为什么能编译通过？该如何解决

为什么能编译通过？？

Java code

public OperatorIF buildOperator(String act)throws BadOperatorException{        if ("add".equalsIgnoreCase(act)) {            return new AddOperatorImpl();        }else if ("sub".equalsIgnoreCase(act)) {            return new SubOperatorImpl();        }else if ("div".equalsIgnoreCase(act)) {            return new DivOperatorImpl();        }else if ("mul".equalsIgnoreCase(act)) {            return new MulOperatorImpl();        }else {            throw new BadOperatorException("Bad Operator!!");        }    }