How to dismiss an alert box in selenium rc using java code.

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Java

how to dismiss the alert box using selenium RC in Eclipse Editor
i am doing a coding for testing web application using java code in Eclipse editor according to selenium RC.

i done a code like this.....

Boolean b=selenium.isAlertPresent();
      if(b.equals(true))
      {

          Alert alert= wd.switchTo().alert();
          alert.dismiss();
         String error=selenium.getAlert();
          return "Error:"+error;
      }

But it is showing the errror like UNABLE TO DISMISS THE ALERT?IS THE ALERT PRESENT?