When we extends class then what comes default

Question

class base
void one()throws IOException
class derived extends base
     void one()
when derived class override the function of one of base class then why it not give errror ?

Answer 1

Oh, is that what you're asking--why the child class doesn't have to declare that it throws the exception? If that's what you're asking...
The child has to honor its parent's contracts. When the parent's method declares throws IOException, it's not promising that it does throw that exception. Rather, it is promising that it won't throw any exception except that one (and unchecked exceptions--RuntimeException, Error, and their descendants).
Since the parent promises not to throw any checked exception other than IOException, the child must also not throw any other checked exception. For instance, the child can't throw SQLException. However the child is free to not throw the exception--that doesn't violate the contract, because the parent isn't promising to throw that exception.