Introduction to Exceptions

Objectives

1. Understand what exceptions are
2. Understand the mechanism for handling exceptions in Java.
3. Write a simple program that handles exceptions
4. Discuss the use of try-catch and exceptions.

Notes

• This material is from Section 3.7.
• This is a complex topic, we will not cover it fully.
  • But we need it to handle a problem associated with files.
  • He discusses this in way too much depth.
• Exceptions are exceptional but not unexpected events that occur in the execution of a program.
  • A simple example is division by 0.
• When an error is encountered, an exception will be thrown.
• We can check for exceptions in our program using a try-catch block.

  • try { 
        // code that might contain an exception
    } catch (ExceptionType e) {
        // code to deal with the exception
    }

• Exception types:
  • There are many built in exception types.
  • These include
    • A generic exception (Exception)
    • Opening a file that does not exist ( FileNotFoundException)
    • Trying to change a string into a number when the string does not contain a number (NumberFormatException)
    • Division by 0: (ArithmeticException)
  • And you can make your own.https://docs.oracle.com/javase/8/docs/api/java/lang/Exception.html
• Exceptions have associated methods including:
  • getMessage()
  • toString()
  • There are others but that will do for now.
• An example:

  • public class ExceptionExample {
        public static void main(String[] args)  {
             int i;
             
             i = -1;
             double f;
             while (i < 2) {
                System.out.println();
                try {
                    System.out.printf("Computing 1/%d\n",i);
                    f = 1/i;
                    System.out.printf("1/%d = %f\n",i, f); 
                } catch (Exception e) {
                    System.out.printf("Exception Caught: %s\n",e.getMessage());
                    System.out.printf("That is also: %s\n", e.toString());
                }
                ++i;
             }
        }
    } 

• I can catch more specific exceptions first, but not the other way around.
• What do do when there is an exception
  • This is tough.
  • Recover if you can.
  • But exit the program if you can't
  • It depends on the situation.
• In general, we want to be careful with try/catch
  • This is not an excuse for bad programming.
  • Clean up logic errors with out exceptions.
  • Use exceptions only when you have no choice.
• We will revisit exceptions as needed.