1. Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 1 Chapter 12 Exception Handling and Text IO

2. Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 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, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 3 Objectives F To get an overview of exceptions and exception handling. F To explore the advantages of using exception handling. F To distinguish exception types: Error (fatal) vs. Exception (nonfatal) and checked vs. unchecked. F To declare exceptions in a method header. F To throw exceptions in a method. F To write a try-catch block to handle exceptions. F To obtain information from an exception object. F To use the finally clause in a try-catch block. F To define custom exception classes. F To discover file/directory properties, to delete and rename files/directories, and to create directories using the File class. F To write data to a file using the PrintWriter class. F To use try-with-resources to ensure that the resources are closed automatically. F To read data from a file using the Scanner class. F To understand how data is read using a Scanner. F To develop a program that replaces text in a file. F To read data from the Web. F To develop a Web crawler.

4. Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 4 Exception-Handling Overview Show runtime error: Program 1 Fix it using an if statement: Program 2 Fix it using Exception Handling: Program 3

5. Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 5 Exception Advantages So you see the advantages of using exception handling. Exception handling enables a program to deal with exceptional situations and continue its normal execution It enables a method to throw an exception to its caller. Without this capability, a method must handle the exception or terminate the program. You should not let the method terminate the program – the caller should decide whether to terminate the program

6. Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 6 Exception Types The Object class, in the java.lang package, sits at the top of the class hierarchy tree. Every class is a descendant, direct or indirect, of the Object class.

7. Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 7 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.

8. Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 8 Exceptions Exception describes errors caused by your program and external circumstances. These errors can be caught and handled by your program.

9. Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 9 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.

10. Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 10 Unchecked Exceptions In most cases, unchecked exceptions reflect programming logic errors that are not recoverable. For example, 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.

11. Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 11 Unchecked Exceptions Unchecked exception.

12. Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 12 Declaring, Throwing, and Catching Exceptions

13. Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 13 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

14. Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 14 Throwing Exceptions When the program detects an error, the program can create an instance of an appropriate exception type and throw it. This is known as throwing an exception. Here is an example, throw new TheException(); TheException ex = new TheException(); throw ex;

15. Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 15 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"); }

16. Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 16 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; }

17. Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 17 The finally Clause try { statements; } catch(TheException ex) { handling ex; } finally { finalStatements; }

18. Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 18 Trace a Program Execution animation try { statements; } catch(TheException ex) { handling ex; } finally { finalStatements; } Next statement; Suppose no exceptions in the statements

19. Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 19 Trace a Program Execution animation try { statements; } catch(TheException ex) { handling ex; } finally { finalStatements; } Next statement; The final block is always executed

20. Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 20 Trace a Program Execution animation try { statements; } catch(TheException ex) { handling ex; } finally { finalStatements; } Next statement; Next statement in the method is executed

21. Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 21 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

22. Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 22 Trace a Program Execution animation try { statement1; statement2; statement3; } catch(Exception1 ex) { handling ex; } finally { finalStatements; } Next statement; The exception is handled.

23. Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 23 Trace a Program Execution animation try { statement1; statement2; statement3; } catch(Exception1 ex) { handling ex; } finally { finalStatements; } Next statement; The final block is always executed.

24. Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 24 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.

25. Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 25 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.

26. Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 26 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.

27. Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 27 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"); }

28. Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 28 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");

29. Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 29 Defining Custom Exception Classes F Use the exception classes in the API whenever possible. F Define custom exception classes if the predefined classes are not sufficient. F Define custom exception classes by extending Exception or a subclass of Exception.

30. Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 30 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.

31. Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 31 Obtaining file properties and manipulating file

32. Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 32 Problem: Explore File Properties 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. The following figures show a sample run of the program on Windows and on Unix.

33. Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 33 Text I/O 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. The objects contain the methods for reading/writing data from/to a file. This section introduces how to read/write strings and numeric values from/to a text file using the Scanner and PrintWriter classes.

34. Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 34 Writing Data Using PrintWriter WriteData Run

35. Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 35 Try-with-resources Programmers often forget to close the file. JDK 7 provides the new try-with-resources syntax that automatically closes the files. try (declare and create resources) { Use the resource to process the file; } WriteDataWithAutoClose Run

36. Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 36 Reading Data Using Scanner ReadData Run

37. Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 37 Exercise: Changing the Delimiter String s = "Hello World Hello_World"; // create a new scanner with the specified String Object java.util.Scanner scanner = new java.util.Scanner(s); while(scanner.hasNext()){ System.out.println(scanner.next()); } scanner = new java.util.Scanner(s); // change the delimiter of this scanner, underscore scanner.useDelimiter("_"); while(scanner.hasNext()){ System.out.println(scanner.next()); } scanner = new java.util.Scanner(s); // change the delimiter of this scanner,underscore or a space scanner.useDelimiter("_| "); while(scanner.hasNext()){ System.out.println(scanner.next()); } // close the scanner scanner.close();

38. Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 38 Note: How does Scanner Work? Note 1: What happens when you enter 55, press the enter key and then enter 65 and press the enter key for the following code? Scanner input = new Scanner(System.in); int intValue = input.nextInt(); String line = input.nextLine(); Note 2: Besides reading data from a file or a keyboard, you can also scan data from a string. Scanner input = new Scanner(“13 14”); int sum = input.nextInt() + input.nextInt(); System.out.println(sum);

39. Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 39 Exercise: Replacing Text Write a class named ReplaceText that replaces a string in a text file with a new string. java.io.File file1 = new java.io.File("C:\\Users\\AA\\Desktop\\test.txt"); java.io.File file2 = new java.io.File("C:\\Users\\AA\\Desktop\\test1.txt"); try ( java.util.Scanner input = new java.util.Scanner(file1); java.io.PrintWriter output = new java.io.PrintWriter(file2); ) { while (input.hasNext()) { String s1 = input.nextLine(); String s2 = s1.replaceAll(“number", “value"); output.println(s2); }

40. Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 40 Reading Data from the Web Just like you can read data from a file on your computer, you can read data from a file on the Web.

41. Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 41 Reading Data from the Web URL url = new URL("www.google.com/index.html"); After a URL object is created, you can use the openStream() method defined in the URL class to open an input stream and use this stream to create a Scanner object as follows: Scanner input = new Scanner(url.openStream()); ReadFileFromURL Run

42. Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 42 Case Study: Web Crawler This case study develops a program that travels the Web by following hyperlinks.

43. Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 43 Case Study: Web Crawler The program follows the URLs to traverse the Web. To avoid that each URL is traversed only once, the program maintains two lists of URLs. One list stores the URLs pending for traversing and the other stores the URLs that have already been traversed. The algorithm for this program can be described as follows:

44. Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved. 44 Case Study: Web Crawler Add the starting URL to a list named listOfPendingURLs; while listOfPendingURLs is not empty { Remove a URL from listOfPendingURLs; if this URL is not in listOfTraversedURLs { Add it to listOfTraversedURLs; Display this URL; Exit the while loop when the size of S is equal to 100. Read the page from this URL and for each URL contained in the page { Add it to listOfPendingURLs if it is not is listOfTraversedURLs; } WebCrawler Run