Exception Handling

Chapter Goals

• 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

Error Codes

• Traditional approach to error handling: method returns error code
  
• Example: JOptionPane.showInputDialog returns null if user hits Cancel
  
• Problem: Calling method may forget to check for error code
• Problem: Calling method may not know how to fix error--then it needs to return an error code
• Symptom: Programming for success
  x.doSomething()
  is replaced by programming for failure
  if (!x.doSomething()) return false;

Exceptions

• Can't be overlooked
• Can be handled by a competent handler, not necessarily the calling method
  
• Throw an exception object to indicate failure
  if (failure)
  {
     XxxException e = new XxxException(. . .);
     throw e;
  }
• More concisely
  throw new XxxException(. . .);
  

Exceptions

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

Hierarchy of Exception Classes

Syntax 14.1: Throwing an Exception

throw new IllegalArgumentException();

Checked Exceptions

• Compiler checks that you are aware of the exception
• Generally used for errors that can happen even in correct programs
• IOException and its sublcasses are checked exceptions
• NullPointerException, ArrayIndexOutOfBoundsException , . . .  are unchecked--they are your fault :-)
• Virtual machine errors (e.g. OutOfMemoryError) are unchecked
  
• Classification not perfect. For example, Integer.parseInt throws unchecked NumberFormatException
• Checked exceptions are subclasses of Exception that are not subclasses of RuntimeException

Checked and Unchecked Exceptions

Exception Specifications

• BufferedReader.readLine may throw IOException
• Tag calling method with throws IOException
  

public class Coin 
{ 
   public void read(BufferedReader in) throws IOException 
   { 
      value = Double.parseDouble(in.readLine()); 
      name =in.readLine(); 
   } 
   ... 
}

Exception Specifications

• Need to tag caller of Coin.read as well
  
  
• Stop at main or with handler (see below)
• Can have multiple exception types
  public void read()
     throws IOException, ClassNotFoundException
• throws specifier not a sign of irresponsible programming
• Better to declare exception than to handle it incompetently

Syntax 14.2: Exception Specification

Designing Your Own Exception Types

• if (amount > balance)
     throw new InsufficientFundsException(. . .);
• Make it an unchecked exception--programmer could have avoided it by calling getBalance first
• Extend RuntimeException
• Supply two constructors
  

Designing Your Own Exception Types

public class InsufficientFundsException 
   extends RuntimeException 
{ 
   public InsufficientFundsException() 
   { 
   }
 
   public InsufficientFundsException(String reason) 
   { 
      super(reason); 
   }
}

Catching Exceptions

try 
{ 
   BufferedReader in = new BufferedReader( 
      new InputStreamReader(System.in)); 
   System.out.println("How old are you?"); 
   String inputLine = in.readLine(); 
   int age = Integer.parseInt(inputLine); 
   age++; 
   System.out.println("Next year,you'll be " + age); 
} 
catch (IOException exception) 
{ 
   System.out.println("Input/output error " +exception);
} 
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 to the calling method
• If main doesn't catch an exception, the program terminates with a stack trace

Syntax 14.3: General Try Block

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

try 
{ 
   System.out.println("What is your name?"); 
   String name = console.readLine(); 
   System.out.println("Hello,"+name +"!"); 
} 
catch (IOException exception) 
{  
   exception.printStackTrace(); 
   System.exit(1); 
}

The finally Clause

• Exception terminates current method
• Danger: Can skip over essential code
• Example:
  BufferedReader in;
  in = new BufferedReader(
     new FileReader(filename));
  purse.read(in);
  in.close(); 
• Must execute in.close() even if exception happens
• Use finally clause for code that must be executed "no matter what"

The finally Clause

BufferedReader in = null; 
try 
{ 
   in = new BufferedReader(
      new FileReader(filename)); 
   purse.read(in); 
} 
finally 
{ 
   if (in !=null) in.close(); 
} 

The finally Clause

• Executed when try block comes to normal end
  
• Executed if a statement in try block throws an exception, before exception is thrown out of try block
• Can also be combined with catch clauses

Syntax 14.4: The finally Clause

BufferedReader in = null; 
try 
{ 
   in = new BufferedReader(
      new FileReader(filename)); 
   purse.read(in); 
} 
finally 
{ 
   if (in !=null) in.close(); 
} 

A Complete Example

• Program 
• reads coin descriptions from file
•  adds coins to purse
• prints total
  
• What can go wrong?
• File might not exist
• File might have data in wrong format
• Who can detect the faults?
• main method of PurseTest interacts with user
• main method can report errors
• Other methods pass exceptions to caller

The read method of the Coin class

• Distinguishes between expected and unexpected end of file
  

public boolean read(BufferedReader in) throws IOException 
{ 
   String input =in.readLine(); 
   if (input == null) // normal end of file
      return false; 
   value = Double.parseDouble(input);
      // may throw unchecked NumberFormatException 
   name = in.readLine(); 
   if (name == null) // unexpected end of file 
      throw new EOFException("Coin name expected"); 
   return true; 
}

The read method of the Purse class

• Unconcerned with exceptions
• Just passes them to caller

public void read(BufferedReader in) 
   throws IOException 
{ 
   boolean done = false; 
   while (!done)
   {
      Coin c = new Coin(); 
      if (c.read(in)) add(c); 
      else done =true;
   }
}

The readFile method of the Purse class

• finally clause closes files if exception happens

public void readFile(String filename) 
   throws IOException 
{ 
   BufferedReader in = null; 
   try 
   { 
      in = new BufferedReader(
         new FileReader(filename)); 
      read(in); 
   } 
   finally 
   { 
      if (in != null)
      in.close(); 
   }
} 

User interaction in main

• If an exception occurs, user can specify another file name

boolean done = false; 
String filename = JOptionPane.showInputDialog("Enter file name"); 
while (!done) 
{ 
   try 
   { 
      Purse myPurse = new Purse(); 
      myPurse.readFile(filename); 
      System.out.println("total=" + myPurse.getTotal()); 
      done =true; 
   } 
   catch (IOException exception) 
   { 
      System.out.println("Input/output error " + exception);
   } 
   catch (NumberFormatException exception) 
   { 
      exception.printStackTrace(); // error in file format
   } 
   if (!done) 
   { 
      filename = JOptionPane.showInputDialog( "Try another file:"); 
      if (filename == null)
         done =true; 
   } 
}

Scenario

1.  PurseTest.main calls Purse.readFile
2. Purse.readFile calls Purse.read
3. Purse.read calls Coin.read
4. Coin.read throws an EOFException
5. Coin.read has no handler for the exception and terminates immediately. 
6. Purse.read has no handler for the exception and terminates immediately
7. Purse.readFile has no handler for the exception and terminates immediately after executing the finally clause and closing the file. 
8. PurseTest.main has a handler for an IOException , a superclass of EOFException. That handler prints a message to the user. Afterwards, the user is given another chance to enter a file name. Note that the statement printing the purse total has been skipped.
   

File PurseTest.java

File Purse.java

File Coin.java