chapter 81 exception handling chapter 8. 2 reminders project 5 due oct 20 @ 10:30 pm project 3...
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Chapter 8 1
Exception Handling
Chapter 8
Chapter 8 2
Reminders
• Project 5 due Oct 20 @ 10:30 pm
• Project 3 regrades due by midnight tonight
• Discussion groups now twice a week (7-9 pm: M in CS G066, W in Phys 11)
Chapter 8 3
Exam 2
• Tuesday, October 25
• 7:00 – 8:00 PM
• Physics 112
• Covers chapters 5-9 (and material from 1-4 as well)
• Same format as exam 1– 20 MC– 5 programming
Chapter 8 4
Introduction
• A program can be written assuming that nothing unusual or incorrect will happen.– The user will always enter an integer when
prompted to do so. – There will always be a nonempty list for a
program that takes an entry from the list.– The file containing the needed information
will always exist.• Unfortunately, it isn’t always so.
Chapter 8 5
Exception Handling
• Either your code or Java signals when something unusual happens.
• The signaling process is called throwing an exception.
• Somewhere in your program, you can place code to handle the exception.
Chapter 8 6
Exceptions in Java: Example, cont.
• simple example, cont.– But what if there is no milk?– An attempt to divide the number of donuts
by the number of glasses of milk will result in an attempt to divide by zero.
– This would be an utter (udder?) disaster, known in Java as an exception.
Chapter 8 7
Exceptions in Java: Example, cont.
• In Java, it is possible to test for this unusual situation using an if-else statement, for example.
Chapter 8 8
Chapter 8 10
Exceptions in Java: Example, cont.
• In Java, it is also possible to throw an exception.
Chapter 8 11
Chapter 8 12
Exceptions in Java: Example, cont.
Chapter 8 13
Defining Exception Classes
• You can define your own exception classes, but they must be derived from an existing exception class.
• Constructors are the most important, and often the only methods (except for methods inherited from the base class).
Chapter 8 14
Defining Exception Classes, cont.
• class DivideByZeroException
Chapter 8 15
Chapter 8 16
Java Tip: Preserve getMessage
• For all predefined exception classes, method getMessage returns either– the string that is passed as an argument to
the constructor or– a default string if no argument is passed to
the constructor.• The behavior of method getMessage should be
preserved in any exception class you define.
Chapter 8 17
Java Tip: Preserve getMessage, cont.
• This is done by including a string parameter that begins with a call to super.public MyException(String message)
{
super(message);
More_Code_If_Appropriate
}
Chapter 8 18
Java Tip: Preserve getMessage, cont.
• Also include a default constructor.public MyException()
{
super(“MyException thrown”);
More_Code_If_Appropriate
}
Chapter 8 19
Programming Tip: When to Define an Exception Class
• In general, define an exception class if you are going to insert a throw statement in your code.
• This permits catch blocks to distinguish between your exceptions and exceptions thrown by predefined methods.
Chapter 8 20
Guidelines
• Use class Exception as the base class unless there is a compelling reason to do otherwise.
• Define at least two constructors. Typically, no other methods are needed.
• Begin each constructor definition with a call to the constructor of the base class.
Chapter 8 21
Declaring Exceptions (Passing the Buck)
• Sometimes is it appropriate to handle an exception other than in the method where the exception occurred.
• For example, it might be better to handle the exception in the method that called the method that called the method… that threw the exception
Chapter 8 22
Declaring Exceptions, cont.
• If a method can throw an exception but does not catch it, it must alert the programmer to the possibility of an exception by including a throws clause.
• Examplepublic void someMethod()
throws DivideByZeroException
Chapter 8 23
Accounting for Exceptions
• An exception can be caught in a catch block within a method definition.
• Alternatively, the possibility of an exception can be declared at the start of the method definition by placing the exception-class name in a throws clause.
• These two approaches can be mixed in a method, catching some exceptions and declaring others in a throws clause.
Chapter 8 24
Accounting for Exceptions, cont.
• If method_A uses a throws clause instead of handling an exception and method_B calls method_A, then method_B either must handle the exception or must also include a throws clause.
Chapter 8 25
Chapter 8 26
Accounting for Exceptions, cont.
• A throws clause can include more than one exception type.
• examplepublic int someMethod()throws
IOException, DivideByZeroException
Chapter 8 27
Accounting for Exceptions, cont.
• Some method in the calling hierarchy should handle the exception.
• If an exception is thrown, but never caught, either the program terminates or its behavior becomes unreliable.
Chapter 8 28
Exceptions That Need Not Be Caught
• Some exceptions do not need to be accounted for in any way.– (Perhaps these are the exceptions that
prove the rule about needing to account for exceptions.)
• Exceptions that do not need to be accounted for result from errors and usually are thrown by methods in predefined classes.
Chapter 8 29
throws Clauses in Derived Classes
• When a method is redefined in a derived class, the redefined method cannot contain any exception classes that are not in the throws clause of the same method in the base class (though the derived class can list fewer exceptions in its throws clause).
• Any exceptions thrown in the derived class must be caught or thrown by the base class.
Chapter 8 30
Multiple Throws and Catches
• A try block can throw any number of different types of exceptions.
• Each catch block can catch only one type of exception.
• Multiple catch blocks after a try block can catch multiple types of exceptions.
Chapter 8 31
Chapter 8 32
Java Tip: Catch the More Specific Exceptions First
• catch blocks are examined in order.• The first matching catch block is executed.• More specific exceptions should precede less
specific exceptions, i.e. exceptions lower in the exception hierarchy should come before exceptions higher in the exception hierarchy.
Chapter 8 33
Keep It Simple
• Attempt to modify a program or class definition so that it does not need a throw statement.
• In general, use exceptions sparingly.• If the way the exception is handled depends
on the calling method, let the calling method handle the exception.
• Consider throwing the exception and catching the exception in separate methods.
Chapter 8 34
Keep It Simple, cont.
public void methodB()
{
...
try
{
...
methodA();
...
}
catch (MyException e)
...
Chapter 8 35
The finally Block
• A finally block can be added after a try block and its catch blocks.
• The finally block is executed– if the try block throws no exceptions– if the try block throws an exception which
is caught by a catch block– if an exception is thrown but not caught
i.e. it is always executed.
Chapter 8 36
The finally Block, cont.
• syntaxtry
{
…
}
catch Block(s)
finally
{
…
}
Chapter 8 37
try {
doMethod(“test”);
}
catch (Exception e) {
System.out.println(“Exception”);
return;
}
finally {
System.out.println(“Done”);
}
return;
If doMethod() throws an exception, what is the output?
Chapter 8 38
Output
Exception
Done
Chapter 8 39
Output
Exception
Done
why?
Chapter 8 40
Graphics Supplement: Exceptions in GUIs
• An uncaught exception in a (non-GUI) application will end the program.
• An uncaught exception in a GUI program (either a JFrame GUI or an applet) will not end the program.– However, unless the exception is caught,
the GUI may not cope correctly with the exception or the user may receive insufficient instructions.
Chapter 8 41
Programming Example: A JFrame Using Exceptions
• The example JFrame GUI allows the user to write the name of a color in a text field and to click the “Show Color” button.– If the GUI recognizes the color, the
background color changes to the named color.
– Otherwise, the text field displays “Unknown Color” and the background changes to gray.
Chapter 8 42
Programming Example: A JFrame Using Exceptions, cont.• class ColorDemo
Chapter 8 43
• class UnknownColorException
Programming Example: A JFrame Using Exceptions, cont.
Chapter 8 44
• class ShowColorDemo
Programming Example: A JFrame Using Exceptions, cont.
Chapter 8 45
Programming Example: A JFrame Using Exceptions, cont.
Chapter 8 46
throws Clause Not Allowed in actionPerformed
• A throws clause cannot be added to method actionPerformed in any action listener class.
• Any exception thrown in method actionPerformed must be caught in method actionPerformed.
• Similarly, if method windowClosing is redefined in a window listener class, you may not add a throws clause to method windowClosing.
Chapter 8 47
Summary
• You have become familiar with the notion of exception handling.
• You have learned Java syntax for exception handling.
• You have learned to use exception handling effectively in classes and programs.