Horstmann Chapter 11

Big Java 4

Chapter 11 – Input/Output and Exception Handling

Chapter Goals

a keyboard

  • To read and write text files
  • To process command line arguments
  • To throw and catch exceptions
  • To implement programs that propagate checked exceptions

Reading and Writing Text Files - Reading

  • Use Scanner class for reading text files
  • To read from a disk file:
    • Construct a File object representing the input file
    File inputFile = new File("input.txt");
  • Use this File object to construct a Scanner object:
 Scanner in = new Scanner(reader);
  • Use the Scanner methods to read data from file
    • next, nextInt, and nextDouble
  • Reading and Writing Text Files - Reading

    • A loop to process numbers in the input file:
      while (in.hasNextDouble())
      {
         double value = in.nextDouble();
         Process value.
      }

    Reading and Writing Text Files - Writing

    • To write to a file, construct a PrintWriter object:
      PrintWriter out = new PrintWriter("output.txt");
    • If file already exists, it is emptied before the new data are written into it.
    • If file doesn't exist, an empty file is created.
    • Use print and println to write into a PrintWriter:
      out.println("Hello, World!");
      out.printf("Total: %8.2f\n", total);
    • You must close a file when you are done processing it:
      in.close();
      out.close();
      Otherwise, not all of the output may be written to the disk file.
    • Always specify "UTF-8" as the second parameter when construction a Scanner or a PrintWriter.

    FileNotFoundException

    • When the input or output file doesn't exist, a FileNotFoundException can occur.
    • To handle the exception, label the main method like this:
      public static void main(String[] args) throws FileNotFoundException

    Reading and Writing Text Files - Example

    • Read a file containing numbers
      • Sample input
      32 54 67.5 29 35 80
      115 44.5 100 65
  • Write the numbers in a column followed by their total
    • Output from sample input
              32.00
              54.00
              67.50
              29.00
              35.00
              80.00
             115.00
              44.50
             100.00
              65.00
    Total:   622.00
  • section_1/Total.java

    Your browser does not support the <object> tag.

    Self Check 11.1

    What happens when you supply the same name for the input and output files to the Total program? Try it out if you are not sure.
    • Answer: When the PrintWriter object is created, the output file is emptied. Sadly, that is the same file as the input file. The input file is now empty and the while loop exits immediately.

    Self Check 11.2

    What happens when you supply the name of a nonexistent input file to the Total program? Try it out if you are not sure.
    • Answer: The program throws a FileNotFoundException and terminates.

    Self Check 11.3

    Suppose you wanted to add the total to an existing file instead of writing a new file. Self Check 1 indicates that you cannot simply do this by specifying the same file for input and output. How can you achieve this task? Provide the pseudocode for the solution.
    • Answer:
      Open a scanner for the file.
      For each number in the scanner
           Add the number to an array.
      Close the scanner.
      Set total to 0.
      Open a print writer for the file.
      For each number in the array
          Write the number to the print writer.
         Add the number to total.
      Write total to the print writer.
      Close the print writer.

    Self Check 11.4

    How do you modify the program so that it shows the average, not the total, of the inputs?
    • Answer: Add a variable count that is incremented whenever a number is read. At the end, print the average, not the total, as
      out.printf("Average: %8.2f\n", total / count);
      Because the string "Average" is three characters longer than "Total", change the other output to out.printf("%18.2f\n", value).

    Self Check 11.5

    How can you modify the Total program so that it writes the values in two columns, like this:
       32.00  54.00
       67.50  29.00
       35.00  80.00
      115.00  44.50
      100.00  65.00
    Total:    622.00
    • Answer: Add a variable count that is incremented whenever a number is read. Only write a new line when it is even.
      count++;
      out.printf("%8.2f", value);
      if (count % 2 == 0) { out.println(); }
      At the end of the loop, write a new line if count is odd, then write the total:
      if (count % 2 == 1) { out.println(); }
      out.printf("Total: %10.2f\n", total); 

    Text Input and Output

    • The next method of the Scanner class reads a string that is delimited by white space.
    • A loop for processing a file
      while (in.hasNext()) 
      { 
         String input = in.next(); 
         System.out.println(input);
      }
    • If the input is "Mary had a little lamb", the loop prints each word on a separate line
      Mary 
      had 
      a 
      little 
      lamb

    Text Input and Output

    • The next method returns any sequence of characters that is not white space.
    • White space includes: spaces, tab characters, and the newline characters that separate lines.
    • These strings are considered "words" by the next method
          snow.
          1729
          C++
    • When next is called:
      • Input characters that are white space are consumed - removed from the input
      • They do not become part of the word
      • The first character that is not white space becomes the first character of the word
      • More characters are added until
        • Either another white space character occurs
        • Or the end of the input file has been reached
    • If the end of the input file is reached before any character was added to the word
      • a “no such element exception” occurs.

    Text Input and Output

    • To read just words and discard anything that isn't a letter:
      • Call useDelimiter method of the Scanner class
      Scanner in = new Scanner(. . .); 
      in.useDelimiter("[^A-Za-z]+");
      . . .
    • The word separator becomes any character that is not a letter.
    • Punctuation and numbers are not included in the words returned by the next method.

    Text Input and Output - Reading Characters

    • To read one character at a time, set the delimiter pattern to the empty string:
      Scanner in = new Scanner(. . .); 
      in.useDelimiter(""); 
      
    • Now each call to next returns a string consisting of a single character.
    • To process the characters:
      while (in.hasNext()) 
      {
         char ch = in.next().charAt(0); 
         Process ch 
      }

    Text Input and Output - Classifying Characters

    The Character class has methods for classifying characters.

    metods of Character class

    Text Input and Output - Reading Lines

    • The nextLine method reads a line of input and consumes the newline character at the end of the line:
      String line = in.nextLine();
    • The hasNextLine method returns true if there are more input lines, false when all lines have been read.
    • Example: process a file with population data from the CIA Fact Book with lines like this:
      China  1330044605 
      India  1147995898 
      United States 303824646
      ...
    • Read each input line into a string
      while (in.hasNextLine())
      {
         String line = nextLine();
         Process  line.
      }

    Text Input and Output - Reading Lines

    • Then use the isDigit and isWhitespace methods to find out where the name ends and the number starts.
    • To locate the first digit:
      int i = 0; 
      while (!Character.isDigit(line.charAt(i))) { i++; }
    • To extract the country name and population:
      String countryName = line.substring(0, i);
      String population = line.substring(i);
    • Use trim to remove spaces at the beginning and end of string:
      countryName = countryName.trim();
      Characters in a string
    • Note that the population is stored in a string.

    Text Input and Output - Scanning a String

    • Occasionally easier to construct a new Scanner object to read the characters from a string:
      Scanner lineScanner = new Scanner(line);
    • Then you can use lineScanner like any other Scanner object, reading words and numbers:
      String countryName = lineScanner.next(); 
      while (!lineScanner.hasNextInt()) 
      { 
         countryName = countryName + " " + lineScanner.next(); 
      } 
      int populationValue = lineScanner.nextInt();

    Text Input and Output - Converting Strings to Numbers

    • If a string contains the digits of a number.
      • Use the Integer.parseInt or Double.parseDouble method to obtain the number value.
    • If the string contains "303824646"
      • Use Integer.parseInt method to get the integer value
      int populationValue = Integer.parseInt(population);
         // populationValue is the integer 303824646
  • If the string contains "3.95"
    • Use Double.parseDouble
    double price = Double.parseDouble(input);
       // price is the floating-point number 3.95
  • The string must not contain spaces or other non-digits. Use trim:
    int populationValue = Integer.parseInt(population.trim());
  • Avoiding Errors When Reading Numbers

    • If the input is not a properly formatted number when calling nextInt or nextDouble method
      • input mismatch exception occurs
    • For example, if the input contains characters:
      A non-integer
      • White space is consumed and the word 21st is read.
      • 21st is not a properly formatted number
      • Causes an input mismatch exception in the nextInt method.
    • If there is no input at all when you call nextInt or nextDouble,
      • A “no such element exception” occurs.
    • To avoid exceptions, use the hasNextInt method
      if (in.hasNextInt())
      {
         int value = in.nextInt();
         . . .
      }

    Mixing Number, Word, and Line Input

    • The nextInt, nextDouble, and next methods do not consume the white space that follows the number or word.
    • This can be a problem if you alternate between calling nextInt/ nextDouble/next and nextLine.
    • Example: a file contains country names and populations in this format:
      China
      1330044605
      India
      1147995898
      United States
      303824646
    • The file is read with these instructions:
      while (in.hasNextLine())
      {
         String countryName = in.nextLine();
         int population = in.nextInt();
         Process the country name and population.
      }

    Mixing Number, Word, and Line Input

    • Initial input
      initial input
    • Input after first call to nextLine
      input after call to nextLine
    • Input after call to nextInt
      input afer nextInt
      • nextInt did not consume the newline character
    • The second call to nextLine reads an empty string!
    • The remedy is to add a call to nextLine after reading the population value:
      String countryName = in.nextLine();
      int population = in.nextInt();
      in.nextLine(); // Consume the newline

    Formatting Output

    • There are additional options for printf method.
    • Format flags
      table of formatting flags
    • Example: print a table of items and prices, each stored in an array
      Cookies:       3.20
      Linguine:      2.95
      Clams:        17.29
    • The item strings line up to the left; the numbers line up to the right.

    Formatting Output

    • To specify left alignment, add a hyphen (-) before the field width:
      System.out.printf(“%-10s%10.2f”, items[i] + “:”, prices[i]);
    • There are two format specifiers: "%-10s%10.2f "
    • %-10s
      • Formats a left-justified string.
      • Padded with spaces so it becomes ten characters wide
    • %10.2f
      • Formats a floating-point number
      • The field that is ten characters wide.
      • Spaces appear to the left and the value to the right
    • The output
      two format specifiers

    Formatting Output

    A format specifier has the following structure:
    • The first character is a %.
    • Next are optional “flags” that modify the format, such as - to indicate left alignment.
    • Next is the field width, the total number of characters in the field (including the spaces used for padding), followed by an optional precision for floating-point numbers.
    • The format specifier ends with the format type, such as f for floating-point values or s for strings.
    • Format types
      Format types

    Self Check 11.6

    Suppose the input contains the characters Hello, World!. What are the values of word and input after this code fragment?
    String word = in.next();
    String input = in.nextLine();
    • Answer: word is "Hello", and input is "World!"

    Self Check 11.7

    Suppose the input contains the characters 995.0 Fred. What are the values of number and input after this code fragment?
    int number = 0;
    if (in.hasNextInt()) { number = in.nextInt(); }
    String input = in.next();
    • Answer: Because 995.0 is not an integer, the call in.hasNextInt() returns false, and the call in.nextInt() is skipped. The value of number stays 0, and input is set to the string "995.0".

    Self Check 11.8

    Suppose the input contains the characters 6E6 6,995.00. What are the values of x1 and x2 after this code fragment?
    double x1 = in.nextDouble();
    double x2 = in.nextDouble();
    • Answer: x1 is set to 6000000. Because a comma is not considered a part of a floating-point number in Java, the second call to nextDouble causes an input mismatch exception and x2 is not set.

    Self Check 11.9

    Your input file contains a sequence of numbers, but sometimes a value is not available and marked as N/A. How can you read the numbers and skip over the markers?
    • Answer: Read them as strings, and convert those strings to numbers that are not equal to N/A:
      String input = in.next(); 
      if (!input.equals("N/A")) 
      { 
         double value = Double.parseDouble(input); 
         Process value 
      }

    Self Check 11.10

    How can you remove spaces from the country name in Section 11.2.4 without using the trim method?
    • Answer: Locate the last character of the country name:
      int j = i - 1;
      while (!Character.isWhiteSpace(line.charAt(j)))
      {
         j--;
      }
      Then extract the country name:
      String countryName = line.substring(0, j + 1);

    Command Line Arguments

    • You can run a Java program by typing a command at the prompt in the command shell window
      • Called “invoking the program from the command line”
    • With this method, you can add extra information for the program to use
      • Called command line arguments
    • Example: start a program with a command line
      java ProgramClass -v input.dat
      The program receives the strings "-v" and "input.dat" as command line arguments
    • Useful for automating tasks
    • Your program receives its command line arguments in the args parameter of the main method:
      public static void main(String[] args)
    • In the example, args is an array of length 2, containing the strings
      args[0]: "-v"
      args[1]: "input.dat"

    Command Line Arguments

    • Example: a program that encrypts a file
      • Use a Caesar Cipher that replaces A with a D, B with an E, and so on
      • Sample text
        cearsar cipher
    • The program will take command line arguments
      • An optional -d flag to indicate decryption instead of encryption
      • The input file name
      • The output file name
    • To encrypt the file input.txt and place the result into encrypt.txt
      java CaesarCipher input.txt encrypt.txt
    • To decrypt the file encrypt.txt and place the result into output.txt
      java CaesarCipher -d encrypt.txt output.txt
      .

    section_3/CaesarCipher.java

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    Self Check 11.11

    If the program is invoked with java CaesarCipher -d file1.txt, what are the elements of args?
    • Answer: args[0] is "-d" and args[1] is "file1.txt"

    Self Check 11.12

    Trace the program when it is invoked as in Self Check 11.
    • Answer:
      trace of self check 11
      Then the program prints a message
      Usage: java CaesarCipher [-d] infile outfile

    Self Check 11.13

    Will the program run correctly if the program is invoked with java CaesarCipher file1.txt file2.txt -d? If so, why? If not, why not?
    • Answer: The program will run correctly. The loop that parses the options does not depend on the positions in which the options appear.

    Self Check 11.14

    Encrypt CAESAR using the Caesar cipher.
    • Answer: FDHVDU

    Self Check 11.15

    How can you modify the program so that the user can specify an encryption key other than 3 with a -k option, for example
    java CaesarCipher -k15 input.txt output.txt
    • Answer: Add the lines
      else if (option == 'k')
      {
         key = Integer.parseInt(
         args[i].substring(2));
      }
      after line 27 and update the usage information.

    Exception Handling - Throwing Exceptions

    • Exception handling provides a flexible mechanism for passing control from the point of error detection to a handler that can deal with the error.
    • When you detect an error condition, throw an exception object to signal an exceptional condition
    • If someone tries to withdraw too much money from a bank account
      • Throw an IllegalArgumentException
      IllegalArgumentException exception
            = new IllegalArgumentException("Amount exceeds balance");
      throw exception;
  • When an exception is thrown, method terminates immediately
    • Execution continues with an exception handler
  • When you throw an exception, the normal control flow is terminated. This is similar to a circuit breaker that cuts off the flow of electricity in a dangerous situation.
    circuit breaker
  • Syntax 11.1 Throwing an Exception

    Syntax for throwing an exception

    Hierarchy of Exception Classes

    Figure 2 A Part of the Hierarchy of Exception Classes

    exception hierarchy

    Catching Exceptions

    • Every exception should be handled somewhere in your program
    • Place the statements that can cause an exception inside a try block, and the handler inside a catch clause.
      try
      {
         String filename = . . .;
         Scanner in = new Scanner(new File(filename));
         String input = in.next();
         int value = Integer.parseInt(input);
         . . .
      }
      catch (IOException exception)
      {
         exception.printStackTrace();
      }
      catch (NumberFormatException exception)
      {
         System.out.println(exception.getMessage());
      }

    Catching Exceptions

    • Three exceptions may be thrown in the try block:
      • The Scanner constructor can throw a FileNotFoundException.
      • Scanner.next can throw a NoSuchElementException.
      • Integer.parseInt can throw a NumberFormatException.
    • If any of these exceptions is actually thrown, then the rest of the instructions in the try block are skipped.

    Catching Exceptions

    • What happens when each exception is thrown:
    • If a FileNotFoundException is thrown,
      • then the catch clause for the IOException is executed because FileNotFoundException is a descendant of IOException.
      • If you want to show the user a different message for a FileNotFoundException, you must place the catch clause before the clause for an IOException
    • . If a NumberFormatException occurs,
      • then the second catch clause is executed.
    • A NoSuchElementException is not caught by any of the catch clauses.
      • The exception remains thrown until it is caught by another try block.

    Catching Exceptions

    • Each catch clause contains a handler.
    • Our example just informed the user of a problem.
    • Often better to give the user another chance.
    • When you throw an exception, you can provide your own message string.
    • For example, when you call
       throw new IllegalArgumentException("Amount exceeds balance");
      the message of the exception is the string provided in the constructor.
    • You should only catch those exceptions that you can handle.
      Dog catching a frisbee

    Syntax 11.2 Catching Exceptions

    Checked Exceptions

    Exceptions fall into three categories

    • Internal errors are reported by descendants of the type Error.
      • Example: OutOfMemoryError
    • Descendants of RuntimeException,
      • Example: IndexOutOfBoundsException or IllegalArgumentException
      • Indicate errors in your code.
      • They are called unchecked exceptions.
    • All other exceptions are checked exceptions.
      • Indicate that something has gone wrong for some external reason beyond your control
      • Example: IOException

    Checked Exceptions

    • Checked exceptions are due to external circumstances that the programmer cannot prevent.
      • The compiler checks that your program handles these exceptions.
    • The unchecked exceptions are your fault.
      • The compiler does not check whether you handle an unchecked exception.

    Checked Exceptions - throws

    • You can handle the checked exception in the same method that throws it
      try
      {
         File inFile = new File(filename);
         Scanner in = new Scanner(inFile); // Throws FileNotFoundException
         . . .
      }
      catch (FileNotFoundException exception) // Exception caught here
      {
         . . .
      }
    • Often the current method cannot handle the exception.
      • Tell the compiler you are aware of the exception
      • You want the method to terminate if the exception occurs
      • Add a throws clause to the method header
      public void readData(String filename) throws FileNotFoundException
      {
         File inFile = new File(filename);
         Scanner in = new Scanner(inFile);
      . . .
      }

    Checked Exceptions - throws

    • The throws clause signals to the caller of your method that it may encounter a FileNotFoundException.
      • The caller must decide
        • To handle the exception
        • Or declare the exception may be thrown
    • Throw early, catch late
      • Throw an exception as soon as a problem is detected.
      • Catch it only when the problem can be handled
    • Just as trucks with large or hazardous loads carry warning signs, the throws clause warns the caller that an exception may occur.
      a truck with a wide load warning

    Syntax 11.3 throws Clause

    Syntax of throws

    The finally Clause

    • Once a try block is entered, the statements in a finally clause are guaranteed to be executed - whether or not an exception is thrown.
    • Use when you do to do some clean up
    • Example - closing files
      PrintWriter out = new PrintWriter(filename);
      try
      {
         writeData(out);
      }
         finally
      {
      out.close();
      }
    • Executes the close even if an exception is thrown.

    The finally Clause

    passport control

     

    All visitors to a foreign country have to go through passport control, no matter what happened on their trip. Similarly, the code in a finally clause is always executed, even when an exception has occurred.

    Syntax 11.4 finally Clause

    syntax for throws

    Designing Your Own Exception Types

    • You can design your own exception types — subclasses of Exception or RuntimeException.
    • Throw an InsufficientFundsException when the amount to withdraw an amount from a bank account exceeds the current balance.
      if (amount > balance)
      {
         throw new InsufficientFundsException(
            "withdrawal of " + amount + " exceeds balance of " + balance);
      }
    • Make InsufficientFundsException an unchecked exception
      • Programmer could have avoided it by calling getBalance first
      • Extend RuntimeException or one of its subclasses

    Designing Your Own Exception Types

    • Supply two constructors for the class
      • A constructor with no arguments
      • A constructor that accepts a message string describing reason for exception
      public class InsufficientFundsException
            extends RuntimeException
      {
         public InsufficientFundsException() {}
      
         public InsufficientFundsException(String message)
         {
            super(message);
         }
      }
  • When the exception is caught, its message string can be retrieved
    • Using the getMessage method of the Throwable class.
  • Self Check 11.16

    Suppose balance is 100 and amount is 200. What is the value of balance after these statements?
    if (amount > balance)
    {
       throw new IllegalArgumentException("Amount exceeds balance");
    }
    balance = balance - amount;
    • Answer: It is still 100. The last statement was not executed because the exception was thrown.

    Self Check 11.17

    When depositing an amount into a bank account, we don’t have to worry about overdrafts—except when the amount is negative. Write a statement that throws an appropriate exception in that case.
    • Answer:
      if (amount < 0)
      {
         throw new IllegalArgumentException("Negative amount");
      }

    Self Check 11.18

    Consider the method
    public static void main(String[] args)
    {
       try
       {
          Scanner in = new Scanner(new File("input.txt"));
          int value = in.nextInt();
          System.out.println(value);
       }
       catch (IOException exception)
       {
          System.out.println("Error opening file.");
       }
    }
    Suppose the file with the given file name exists and has no contents. Trace the flow of execution.
    • Answer: The Scanner constructor succeeds because the file exists. The nextInt method throws a NoSuchElementException. This is not an IOException. Therefore, the error is not caught. Because there is no other handler, an error message is printed and the program terminates.

    Self Check 11.19

    Why is an ArrayIndexOutOfBoundsException not a checked exception?
    • Answer: Because programmers should simply check that their array index values are valid instead of trying to handle an ArrayIndexOutOfBoundsException.

    Self Check 11.20

    Is there a difference between catching checked and unchecked exceptions?
    • Answer: No. You can catch both exception types in the same way, as you can see in the code example on page 536.

    Self Check 11.21

    What is wrong with the following code, and how can you fix it?
    public static void writeAll(String[] lines, String filename)
    {
       PrintWriter out = new PrintWriter(filename);
       for (String line : lines)
       {
          out.println(line.toUpperCase());
       }
       out.close();
    }
    • Answer: There are two mistakes. The PrintWriter constructor can throw a FileNotFoundException. You should supply a throws clause. And if one of the array elements is null, a NullPointerException is thrown. In that case, the out.close() statement is never executed. You should use a try/finally statement.

    Self Check 11.22

    What is the purpose of the call super(message) in the second InsufficientFundsException
    constructor?
    • Answer: To pass the exception message string to the IllegalArgumentException superclass.

    Self Check 11.23

    Suppose you read bank account data from a file. Contrary to your expectation, the next input value is not of type double. You decide to implement a BadDataException. Which exception class should you extend?
    • Answer: Because file corruption is beyond the control of the programmer, this should be a checked exception, so it would be wrong to extend RuntimeException or IllegalArgumentException. Because the error is related to input, IOException would be a good choice.

    Application: Handling Input Errors

    • Program
      • Asks user for name of file
      • File expected to contain data values
      • First line of file contains total number of values
      • Remaining lines contain the data
      • Typical input file:
      3
      1.45
      -2.1
      0.05

    Case Study: A Complete Example

    • What can go wrong?
      • File might not exist
      • File might have data in wrong format
    • Who can detect the faults?
      • Scanner constructor will throw an exception when file does not exist
      • Methods that process input need to throw exception if they find error in data format
    • What exceptions can be thrown?
      • FileNotFoundException can be thrown by Scanner constructor
      • BadDataException, a custom checked exception class for reporting wrong data format
    • Who can remedy the faults that the exceptions report?
      • Only the main method of DataAnalyzer program interacts with user
        • Catches exceptions
        • Prints appropriate error messages
        • Gives user another chance to enter a correct file

    section_5/DataAnalyzer.java

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    The readFile Method of the DataSetReader Class

    • Constructs Scanner object
    • Calls readData method
    • Completely unconcerned with any exceptions
    • If there is a problem with input file, it simply passes the exception to caller:
      public double[] readFile(String filename) throws IOException
      {
         File inFile = new File(filename);
         Scanner in = new Scanner(inFile);
         try
         {
            readData(in);
            return data;
         }
         finally
         {
            in.close();
         }
      }

    The readData Method of the DataSetReader Class

    • Reads the number of values
    • Constructs an array
    • Calls readValue for each data value:
      private void readData(Scanner in) throws BadDataException
      {
         if (!in.hasNextInt())
         {
            throw new BadDataException("Length expected");
         }
         int numberOfValues = in.nextInt();
         data = new double[numberOfValues];
      
         for (int i = 0; i < numberOfValues; i++)
            readValue(in, i);
      
         if (in.hasNext())
            throw new BadDataException("End of file expected");
      }
    • Checks for two potential errors: File might not start with an integer and File might have additional data after reading all values.
    • Makes no attempt to catch any exceptions.

    The readValue method of the DataSetReader class

    private void readValue(Scanner in, int i) throws BadDataException
    {
       if (!in.hasNextDouble())
          throw new BadDataException("Data value expected");
       data[i] = in.nextDouble();
    }

    Error Scenario

    1. DataAnalyzer.main calls DataSetReader.readFile
    2. readFile calls readData
    3. readData calls readValue
    4. readValue doesn't find expected value and throws BadDataException
    5. readValue has no handler for exception and terminates
    6. readData has no handler for exception and terminates
    7. readFile has no handler for exception and terminates after executing finally clause and closing the Scanner object
    8. DataAnalyzer.main has handler for BadDataException
      1. Handler prints a message
      2. User is given another chance to enter file name

    section_5/DataSetReader.java

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    section_5/BadDataException.java

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    Self Check 11.24

    Why doesn't the DataSetReader.readFile method catch any exceptions?
    • Answer: It would not be able to do much with them. The DataSetReader class is a reusable class that may be used for systems with different languages and different user interfaces. Thus, it cannot engage in a dialog with the program user.

    Self Check 11.25

    Suppose the user specifies a file that exists and is empty. Trace the flow of execution.
    • Answer: DataAnalyzer.main calls DataSetReader.readFile, which calls readData. The call in.hasNextInt() returns false, and readData throws a BadDataException. The readFile method doesn't catch it, so it propagates back to main, where it is caught.

    Self Check 11.26

    If the readValue method had to throw a NoSuchElementException instead of a BadDataException when the next input isn’t a floating-point number, how would the implementation change?
    • Answer: It could simply be
      private void readValue(Scanner in, int i)
      {
         data[i] = in.nextDouble();
      }
      The nextDouble method throws a NoSuchElementException or a InputMismatchException (which is a subclass of NoSuchElementException) when the next input isn’t a floating-point number. That exception isn’t a checked exception, so it need not be declared.

    Self Check 11.x27

    Consider the try/finally statement in the readFile method. Why was the in variable declared outside the try block?
    • Answer: If it had been declared inside the try block, its scope would only have extended until the end of the try block, and it would not have been accessible in the finally clause.

    Self Check 11.28

    How can the program be simplified when you use the “automatic resource management” feature described in Special Topic 11.6?
    • Answer: The try/finally statement in the readFile method can be rewritten as
      try (Scanner in = new Scanner(inFile))
      {
         readData(in);
         return data;
      }