Exceptions

Exceptions

[Only cover first 4 sections of this chapter now. Other parts will come when we talk about streams.]

We have seen exceptions all semester long:

• NullPointerException
• ArrayIndexOutOfBoundsException
• NumberFormatException

These are just a few examples of a more general system provided by Java (and some other programming languages) to deal with errors called exceptions.

We will look at what exceptions can do for us and how Java implements them using variants of this Color Mixer program covered earlier:

In this simple version, things work fine, until we enter non-numerical text. When this is passed to Integer.parseInt, a NumberFormatException is thrown.

• Consider the version that included the check to make sure the String in each text field contained only characters '0' through '9'. Demo: Safer Color Mixer The isInteger method verified the correct contents, but there's also code to make sure the value is between 0 and 255, the valid range for the RGB values of a color.
• Now consider a version of this program with both the valid input and input range checks removed. Demo: Fragile Color Mixer When we run this and give invalid input, Java notices the problem and "throws" an exception. We see this as the huge mess of error messages, but the start of that mess says there was a NumberFormatException when we put non-numeric characters in a text field, or IllegalArgumentException if we type a number that's out of the 0-255 range. The computer didn't break and our program didn't even crash. Clearly, Java is doing these same kinds of error checks we were doing, and hopefully doing a more thorough job than us.
• Fortunately, Java gives us the ability to "catch" these exceptions before it goes ahead and prints the ugly error messages, with the try..catch construct:

  try {
     bunches of statements 
  } catch { bunches of statements }
  

  The idea is that the "bunch of statements" in try might throw an exception, and if it does, some code in the "bunch of statements" in catch can respond to it.
• A simple example where we handle the exception in the catch block to provide feedback: Demo: Warning Label Color Mixer Here, we create a Swing JLabel that can be displayed when an error is detected. The catch clause now has a statement to add this JLabel to the window. There are two things here we haven't seen before - the remove method is used to remove an Swing component that may previously have been added. And validate shuffles things around and redisplays them nicely in the case where a component was added or removed. Note that the catch clause has the phrase Exception e in parentheses just like parameter list for a method. It essentially is. So:
  • You can pick a name other than e.
  • e is a variable you can reference anywhere in the subsequent block.
  • Exception must be a class. Are there methods for the class? Can you create objects of the class? Can you have variables of the class? We will answer these questions shortly.
  • The name e must be bound to some object. What is it?
• Java's Exception class provides us with a getMessage method that returns a more intelligible error message: Demo: Almost Intelligible Label Color Mixer The message for a value out of range is now pretty good. However, the message for the invalid input isn't very informative. Plus, while we have detected the errors, we haven't responded to them in any useful way, like the way we took values that were too high or too low and forced them back into the 0-255 range.
• It would be good to associate different "bunches of statements" with different error conditions. We can use specialized Exception classes to catch only certain types of exceptions. In our case, NumberFormatException for invalid characters in the text field, and IllegalArgumentException for color values out of range. Demo: Intelligible Label Color Mixer
• Perhaps we want to deal with the out of range error directly, but have the non-numeric input problem detected using exceptions. In this case, we could have a try/catch in one method to catch an exception that happens in another.

Exceptions