When writing a program, you can anticipate on an exception by using clauses with the keywords try and catch. In the try-clause, you specify the statements that might throw an exception, while in the catch-clause you tell what to do if something goes wrong.

In the next applet, we adapt the above applet by catching the exceptions and sending them to an extra text area.

  public boolean action (Event evt, Object arg) {
    if (evt.target == aField || evt.target == bField) {
      try {
        a = Integer.parseInt(aField.getText());
        b = Integer.parseInt(bField.getText());
        output = a/b;
        outputField.setText(String.valueOf(output));
        exceptionArea.setText("");
      }
      catch (Exception e) {
        outputField.setText("");
        exceptionArea.setText(e.toString());
      }
    }
    return true;
  }

• Exceptions are organized in the hierarchy of classes. In the catch clause you can specify exactly what you would like to happen when a particular exception happens. For instance, you can discriminate between the reactions upon the two exceptions by specifying more catch-clauses with different arguments. Any other exception will pass uncaught as before.

  try { ... 
  }
  catch (ArithmeticException e) {
    exceptionArea.setText("You may not divide by zero!");
  }
  catch (NumberFormatException e) {
    exceptionArea.setText("Please enter an integer.");
  }
  

• If the exception is generated in the middle of the try-clause, the program jumps out of the clause at that point, so the program lines after that point will not be executed. You may add a third finally clause that contains code that will be executed in any case, whether an exception is generated or not.

  try { ... }
  catch (...) { ... }
  finally { ... }