“When you make a blessing on a class of food it is assumed that all of that type of food is included.”
That is correct if it is a meal or if you are a guest at someone’s house.
If it’s not a meal and you are not a guest, you have to either:
1. have in mind that the bracha should count for all other foods that you will eat with that bracha, or
2. Have planned to eat that specific food at the time you made the bracha, or
3. the second food was in front of you at the time you made the bracha, or
4. there is still food remaining from the original food you made the bracha on (this only helps if both foods are solids or both are liquids:
5. there is at least one other case, but I have to look it up to make sure I write it correctly. If I have a chance, I will do so, and post it, b”n.
However, there are exceptions to rules 3 & 4. So the best thing to do is to have in mind when you make the original bracha that it will count for all other foods you eat with that bracha, as the Rema says.
If you do have any kind of doubt (for example, you don’t know the exceptions to rules 3 & 4 or you don’t know all the possible cases since there are others, or you’re not sure what you had in mind), then you should either:
a)leave your house and come back in (but that doesn’t work for Mezonos or hamotzi).
or
b: find someone else who wants to eat a food with the same bracha,ask them to have you in mind when they make the bracha, listen to them make the bracha, have in mind to be included in their bracha, and as soon as they finish the bracha and take a bite, you should take a bite from your food.
The most important rule in hilchos brachos is: IF THERE IS ANY DOUBT, DON’T MAKE A BRACHA!!!
I only know all this because I taught it recently, so it is one of the few areas of halacha that I happen to know somewhat well.