Md.1 case Exception Handling

Make sure you study Background materials first before moving on to case assignment.

In Background materials, we learned that there are checked and unchecked exceptions in Java. The Java compiler requires methods that throw a checked exception to have an exception handling statement, yet does not have this requirement for programs that might throw unchecked exceptions. As a good programmer however, it is good practice for us to catch unchecked exceptions even if it is not required. Next, we will view some examples.

1) Unchecked exceptions

Example A

Click here to download a program you have seen in CSC212. This program asks you to select one of the three kinds of gas, and how many gallons of gas you want to buy and then calculate how much you need to pay. Run this program, type in “three” when asked to put the amount of gas you need, and see what happens. Did the program crash? Congratulations, you just got an exception thrown at you. What kind of error message you get? Click here to find out the NumberFormatException class information. You will see thatNumberFormatException is a subclass of IllegalArgumentException which is a subclass of RuntimeException. In the background material, we learned that all exception types that ARE subclasses of RuntimeException are unchecked exceptions, which means that even there is a possibility that these exceptions would occur, and the compiler does not demand that these exceptions are handled. So your program still compiles okay but might still crash under certain conditions.  Although it is not required to write exception handling statements to deal with unchecked exceptions, it is good practice to include these statements to avoid theseproblems.

The program should be modified to include a try/catch block to handle exceptions. An if block should be added before code that might throw a NumberFormatException. This exception will be thrown when the program has attempted to convert a string to one of the numeric types, but the string does not have the appropriate format.  When users type in “three” instead of the number 3, the program is not able to convert the string into a number so a NumberFormatException is thrown.

Next, lets move on to discuss the catch statement:

catch (NumberFormatException e)
{
        JOptionPane.showMessageDialog (null, “Invaid data input, try again”);

}

You must spell NumberFormatException correctly since it is a standard java class. However, you can use whatever you like for the parameter name. In this case, I use name “e” in order to represent exception but you can use other names. Also, make sure you check JOptionPane class, and look for all the methods it has.  Specifically look for the showMessageDialog method. To further experiment with this program, modify the catch block as show below, run the program again, and see what happens.

catch (NumberFormatException e)

{
        JOptionPane.showMessageDialog (null, e.getMessage( ));