings. PrintWriter and PrintStream can be used to output objects, strings and numeric values as text. PrintWriter was introduced in JDK 1.2 to replace PrintStream. Both classes are almost identical in the sense that they provide the same function and same methods for outputting strings and numeric values as text. PrintWriter is more efficient than PrintStream. So, you use PrintWriter rather than PrintStream.

20. Liang,Introduction to Java Programming,revised by Dai-kaiyu 20 Methods in PrintWriter/PrintStream PrintWriter and PrintStream also contain the JDK 1.5 printf method for printing formatted output, which was introduced in Section 2.17, “Formatted Output.”

21. Liang,Introduction to Java Programming,revised by Dai-kaiyu 21 Constructing PrintWriter This section introduces PrintWriter, but PrintStream can be used in the same way. To construct a PrintWriter, use the following constructors: public PrintWriter(Writer out) public PrintWriter(Writer out, boolean autoFlush) If autoFlush is true, the println methods will cause the buffer to be flushed. The constructors and methods in PrintWriter and PrintStream do not throw an IOException. So you don’t need to invoke them from a try-catch block. TestPrintWriter Run

22. Liang,Introduction to Java Programming,revised by Dai-kaiyu 22 Case Studies: Text Viewer This case study writes a program that views a text file in a text area. The user enters a filename in a text field and clicks the View button; the file is then displayed in a text area. FileViewer Run

23. Liang,Introduction to Java Programming,revised by Dai-kaiyu 23 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. Binary I/O is more efficient

24. Liang,Introduction to Java Programming,revised by Dai-kaiyu 24 Binary I/O Classes

25. Liang,Introduction to Java Programming,revised by Dai-kaiyu 25 The value returned is a byte as an int type. InputStream

26. Liang,Introduction to Java Programming,revised by Dai-kaiyu 26 The value is a byte as an int type. OutputStream

27. Liang,Introduction to Java Programming,revised by Dai-kaiyu 27 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.

28. Liang,Introduction to Java Programming,revised by Dai-kaiyu 28 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.

29. Liang,Introduction to Java Programming,revised by Dai-kaiyu 29 FileOutputStream To construct a FileOutputStream, use the following constructors: public FileOutputStream(String filename) public FileOutputStream(File file) public FileOutputStream(String filename, boolean append) public FileOutputStream(File file, boolean append) If the file does not exist, a new file would be created. If the file already exists, the first two constructors would delete the current contents in the file. To retain the current content and append new data into the file, use the last two constructors by passing true to the append parameter. TestFileStream Run

30. Liang,Introduction to Java Programming,revised by Dai-kaiyu 30 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.

31. Liang,Introduction to Java Programming,revised by Dai-kaiyu 31 DataInputStream/DataOutputStream DataInputStream reads bytes from the stream and converts them into appropriate primitive type values or strings. DataOutputStream converts primitive type values or strings into bytes and output the bytes to the stream.

32. Liang,Introduction to Java Programming,revised by Dai-kaiyu 32 DataInputStream DataInputStream extends FilterInputStream and implements the DataInput interface.

33. Liang,Introduction to Java Programming,revised by Dai-kaiyu 33 DataOutputStream DataOutputStream extends FilterOutputStream and implements the DataOutput interface.

34. Liang,Introduction to Java Programming,revised by Dai-kaiyu 34 Characters and Strings in Binary I/O A Unicode consists of two bytes. The writeChar(char c) method writes the Unicode of character c to the output. The writeChars(String s) method writes the Unicode for each character in the string s to the output. writeBytes(String s) method writes the lower byte of the Unicode to the output. Why UTF? What is UTF? UTF is a coding scheme that allows systems to operate with both ASCII and Unicode efficiently. Most operating systems use ASCII. Java uses Unicode. The ASCII character set is a subset of the Unicode character set. Since most applications need only the ASCII character set, it is a waste to represent an 8-bit ASCII character as a 16-bit Unicode character. The UTF is an alternative scheme that stores a character using 1, 2, or 3 bytes. ASCII values (less than 0x7F) are coded in one byte. Unicode values less than 0x7FF are coded in two bytes. Other Unicode values are coded in three bytes.

35. Liang,Introduction to Java Programming,revised by Dai-kaiyu 35 Using DataInputStream/DataOutputStream Data streams are used as wrappers on existing input and output streams to filter data in the original stream. They are created using the following constructors: public DataInputStream(InputStream instream) public DataOutputStream(OutputStream outstream) The statements given below create data streams. The first statement creates an input stream for file in.dat; the second statement creates an output stream for file out.dat. DataInputStream infile = new DataInputStream(new FileInputStream("in.dat")); DataOutputStream outfile = new DataOutputStream(new FileOutputStream("out.dat")); TestDataStream Run

36. Liang,Introduction to Java Programming,revised by Dai-kaiyu 36 Checking End of File TIP: If you keep reading data at the end of a stream, an EOFException would occur. You can use input.available() to check it. input.available() == 0 indicates that it is the end of a file. Order and Format CAUTION: You have to read the data in the same order and same format in which they are stored. For example, since names are written in UTF using writeUTF, you must read names using readUTF.

37. Liang,Introduction to Java Programming,revised by Dai-kaiyu 37 BufferedInputStream/ BufferedOutputStream Using buffers to speed up I/O BufferedInputStream/BufferedOutputStream does not contain new methods. All the methods BufferedInputStream/BufferedOutputStream are inherited from the InputStream/OutputStream classes.

38. Liang,Introduction to Java Programming,revised by Dai-kaiyu 38 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)

39. Liang,Introduction to Java Programming,revised by Dai-kaiyu 39 Case Studies: Copy File This case study develops a program that copies files. The user needs to provide a source file and a target file as command-line arguments using the following command: java Copy source target The program copies a source file to a target file and displays the number of bytes in the file. If the source does not exist, tell the user the file is not found. If the target file already exists, tell the user the file already exists. Copy Run

40. Liang,Introduction to Java Programming,revised by Dai-kaiyu 40 More on Text Files and Binary Files Computers do not differentiate between a binary file and a text file. All files are stored in binary format. So, all files are essentially binary files. Text I/O is built upon binary I/O to provide a level of abstraction for character encoding and decoding. Encoding and decoding are automatically performed by text I/O. In general, you should use text input to read a file created by a text editor or a text output program, and use binary input to read a file created by a Java binary output program. For binary input, you need to know exactly how data were written in order to read them in correct type and order. Binary I/O also contains methods to read/write a character and string.

41. Liang,Introduction to Java Programming,revised by Dai-kaiyu 41 Write a byte 199 as a numeric value import java.io.*; public class Test { public static void main(String[] args) throws IOException { FileOutputStream output = new FileOutputStream("out.dat"); output.write(199); // Output byte 199 to the stream output.close(); FileInputStream input = new FileInputStream("out.dat"); System.out.println((char)input.read()); // Read and display a byte input.close(); } Here can’t be “199”, otherwise you use DataOutputStream Demo Test.java If no “char”, then the numeric number will be printed

42. Liang,Introduction to Java Programming,revised by Dai-kaiyu 42 Write a byte 199 as characters import java.io.*; public class Test { public static void main(String[] args) throws IOException { FileWriter output = new FileWriter("out.txt"); output.write("199"); // Output string "199" to the stream output.close(); // Read and display three characters FileReader input = new FileReader("out.txt"); System.out.print((char)input.read()); System.out.println((char)input.read()); input.close(); }

43. Liang,Introduction to Java Programming,revised by Dai-kaiyu 43 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. Optional

44. Liang,Introduction to Java Programming,revised by Dai-kaiyu 44 ObjectInputStream ObjectInputStream extends InputStream and implements ObjectInput and ObjectStreamConstants. You can replace DataInputStream/DataOutputStream completely with ObjectInputStream/ObjectOutputStream

45. Liang,Introduction to Java Programming,revised by Dai-kaiyu 45 ObjectOutputStream ObjectOutputStream extends OutputStream and implements ObjectOutput and ObjectStreamConstants.

46. Liang,Introduction to Java Programming,revised by Dai-kaiyu 46

47. Liang,Introduction to Java Programming,revised by Dai-kaiyu 47 Using Object Streams You may wrap an ObjectInputStream/ObjectOutputStream on any InputStream/OutputStream using the following constructors: // Create an ObjectInputStream public ObjectInputStream(InputStream in) // Create an ObjectOutputStream public ObjectOutputStream(OutputStream out) TestObjectOutputStream Run TestObjectInputStream Run

48. Liang,Introduction to Java Programming,revised by Dai-kaiyu 48 Using Object Streams Sample public class Student implements Serializable { int id; String name; int age; String department; transient int number; static int count; }

49. Liang,Introduction to Java Programming,revised by Dai-kaiyu 49 Using Object Streams Sample public class Objectser { public static void main(String args[]) { Student stu=new Student(981036, “Li Ming”, 16, “CSD”); try { FileOutputStream fo = new FileOutputStream(“data.ser”); ObjectOutputStream so = new ObjectOutputStream(fo); so.writeObject(stu); so.close(); } catch(Exception e) { System.err.println(e); }

50. Liang,Introduction to Java Programming,revised by Dai-kaiyu 50 Using Object Streams Sample public class ObjectRecov { public static void main(String args[]) { Student stu; try { FileInputStream fi = new FileInputStream(“data.ser”); ObjectInputStream si = new ObjectInputStream(fi); stu = (Student)si.readObject(); si.close(); System.out.println(“ID: ”+stu.id+“name:”+ stu.name+“age:”+age+“dept.:”+stu.department); } catch(Exception e) { System.out.println(e); } Can this statement put behind catch?

51. Liang,Introduction to Java Programming,revised by Dai-kaiyu 51 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.

52. Liang,Introduction to Java Programming,revised by Dai-kaiyu 52 The transient Keyword The value of the objects’s static variables are not stored If an object is an instance of Serializable, but it contains non-serializable instance data fields, the object can’t be serialized. To enable the object to be serialized, you can use the transient keyword to mark these data fields to tell the JVM to ignore these fields when writing the object to an object stream.

53. Liang,Introduction to Java Programming,revised by Dai-kaiyu 53 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.

54. Liang,Introduction to Java Programming,revised by Dai-kaiyu 54 Serializing Arrays An array is serializable if all its elements are serializable. So an entire array can be saved using writeObject into a file and later restored using readObject. Listing 16.12 stores an array of five int values an array of three strings, and an array of two JButton objects, and reads them back to display on the console. TestObjectStreamForArray Run

55. Liang,Introduction to Java Programming,revised by Dai-kaiyu 55 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.

56. Liang,Introduction to Java Programming,revised by Dai-kaiyu 56 RandomAccessFile

57. Liang,Introduction to Java Programming,revised by Dai-kaiyu 57 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.

58. Liang,Introduction to Java Programming,revised by Dai-kaiyu 58 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.

59. Liang,Introduction to Java Programming,revised by Dai-kaiyu 59 RandomAccessFile Methods, cont. void seek(long pos) throws IOException; Sets the offset from the beginning of the RandomAccessFile stream to where the next read or write occurs. seek(0) & seek(file.length()); long getFilePointer() IOException; Returns the current offset, in bytes, from the beginning of the file to where the next read or write occurs.

60. Liang,Introduction to Java Programming,revised by Dai-kaiyu 60 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.

61. Liang,Introduction to Java Programming,revised by Dai-kaiyu 61 RandomAccessFile Constructor RandomAccessFile raf = new RandomAccessFile("test.dat", "rw"); //allows read and write RandomAccessFile raf = new RandomAccessFile("test.dat", "r"); //read only

62. Liang,Introduction to Java Programming,revised by Dai-kaiyu 62 A Short Example on RandomAccessFile Run TestRandomAccessFile

63. Liang,Introduction to Java Programming,revised by Dai-kaiyu 63 Case Studies: Address Book Optional Now let us use RandomAccessFile to create a useful project for storing and viewing and address book. The user interface of the program is shown in Figure 16.24. The Add button stores a new address to the end of the file. The First, Next, Previous, and Last buttons retrieve the first, next, previous, and last addresses from the file, respectively.

64. Liang,Introduction to Java Programming,revised by Dai-kaiyu 64 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. FixedLengthStringIO

65. Liang,Introduction to Java Programming,revised by Dai-kaiyu 65 Address Implementation The rest of the work can be summarized in the following steps: Create the user interface. Add a record to the file. Read a record from the file. Write the code to implement the button actions. Run AddressBook