1. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 1 Chapter 18 Exception Handling

2. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 2 Motivations When a program runs into a runtime error, the program terminates abnormally. How can you handle the runtime error so that the program can continue to run or terminate gracefully? This is the subject we will introduce in this chapter.

3. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 3 Exception-Handling Overview QuotientRun QuotientWithIfRun QuotientWithExceptionRun

4. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 4 Exception Advantages QuotientWithMethodRun Now you see the advantages of using exception handling. It enables a method to throw an exception to its caller. Without this capability, a method must handle the exception or terminate the program.

5. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 5 System Errors System errors are thrown by JVM and represented in the Error class. The Error class describes internal system errors. Such errors rarely occur. If one does, there is little you can do beyond notifying the user and trying to terminate the program gracefully.

6. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 6 Exceptions Exception describes errors caused by your program and external circumstances. These errors can be caught and handled by your program.

7. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 7 Runtime Exceptions RuntimeException is caused by programming errors, such as bad casting, accessing an out-of-bounds array, and numeric errors.

8. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 8 Checked Exceptions vs. Unchecked Exceptions RuntimeException, Error and their subclasses are known as unchecked exceptions. All other exceptions are known as checked exceptions, meaning that the compiler forces the programmer to check and deal with the exceptions.

9. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 9 Unchecked Exceptions In most cases, unchecked exceptions reflect programming logic errors that are not recoverable. For example, a NullPointerException is thrown if you access an object through a reference variable before an object is assigned to it; an IndexOutOfBoundsException is thrown if you access an element in an array outside the bounds of the array. These are the logic errors that should be corrected in the program. Unchecked exceptions can occur anywhere in the program. To avoid cumbersome overuse of try-catch blocks, Java does not mandate you to write code to catch unchecked exceptions.

10. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 10 Checked or Unchecked Exceptions Unchecked exception.

11. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 11 Declaring, Throwing, and Catching Exceptions

12. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 12 Declaring Exceptions Every method must state the types of checked exceptions it might throw. This is known as declaring exceptions. public void myMethod() throws IOException public void myMethod() throws IOException, OtherException

13. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 13 Throwing Exceptions Example /** Set a new radius */ public void setRadius(double newRadius) throws IllegalArgumentException throws IllegalArgumentException { if (newRadius >= 0) radius = newRadius; else throw new IllegalArgumentException( "Radius cannot be negative"); }

14. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 14 Catching Exceptions try { statements; // Statements that may throw exceptions } catch (Exception1 exVar1) { handler for exception1; } catch (Exception2 exVar2) { handler for exception2; }... catch (ExceptionN exVar3) { handler for exceptionN; }

15. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 15 Catching Exceptions

16. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 16 Catch or Declare Checked Exceptions Java forces you to deal with checked exceptions. If a method declares a checked exception (i.e., an exception other than Error or RuntimeException), you must invoke it in a try-catch block or declare to throw the exception in the calling method. For example, suppose that method p1 invokes method p2 and p2 may throw a checked exception (e.g., IOException), you have to write the code as shown in (a) or (b).

17. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 17 Example: Declaring, Throwing, and Catching Exceptions F Objective: This example demonstrates declaring, throwing, and catching exceptions by modifying the setRadius method in the Circle class defined in Chapter 6. The new setRadius method throws an exception if radius is negative. TestCircleWithException Run CircleWithException

18. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 18 Example: Exceptions in GUI Applications F An error message appears on the console, but the GUI application continues running. F Write a program that creates a user interface to perform integer divisions. The user enters two numbers in the text fields Number 1 and Number 2. The division of Number 1 and Number 2 is displayed in the Result field when the Divide button is clicked. IntegerDivision Run

19. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 19 Rethrowing Exceptions try { statements; } catch(TheException ex) { perform operations before exits; throw ex; }

20. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 20 The finally Clause try { statements; } catch(TheException ex) { handling ex; } finally { finalStatements; }

21. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 21 Trace a Program Execution animation try { statements; } catch(TheException ex) { handling ex; } finally { finalStatements; } Next statement; Suppose no exceptions in the statements

22. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 22 Trace a Program Execution animation try { statements; } catch(TheException ex) { handling ex; } finally { finalStatements; } Next statement; The final block is always executed

23. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 23 Trace a Program Execution animation try { statements; } catch(TheException ex) { handling ex; } finally { finalStatements; } Next statement; Next statement in the method is executed

24. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 24 Trace a Program Execution animation try { statement1; statement2; statement3; } catch(Exception1 ex) { handling ex; } finally { finalStatements; } Next statement; Suppose an exception of type Exception1 is thrown in statement2

25. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 25 Trace a Program Execution animation try { statement1; statement2; statement3; } catch(Exception1 ex) { handling ex; } finally { finalStatements; } Next statement; The exception is handled.

26. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 26 Trace a Program Execution animation try { statement1; statement2; statement3; } catch(Exception1 ex) { handling ex; } finally { finalStatements; } Next statement; The final block is always executed.

27. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 27 Trace a Program Execution animation try { statement1; statement2; statement3; } catch(Exception1 ex) { handling ex; } finally { finalStatements; } Next statement; The next statement in the method is now executed.

28. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 28 Cautions When Using Exceptions F Exception handling separates error-handling code from normal programming tasks, thus making programs easier to read and to modify. Be aware, however, that exception handling usually requires more time and resources because it requires instantiating a new exception object, rolling back the call stack, and propagating the errors to the calling methods.

29. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 29 When to Throw Exceptions F An exception occurs in a method. If you want the exception to be processed by its caller, you should create an exception object and throw it. If you can handle the exception in the method where it occurs, there is no need to throw it.

30. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 30 When to Use Exceptions When should you use the try-catch block in the code? You should use it to deal with unexpected error conditions. Do not use it to deal with simple, expected situations. For example, the following code try { System.out.println(refVar.toString()); } catch (NullPointerException ex) { System.out.println("refVar is null"); }

31. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 31 When to Use Exceptions is better to be replaced by if (refVar != null) System.out.println(refVar.toString()); else System.out.println("refVar is null");

32. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 32 Creating Custom Exception Classes F Use the exception classes in the API whenever possible. F Create custom exception classes if the predefined classes are not sufficient. F Declare custom exception classes by extending Exception or a subclass of Exception.

33. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 33 Custom Exception Class Example Run InvalidRadiusException In Listing 18.1, the setRadius method throws an exception if the radius is negative. Suppose you wish to pass the radius to the handler, you have to create a custom exception class. CircleWithRadiusException TestCircleWithRadiusException

34. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 34 Assertions An assertion is a Java statement that enables you to assert an assumption about your program. An assertion contains a Boolean expression that should be true during program execution. Assertions can be used to assure program correctness and avoid logic errors. Companion Website

35. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 35 Executing Assertions When an assertion statement is executed, Java evaluates the assertion. If it is false, an AssertionError will be thrown. The AssertionError class has a no-arg constructor and seven overloaded single-argument constructors of type int, long, float, double, boolean, char, and Object. For the first assert statement with no detail message, the no-arg constructor of AssertionError is used. For the second assert statement with a detail message, an appropriate AssertionError constructor is used to match the data type of the message. Since AssertionError is a subclass of Error, when an assertion becomes false, the program displays a message on the console and exits. Companion Website

36. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 36 Executing Assertions Example public class AssertionDemo { public static void main(String[] args) { int i; int sum = 0; for (i = 0; i < 10; i++) { sum += i; } assert i == 10; assert sum > 10 && sum < 5 * 10 : "sum is " + sum; } Companion Website

37. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 37 Running Programs with Assertions By default, the assertions are disabled at runtime. To enable it, use the switch –enableassertions, or –ea for short, as follows: java –ea AssertionDemo Assertions can be selectively enabled or disabled at class level or package level. The disable switch is – disableassertions or –da for short. For example, the following command enables assertions in package package1 and disables assertions in class Class1. java –ea:package1 –da:Class1 AssertionDemo Companion Website

38. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 38 Using Exception Handling or Assertions Assertion should not be used to replace exception handling. Exception handling deals with unusual circumstances during program execution. Assertions are to assure the correctness of the program. Exception handling addresses robustness and assertion addresses correctness. Like exception handling, assertions are not used for normal tests, but for internal consistency and validity checks. Assertions are checked at runtime and can be turned on or off at startup time. Companion Website

39. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 39 Using Exception Handling or Assertions, cont. Do not use assertions for argument checking in public methods. Valid arguments that may be passed to a public method are considered to be part of the method’s contract. The contract must always be obeyed whether assertions are enabled or disabled. For example, the following code should be rewritten using exception handling as shown in Lines 28-35 in Circle.java in Listing 18.1. public void setRadius(double newRadius) { assert newRadius >= 0; radius = newRadius; } Companion Website

40. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 40 Using Exception Handling or Assertions, cont. Use assertions to reaffirm assumptions. This gives you more confidence to assure correctness of the program. A common use of assertions is to replace assumptions with assertions in the code. Companion Website

41. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 41 Using Exception Handling or Assertions, cont. Use assertions to reaffirm assumptions. This gives you more confidence to assure correctness of the program. A common use of assertions is to replace assumptions with assertions in the code (only if the assumptions are enforced by the caller!). switch (month) { case 1:... ; break; case 2:... ; break;... case 12:... ; break; default: assert false : "Invalid month: " + month } Companion Website

42. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 42 Chapter 19 Binary I/O

43. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 43 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. Formatter output = new Formatter("temp.txt"); output.format("%s", "Java 101"); output.close(); Scanner input = new Scanner(new File("temp.txt")); System.out.println(input.nextLine());

44. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 44 Text File vs. Binary File F 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. F Although it is not technically precise and correct, 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.

45. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 45 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.

46. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 46 The value returned is a byte as an int type. InputStream

47. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 47 The value is a byte as an int type. OutputStream

48. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 48 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.

49. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 49 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.

50. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 50 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

51. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 51 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.

52. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 52 DataInputStream DataInputStream extends FilterInputStream and implements the DataInput interface.

53. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 53 DataOutputStream DataOutputStream extends FilterOutputStream and implements the DataOutput interface.

54. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 54 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. Why UTF-8? What is UTF-8? UTF-8 is a coding scheme that allows systems to operate with both ASCII and Unicode efficiently. Most operating systems use ASCII. Java uses Unicode. The ASCII character set is a subset of the Unicode character set. Since most applications need only the ASCII character set, it is a waste to represent an 8-bit ASCII character as a 16-bit Unicode character. The UTF-8 is an alternative scheme that stores a character using 1, 2, or 3 bytes. ASCII values (less than 0x7F) are coded in one byte. Unicode values less than 0x7FF are coded in two bytes. Other Unicode values are coded in three bytes.

55. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 55 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

56. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 56 Checking End of File TIP: If you keep reading data at the end of a stream, an EOFException would occur. So how do you check the end of a file? You can use input.available() to check it. input.available() == 0 indicates that it is the end of a file. Order and Format CAUTION: You have to read the data in the same order and same format in which they are stored. For example, since names are written in UTF-8 using writeUTF, you must read names using readUTF.

57. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 57 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.

58. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 58 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)

59. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 59 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

60. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 60 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

61. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 61 ObjectInputStream ObjectInputStream extends InputStream and implements ObjectInput and ObjectStreamConstants.

62. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 62 ObjectOutputStream ObjectOutputStream extends OutputStream and implements ObjectOutput and ObjectStreamConstants.

63. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 63 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

64. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 64 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.

65. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 65 The transient Keyword If an object is an instance of Serializable, but it contains non-serializable instance data fields, can the object be serialized? The answer is no. To enable the object to be serialized, you can use the transient keyword to mark these data fields to tell the JVM to ignore these fields when writing the object to an object stream.

66. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 66 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.

67. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 67 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

68. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 68 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.

69. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 69 RandomAccessFile

70. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 70 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.

71. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 71 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.

72. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 72 RandomAccessFile Methods, cont. F void seek(long pos) throws IOException; Sets the offset from the beginning of the RandomAccessFile stream to where the next read or write occurs. F long getFilePointer() IOException; Returns the current offset, in bytes, from the beginning of the file to where the next read or write occurs.

73. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 73 RandomAccessFile Methods, cont. F long length()IOException Returns the length of the file. F 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.

74. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 74 RandomAccessFile Constructor RandomAccessFile raf = new RandomAccessFile("test.dat", "rw"); //allows read and write RandomAccessFile raf = new RandomAccessFile("test.dat", "r"); //read only

75. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 75 A Short Example on RandomAccessFile Run TestRandomAccessFile

76. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 76 Case Studies: Address Book Now let us use RandomAccessFile to create a useful project for storing and viewing and address book. The user interface of the program is shown in Figure 16.24. The Add button stores a new address to the end of the file. The First, Next, Previous, and Last buttons retrieve the first, next, previous, and last addresses from the file, respectively.