You make a good point!
I didn't really think of that. The thing is, for now at least and to narrow down my project, I have decided to work only with one set of Pokemon cards. Those cards exist in <cardID>.json files in a resource. Those JSON documents contain all the variable values for the cards such name, hp, attacks and so on. The card images exist in another folder in the resources as <cardID>.jpg. I have done this so that I have something to work with and not risk spending a month or two working with network, ssl, cache, json classes and risk not having anything to present as the deadline aproaches. I do like the project I am working with and I am planning to add those functionalities in the future, maybe even a function that adds cards by scanning them with a phone camera. The JSON documents are retreived from: pokemontcg.io but that is something that I won't touch until the main application fully works.

My line of thinking has been that when the user wants to add a card to an album. He or she will type the name or the id of the card and the program creates an instance of that card in the album. If he doesn't add it to the album and decided just to see it, it will be deleted once he exits the view. I also thought of having an instance of all the cards pre-loaded in a hidden default album that the App uses to display the cards before the user adds them to his or her Album.

You make a good point about a Card being available in more than one album. I guess there should be a default "All my cards" Album. I guess cards in that Album determines what cards are made available to the other albums. I assume that this would be a bit problematic if the user wants to create a "My wishlist" or "To buy" Album. There is also the problem of duplicate cards.

I will try to rethink the relationships between the Albums and between the Cards and Albums. I think that having an Album as an isolated container of cards from other Cards in other Albums might make the implementation a bit easier assuming that the user will manually add Cards to Albums and not expect them to be shared among the Albums. I can however create a "view" that shows all the cards in all the albums or multiple Albums.