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Jb0250: Java OOP: Brief Introduction to Exceptions

Module by: R.G. (Dick) Baldwin. E-mail the author

Summary: This module provides a very brief treatment of exception handling. The topic is discussed in detail in the module titled Java OOP: Exception Handling by Richard Baldwin

Preface

This module provides a very brief treatment of exception handling. The topic is discussed in detail in the module titled Java OOP: Exception Handling . The topic is included in this Programming Fundamentals section simply to introduce you to the concept.

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Listings

Discussion

What is an exception?

According to The Java Tutorials , "An exception is an event that occurs during the execution of a program that disrupts the normal flow of instructions."

A very common example of an exception given in textbooks is code that attempts to divide by zero (this is easy to demonstrate) .

Throwing an exception

Common terminology states that when this happens, the system throws an exception . If a thrown exception is not caught , a runtime error may occur.

Purpose of exception handling

The purpose of exception handling is to make it possible for the program to either attempt to recover from the problem, or at worst shut down the program in a graceful manner, whenever an exception occurs.

Java supports exception handling

Java, C++, and some other programming languages support exception handling in similar ways.

In Java, the exception can be thrown either by the system or by code created by the programmer. There is a fairly long list of exceptions that will be thrown automatically by the Java runtime system.

Checked exceptions cannot be ignored

Included in that long list of automatic exceptions is a subset known as "checked" exceptions. Checked exceptions cannot be ignored by the programmer. A method must either specify (declare) or catch all "checked" exceptions that can be thrown in order for the program to compile.

An example of specifying an exception

I explain the difference between specifying and catching an exception in Java OOP: Exception Handling . For now, suffice it to say that the code that begins with the word "throws" in Listing 1 specifies (declares) an exception that can be thrown by the code inside the main method.

If this specification is not made, the program will not compile.

1
Listing 1: The program named simple1.

/*File simple1.java Copyright 1997, R.G.Baldwin
**********************************************************/
class simple1 { //define the controlling class
  public static void main(String[] args) 
                               throws java.io.IOException {
    int ch1, ch2 = '0';

    System.out.println(
      "Enter some text, terminate with #");

    //Get and save individual bytes
    while( (ch1 = System.in.read() ) != '#') ch2 = ch1;

    //Display the character immediately before the #
    System.out.println("The char before the # was "
      + (char)ch2);
  }//end main
}//End simple1 class.

The program in Listing 1 does not throw any exceptions directly nor does it attempt to catch any exceptions. However, it can throw exceptions indirectly through its call to System.in.read .

Because IOException is a checked exception, the main method must either specify it or catch it . Otherwise the program won't compile. In this case, the main method specifies the exception as opposed to catching it.

Very brief treatment

As mentioned earlier, this is a very brief treatment of a fairly complex topic that is discussed in much more detail in the module titled Java OOP: Exception Handling . The topic was included at this point simply to introduce you to the concept of exceptions.

Run the program

II encourage you to copy the code from Listing 1 . Compile the code and execute it. Experiment with the code, making changes, and observing the results of your changes. Make certain that you can explain why your changes behave as they do.

Looking ahead

As you approach the end of this group of Programming Fundamentals modules, you should be preparing yourself for the more challenging ITSE 2321 OOP tracks identified below:

Miscellaneous

This section contains a variety of miscellaneous information.

Note:

Housekeeping material
  • Module name: Jb0250: Java OOP: Brief Introduction to Exceptions
  • File: Jb0250.htm
  • Originally published: 1997
  • Published at cnx.org: 11/26/12
  • Revised: 01/02/13

Note:

Disclaimers:

Financial : Although the Connexions site makes it possible for you to download a PDF file for this module at no charge, and also makes it possible for you to purchase a pre-printed version of the PDF file, you should be aware that some of the HTML elements in this module may not translate well into PDF.

I also want you to know that, I receive no financial compensation from the Connexions website even if you purchase the PDF version of the module.

In the past, unknown individuals have copied my modules from cnx.org, converted them to Kindle books, and placed them for sale on Amazon.com showing me as the author. I neither receive compensation for those sales nor do I know who does receive compensation. If you purchase such a book, please be aware that it is a copy of a module that is freely available on cnx.org and that it was made and published without my prior knowledge.

Affiliation :: I am a professor of Computer Information Technology at Austin Community College in Austin, TX.

-end-

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