Exception Handling in Java

Exception Handling

You may or may not have seen Exception Handling before. As part of this course, you will learn how to use Java's exception handling mechanism and to utilize it in your software solution to all your Cmpt 225 assignments. This is something you will need to learn on your own as we will not be mentioning this topic in class. More specifically, you need to learn:

what an exception is,
the Java exception classes (already available) and when these exceptions are thrown by the Java Virtual Machine (JVM) as well as why you should catch them,
how to create your own exception classes (These "home made" exception classes can be created as subclasses of the Java class Exception using inheritance.),
how to use Java exception handling mechanism, for example, how to throw and catch an exception, etc.
difference between checked and unchecked (runtime) exceptions.

Here is an example of how to create your own exception class as a subclass of Java class Exception:

class DuplicateEntryException extends Exception
{
   // Constructor
   public DuplicateEntryException( String aString )
   {
      super( aString );
   }
}

The purpose of the method super( ), in the above constructor is to invoke the constructor of the Exception class (the superclass) and to pass it aStringas a parameter. The Exception class (the superclass) has a String field member which is set to aString by its constructor (the one invoked by super( )) . This string parameter can be a description of the occurring problem (error or exception).

You would then instantiate and throw an object of class DuplicateEntryException, for example, if the user is trying to create an entry which already exists in your data collection. Here is a bit of code that illustrates this:

     try
     {
        createEntry( "BlahBlahBlah", ... );
     }
     catch ( DuplicateEntryException myException )
     {
        System.out.println( myException );
     }

In thecreateEntry( "BlahBlahBlah", ... )method:

// this is pseudocode not Java code:
if ( "BlahBlahBlah" is found in the data collection )

// this is syntactically correct Java code:
throw new DuplicateEntryException( "BlahBlahBlah already exists." );

So when the exception is caught by the above code, the line "BlahBlahBlah already exists" is printed on the screen. Of course, printing error message is not the only thing you can do when you catch an exception. One very useful thing to do would be to ask the user to create a different entry.

Notes:

There are very few comments in the code example above. However, in your assignment, you are to fully comment your code.
All the above information regarding Java exception handling will be subject to examination.

List of sources where you can find information about Java exception handling mechanism:

and more...

C++ has a similar exception handling mechanism. Here are some links you may find useful:

and more ...

Anne Lavergne - Last revised August 2006