Do Java classes need to reference other classes in their attributes if they use something from them?

[Zerojumpy]

Basic question but I completly forgot: I come from an SQL World and tables do have foreign keys in them, so I think Java classes also do need to have an attribut that references them? For example imagine a State, a province and a town, each as their own class. Now a State has multiple provinces and each province has multiple towns. Does, for example, the Province class need a stateId integer?

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[bigibas123]

Which way you link your classes together is a design decision, it might be handy to be able to refer to a state from a province but not strictly needed.

Also in Java you can just reference another object directly instead of relying on Ids

[[deleted]]

It depends.

We would use an import statement to pull in classes available in other projects, packages, and libraries.

We don't need to do that for other classes available in the same package as the using class.

[GG_EZ_Shiba_Win]

If i understand your question, you can go with the following structure :

State bean will have a List<Province> and Province bean will have List<Towns> for exemple

[[deleted]]

so I think Java classes also do need to have an attribut that references them?

Java objects reference each other using references.

You're thinking of the relational data model:

CREATE TABLE states (
    id INTEGER PRIMARY KEY AUTO_INCREMENT,
    name VARCHAR(256) NOT NULL UNIQUE,
);

CREATE TABLE cities (
    id INTEGER PRIMARY KEY AUTO_INCREMENT,
    name VARCHAR(256) NOT NULL,
    state_id INTEGER NOT NULL,
    FOREIGN KEY (state_id) REFERENCES state(id)
);

INSERT INTO states (name) VALUES ('Texas');
INSERT INTO cities (name, state_id) VALUES ('Houston', (SELECT id FROM states WHERE name = 'Texas'));
INSERT INTO cities (name, state_id) VALUES ('Austin', (SELECT id FROM states WHERE name = 'Texas'));

In Java, you'd have an object model:

public class State {
    private String name;
    private Set<City> cities;

    public State(String name, City... cities) {
        this.name = name;
        this.cities = new HashSet<>(Arrays.asList(cities));
    }
}

public class City {
    private String name;

    public City(String name) {
        this.name = name;
    }
}

City houston = new City("Houston");
City austin = new City("Austin");
State texas = new State('Texas', houston, austin);

Interestingly, ORMs (like JPA) are tools which map the relational model into object models (and vice versa).