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

CategoryException