Chapter Goals

• To be able to read and write text files
• To learn how to throw exceptions
• To be able to design your own exception classes
• To understand the difference between checked and unchecked exceptions
• To learn how to catch exceptions
• To know when and where to catch an exception

Reading Text Files

• Simplest way to read text: use Scanner class
• To read from a disk file, construct a FileReader
• Then, use the FileReader to construct a Scanner object

  FileReader reader = new FileReader("input.txt");
  Scanner in = new Scanner(reader);

• Use the Scanner methods to read data from file
  • next, nextLine, nextInt, and nextDouble

Writing Text Files

• 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(29.95);
  out.println(new Rectangle(5, 10, 15, 25));
  out.println("Hello, World!");

• You must close a file when you are done processing it:

  out.close();

  Otherwise, not all of the output may be written to the disk file

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

A Sample Program

• Reads all lines of a file and sends them to the output file, preceded by line numbers
• Sample input file:

  Mary had a little lamb
  Whose fleece was white as snow.
  And everywhere that Mary went,
  The lamb was sure to go!

• Program produces the output file:

  /* 1 */ Mary had a little lamb
  /* 2 */ Whose fleece was white as snow.
  /* 3 */ And everywhere that Mary went,
  /* 4 */ The lamb was sure to go!

• Program can be used for numbering Java source files

ch11/fileio/LineNumberer.java

Self Check 11.1

• 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

• Answer: The program catches a FileNotFoundException, prints an error message, and terminates.

File Dialog Boxes

JFileChooser chooser = new JFileChooser();
FileReader in = null;
if (chooser.showOpenDialog(null) == JFileChooser.APPROVE_OPTION)
{
   File selectedFile = chooser.getSelectedFile();
   reader = new FileReader(selectedFile);
   . . .
}

Throwing Exceptions

• Throw an exception object to signal an exceptional condition
• Example: IllegalArgumentException: illegal parameter value

  IllegalArgumentException exception
        = new IllegalArgumentException("Amount exceeds balance");
  throw exception;

• No need to store exception object in a variable:

  throw new IllegalArgumentException("Amount exceeds balance");

• When an exception is thrown, method terminates immediately
  • Execution continues with an exception handler

Example

public class BankAccount
{
   public void withdraw(double amount)
   {
      if (amount > balance)
      {
         IllegalArgumentException exception
               = new IllegalArgumentException("Amount exceeds balance");
         throw exception;
      }
      balance = balance - amount;
   }
   . . .
}

Hierarchy of Exception Classes

Syntax 11.1 Throwing an Exception

throw exceptionObject;

throw new IllegalArgumentException();

Self Check 11.3

• Answer: Throw an exception if the amount being deposited is less than zero.

Self Check 11.4

• Answer: The balance is still zero because the last statement of the withdraw method was never executed.

Checked and Unchecked Exceptions

• Two types of exceptions:
  • Checked
    • The compiler checks that you don't ignore them
    • Due to external circumstances that the programmer cannot prevent
    • Majority occur when dealing with input and output
    • For example, IOException
  • Unchecked:
    • Extend the class RuntimeException or Error
    • They are the programmer's fault
    • Examples of runtime exceptions:

      NumberFormatException
      IllegalArgumentException
      NullPointerException

    • Example of error:

      OutOfMemoryError

Checked and Unchecked Exceptions

• Categories aren't perfect:
  • Scanner.nextInt throws unchecked InputMismatchException
  • Programmer cannot prevent users from entering incorrect input
  • This choice makes the class easy to use for beginning programmers
• Deal with checked exceptions principally when programming with files and streams
• For example, use a Scanner to read a file

  String filename = . . .;
  FileReader reader = new FileReader(filename);
  Scanner in = new Scanner(reader);

• But, FileReader constructor can throw a FileNotFoundException

Checked and Unchecked Exceptions

• Two choices:
  1. Handle the exception
  2. Tell compiler that you want method to be terminated when the exception occurs
     • Use throws specifier so method can throw a checked exception

       public void read(String filename) throws FileNotFoundException
       {
          FileReader reader = new FileReader(filename);
          Scanner in = new Scanner(reader);
          . . .
       }

     • For multiple exceptions:

       public void read(String filename)
             throws IOException, ClassNotFoundException

     • Keep in mind inheritance hierarchy:
       If method can throw an IOException and FileNotFoundException, only use IOException
• Better to declare exception than to handle it incompetently

Syntax 11.2 Exception Specification

accessSpecifier returnType methodName(parameterType parameterName, . . .)
   throws ExceptionClass, ExceptionClass, . . .

public void read(BufferedReader in)
   throws IOException

Self Check 11.5

• Answer: The specification throws IOException is sufficient because FileNotFoundException is a subclass of IOException.

Self Check 11.6

• Answer: Because programmers should simply check for null pointers instead of trying to handle a NullPointerException.

Catching Exceptions

• Install an exception handler with try/catch statement
• try block contains statements that may cause an exception
• catch clause contains handler for an exception type
• Example:

  try
  {
     String filename = . . .;
     FileReader reader = new FileReader(filename);
     Scanner in = new Scanner(reader);
     String input = in.next();
     int value = Integer.parseInt(input);
     . . .
  }
  catch (IOException exception)
  {
     exception.printStackTrace();
  }
  catch (NumberFormatException exception)
  {
     System.out.println("Input was not a number");
  }

Catching Exceptions

• Statements in try block are executed
• If no exceptions occur, catch clauses are skipped
• If exception of matching type occurs, execution jumps to catch clause
• If exception of another type occurs, it is thrown until it is caught by another try block
• catch (IOException exception) block
  • exception contains reference to the exception object that was thrown
  • catch clause can analyze object to find out more details
  • exception.printStackTrace(): printout of chain of method calls that lead to exception

Syntax 11.3 General Try Block

try
{
   statement
   statement
   . . .
}
catch (ExceptionClass exceptionObject)
{
   statement
   statement
   . . .
}
catch (ExceptionClass exceptionObject)
{
   statement
   statement
   . . .
}
. . .

try
{
   System.out.println("How old are you?");
   int age = in.nextInt();
   System.out.println("Next year, you'll be " + (age + 1));
}
catch (InputMismatchException exception)
{
   exception.printStackTrace();
}

Self Check 11.7

• Answer: The FileReader constructor succeeds, and in is constructed. Then the call in.next() throws a NoSuchElementException, and the try block is aborted. None of the catch clauses match, so none are executed. If none of the enclosing method calls catch the exception, the program terminates.

Self Check 11.8

• Answer: No – you catch both exception types in the same way, as you can see from the code example on page 508. Recall that IOException is a checked exception and NumberFormatException is an unchecked exception.

The finally Clause

• Exception terminates current method
• Danger: Can skip over essential code
• Example:
  

  reader = new FileReader(filename);
  Scanner in = new Scanner(reader);
  readData(in);
  reader.close(); // May never get here

• Must execute reader.close() even if exception happens
• Use finally clause for code that must be executed "no matter what"

The finally Clause

FileReader reader = new FileReader(filename);
try
{
   Scanner in = new Scanner(reader);
   readData(in);
}
finally
{
   reader.close(); // if an exception occurs, finally clause is also
                   // executed before exception is passed to its handler
}

The finally Clause

• Executed when try block is exited in any of three ways:
  1. After last statement of try block
  2. After last statement of catch clause, if this try block caught an exception
  3. When an exception was thrown in try block and not caught
• Recommendation: don't mix catch and finally clauses in same try block

Syntax 11.4 The finally Clause

try
{
   statement
   statement
   . . .
}
finally
{
   statement
   statement
   . . .
}

FileReader reader = new FileReader(filename);
try
{
   readData(reader);
}
finally
{
   reader.close();
}

Self Check 11.9

• Answer: If it had been declared inside the try block, its scope would only have extended to the end of the try block, and the catch clause could not have closed it.

Self Check 11.10

• Answer: The FileReader constructor throws an exception. The finally clause is executed. Since reader is null, the call to close is not executed. Next, a catch clause that matches the FileNotFoundException is located. If none exists, the program terminates.

Designing Your Own Exception Types

• You can design your own exception types – subclasses of Exception or RuntimeException

• if (amount > balance)
  {
     throw new InsufficientFundsException(
        "withdrawal of " + amount + " exceeds balance of " + balance);
  }

• Make it an unchecked exception – programmer could have avoided it by calling getBalance first
• Extend RuntimeException or one of its subclasses
• Supply two constructors
  1. Default constructor
  2. A constructor that accepts a message string describing reason for exception

Designing Your Own Exception Types

public class InsufficientFundsException
      extends RuntimeException
{
   public InsufficientFundsException() {}

   public InsufficientFundsException(String message)
   {
      super(message);
   }
}

Self Check 11.11

• Answer: To pass the exception message string to the RuntimeException superclass.

Self Check 11.12

• Answer: Exception or IOException are both good choices. Because file corruption is beyond the control of the programmer, this should be a checked exception, so it would be wrong to extend RuntimeException.

A Complete Example

• 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

A Complete Example

• What can go wrong?
  • File might not exist
  • File might have data in wrong format
• Who can detect the faults?
  • FileReader 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 FileReader constructor
  • IOException can be thrown by close method of FileReader
  • BadDataException, a custom checked exception class
• Who can remedy the faults that the exceptions report?
  • Only the main method of DataSetTester program interacts with user
    • Catches exceptions
    • Prints appropriate error messages
    • Gives user another chance to enter a correct file

ch11/data/DataAnalyzer.java

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, BadDataException
        // FileNotFoundException is an IOException
  {
     FileReader reader = new FileReader(filename);
     try
     {
        Scanner in = new Scanner(reader);
        readData(in);
     }
     finally
     {
        reader.close();
     }
     return data;
  }

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
  1. File might not start with an integer
  2. 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();
}

Animation 11.1 –

Link to Flash animation

Scenario

1. DataSetTester.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
8. DataSetTester.main has handler for BadDataException; handler prints a message, and user is given another chance to enter file name

ch11/data/DataSetReader.java

Self Check 11.13

• 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 11.14

• Answer: DataSetTester.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.