Let us have a look at the following applet. Note that the disk may leave the box, e.g. take x=200, y=100, size = 50. Suppose this is unwanted behaviour.

We would like to throw our own CircleSizeException if the circle does not fit into the box. In the next applet we include a test for this, which will do the throw-action.

For throwing the exception, we have to do two things:

1. declare the class.

   class CircleSizeException extends Exception {
     
     CircleSizeException ( ) { }
   
     CircleSizeException (String str) {
           super(str);
     }
   }
   

2. throw the exception at the right location

   void checkSize (int x, int y, int size) throws CircleSizeException {
     if (x + size > 200 || x < 0 || 
         y + size > 200 || y < 0) 
       throw new CircleSizeException("The disk will not fit in the box."); 
     } 
   } 
   

• Note that we first have to create a new Exception before we can throw it.
• Note that we have to declare that we are possibly going to throw the exception in the first line.