Java Exception
since May 11, 2000, last modified May 11, 2000

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1. Concept and Terminalogy

• Exception: Exceptional Events
• Exception object: An object which has exception information
• Throwing an exception: Creating an Exception Object and handling it to runtime system (by the method in which the error occurred)
• A Mehtod which has exception handler: A method (in the call stack of the method throwing an exception) which handls sametype of the exception occurred.
• Catch the exception: The method which handle the exception (see "A Method which has exception handler")

2. How to define

• If called method has 'throwing exception', a caller method in the call stack should have 'catching excetpion' routine (see the following example, the navi() caller and canvas() called in navi calss)

An Example of Java Exception

/* Caller: catch exception
=========================*/
class navi
	{
	...
	public navi() /* navi constructor  */
		{
		try
			{
			canvas1 = new canvas(); /* navi() is the new canvas() Caller */
			}
		catch (IOException e) /* CATCH EXCEPTION */
			{
			System.err.println("CheckedIOTest: " + e);
			System.exit(-1);
			}
		}
	}

	/* Called: throw exception
	=========================*/
class canvas
	{
	...
	public canvas() throws IOException 
	/* canvas constructor - 
	THROW EXCEPTION to caller, new navi() method */
		{
		try {
		in = new FileReader("input.txt");
		out = new FileWriter("output.txt");
		} 
		catch (FileNotFoundException e) /* CATCH EXCEPTION, Is it needed ? */
			{
			System.err.println("CheckedIOTest: " + e);
			System.exit(-1);
			} 
		catch (IOException e) 
			{
			System.err.println("CheckedIOTest: " + e);
			System.exit(-1);
			}
		}
	}

For more information of Java Exception, see The Java Tutorial "Essential Java Classes.Handling Errors with Exceptions" section.

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Knowledge and Engineering Databases (c) copyright Namchul Do, 2000