Exception Handling and Event Handling

December 16, 2017

I. Exception Handling

What is Exception Handling in programming language ?

Exception handling is the process of responding to the occurrence, during computation, of exceptions – anomalous or exceptional conditions requiring special processing – often changing the normal flow of program execution. (wikipedia)

• In a language without exception handling

–When an exception occurs, control goes to the operating system, where a message is displayed and the program is terminated

• In a language with exception handling

–Programs are allowed to trap some exceptions, thereby providing the possibility of fixing the problem and continuing

 

Basic Concepts

• Many languages allow programs to trap input/output errors (including EOF)
• An exception is any unusual event, either erroneous or not, detectable by either hardware or software, that may require special processing
• The special processing that may be required after detection of an exception is called exception handling
• The exception handling code unit is called an exception handler

Exception Handling Alternatives

• An exception is raised when its associated event occurs
• A language that does not have exception handling capabilities can still define, detect, raise, and handle exceptions (user defined, software detected)
• Alternatives:

–Send an auxiliary parameter or use the return value to indicate the return status of a subprogram

–Pass a label parameter to all subprograms (error return is to the passed label)

–Pass an exception handling subprogram to all subprograms

 

Advantages of Built-in Exception Handling

• Error detection code is tedious to write and it clutters the program
• Exception handling encourages programmers to consider many different possible errors
• Exception propagation allows a high level of reuse of exception handling code

Exception Handling Control Flow

 

C++ Exception Handlers

• Exception Handlers Form:

  try {

— code that is expected to raise an exception

}

catch (formal parameter) {

— handler code

}

…

catch (formal parameter) {

— handler code

}

 

The catch Function

• catch is the name of all handlers–it is an overloaded name, so the formal parameter of each must be unique
• The formal parameter need not have a variable

–It can be simply a type name to distinguish the handler it is in from others

• The formal parameter can be used to transfer information to the handler
• The formal parameter can be an ellipsis, in which case it handles all exceptions not yet handled

Throwing Exceptions

• Exceptions are all raised explicitly by the statement:

  throw [expression];

• The brackets are metasymbols
• A throw without an operand can only appear in a handler; when it appears, it simply re-raises the exception, which is then handled elsewhere
• The type of the expression disambiguates the intended handler

Unhandled Exceptions

• An unhandled exception is propagated to the caller of the function in which it is raised
• This propagation continues to the main function
• If no handler is found, the default handler is called

Continuation

• After a handler completes its execution, control flows to the first statement after the last handler in the sequence of handlers of which it is an element
• Other design choices

–All exceptions are user-defined

–Exceptions are neither specified nor declared

–The default handler, unexpected, simply terminates the program; unexpected can be redefined by the user

–Functions can list the exceptions they may raise

–Without a specification, a function can raise any exception (the throw clause)

 

II. Event Handling

• An event is a notification that something specific has occurred, such as a mouse click on a graphical button
• The event handler is a segment of code that is executed in response to an event

to understand more about event handling, we take an example from Java and C# Programming language.

Java Swing GUI Components

• Text box is an object of class JTextField
• Radio button is an object of class JRadioButton
• Applet’s display is a frame, a multilayered structure
• Content pane is one layer, where applets put output
• GUI components can be placed in a frame
• Layout manager objects are used to control the placement of components

The Java Event Model

• User interactions with GUI components create events that can be caught by event handlers, called event listeners
• An event generator tells a listener of an event by sending a message
• An interface is used to make event-handling methods conform to a standard protocol
• A class that implements a listener must implement an interface for the listener
• One class of events is ItemEvent, which is associated with the event of clicking a checkbox, a radio button, or a list item
• The ItemListener interface prescribes a method, itemStateChanged, which is a handler for ItemEvent events
• The listener is created with addItemListener

Event Handling in C#

• Event handling in C# (and the other .NET languages) is similar to that in Java
• .NET has two approaches, Windows Forms and Windows Presentation Foundation—we cover only the former (which is the original approach)
• An application subclasses the Form predefined class (defined in System.Windows.Forms)
• There is no need to create a frame or panel in which to place the GUI components
• Label objects are used to place text in the window
• Radio buttons are objects of the RadioButton class

• Components are positioned by assigning a new Point object to the Location property of the component

private RadioButton plain = new RadioButton();

plain.Location = new Point(100, 300);

plain.Text = ″Plain″;

controls.Add(plain);

• All C# event handlers have the same protocol, the return type is void and the two parameters are of types object and EventArgs

• An event handler can have any name
• A radio button is tested with the Boolean Checked property of the button

private void rb_CheckedChanged (object o,

EventArgs e) {

if (plain.Checked) …

…

}

• To register an event, a new EventHandler object must be created and added to the predefined delegate for the event

• When a radio button changes from unchecked to checked, the CheckedChanged event is raised
• The associated delegate is referenced by the name of the event
• If the handler was named rb_CheckedChanged, we could register it on the radio button named plain with:

plain.CheckedChanged +=

new EventHandler (rb_CheckedChanged);