·         Can an exception be rethrown in Java?

d

       Yes.

s    Consider the following example:

  static class FirstException extends Exception { }
  static class SecondException extends Exception { }

  public void rethrowException(String exceptionName) throws Exception {
    try {
      if (exceptionName.equals("First")) {
        throw new FirstException();
      } else {
        throw new SecondException();
      }
    } catch (Exception e) {
      throw e;
    }
  }

This examples's try block could throw either FirstException or SecondException. Suppose you want to specify these exception types in the throws clause of the rethrowException method declaration. In releases prior to Java SE 7, you cannot do so. Because the exception parameter of the catch clause, e, is type Exception, and the catch block rethrows the exception parameter e, you can only specify the exception type Exception in the throws clause of the rethrowException method declaration.

However, in Java SE 7, you can specify the exception types FirstException and SecondException in the throws clause in the rethrowException method declaration. The Java SE 7 compiler can determine that the exception thrown by the statementthrow e must have come from the try block, and the only exceptions thrown by the try block can be FirstException and SecondException. Even though the exception parameter of the catch clause, e, is type Exception, the compiler can determine that it is an instance of either FirstException or SecondException:

  public void rethrowException(String exceptionName)
  throws FirstException, SecondException {
    try {
      // ...
    }
    catch (Exception e) {
      throw e;
    }
  }