Exception Reporter

In the following code,

 public class SomeWidget {
 Text fileContent_;
 void setup() {
  ...
  dialog_.addSelectionListener(new SelectionListener() {
    public void widgetSelected(SelectionEvent e) {
        String fileName = dialog_.open();
        try {
           fileContent_.setText(readFileContents(fileName));
        }
        catch (IOException e) {
           throw new RuntimeException(e);
        }
    }
    ...
  });
 }
 readFileContents(String name) throws IOException {/*Some code*/}
 }

The readFileContents throws an IOException, which cannot be re-thrown because the SelectionListener is an interface which does not declare a throws clause in the method widgetSelected. Hence, it is not possible to let the user of SomeWidget handle the exception, because even if we throw a RuntimeException, it gets thrown in a Thread that is different from the one that uses SomeWidget.

One solution could be to use Publisher-Subscriber technique for error reporting.

For example:

 public class SomeWidget {
   Text fileContent_;

   void setup() {
     ...
     dialog_.addSelectionListener(new SelectionListener() {
       public void widgetSelected(SelectionEvent e) {
         String fileName = dialog_.open();
         try {
           fileContent_.setText(readFileContents(fileName));
         }
         catch (IOException e) {
           reportException(e);
         }
       }
       ...
     });
   }

   readFileContents(String name) throws IOException {/*Some code*/}

   addExceptionListener(ExceptionListener l) {
      excListeners_.add(l);
   }

   reportException(Throwable t) {
     /* Iterate thro' excListeners_ and call exceptionOccurred() */
   }
 }

 public interface ExceptionListener {
   void exceptionOccurred(Throwable t);
 }

 /*
   Clients of SomeWidget.
  */

  ...
  SomeWidget s = new SomeWidget();
  ...
  s.addExceptionListener(new ExceptionListener() {
     void exceptionOccurred(Throwable t) {
        MessageDialog.showDialog(t.getMessage());
     }
  });
  ...

This technique allows for more than one handlers to the same exception, centralized error reporting etc.

-- VhIndukumar with Biresh

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CategoryException