Exceptions

Exceptions

• Java gives us the ability to "catch" 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.
• How does an exception get "thrown"?

  This is a sketch of what the code of the Color constructor looks like:

     public Color ( int r, int g, int b ) {
  
        if ( r < 0 || r > MAXVALUE )
           throw new IllegalArgumentException("Color value out of range- Red");
        if ( g < 0 || g > MAXVALUE)
           throw new IllegalArgumentException("Color value out of range- Green");
        if ( b < 0 || b > MAXVALUE)
           throw new IllegalArgumentException("Color value out of range- Blue");
  
        this.r = r;
        this.g = g;
        this.b = b;
     }
  

  So the throw keyword is used to accomplish this. The program should construct a new Exception of the appropriate type and throw it. If the code is operating inside of a try statement somewhere, the exception delivered at a matching catch clause if there is one.

Exceptions