Gondor's Kings

I'm just a bit confused.
See,Gondor was a city established by Elendil and his people.So it is understood that he, and his heirs are the rightful Kings of Gondor.But then we know that Isildur and his sons perished at the Battle of Gladden Fields.

Also, Faramir tells Frodo that Earndur was the last King of Gondor; who after dying at the feud with the Witch of Angmar, left no heir for Gondor.So began the administration of the Stewwards of Gondor.

Now my question is, were Earndur and Isildur Elendil of the same line?It is not mentioned anywhere.