Problem with logic of book store application in java EE

问题

I'm making book store and so far i only have 3 tables.

Book:
- id:(int) primary key, autoincrement, not null,
- title:(varchar) not null,
- price: (decimal) not null

Cart:
-id:(int) primary key, autoincrement, not null 

and

Article:
- id:(int) primary key, autoincrement, not null,
- title:(varchar) not null,
- price: (decimal) not null,
- cart_id:(int) foreign key referencing cart

For me it's logical when user clicks on "add to cart" button to check if cart already exist, if exist i will return that cart if not create new. And user will put multiple articles in one cart. This is my class for inserting Cart in db:

public class CartDAO {

private static DataSource dataSource;
private static int autoIncKey = -1;

public static void insertCart() {
    Connection conn = null;
    ResultSet rs = null;
    
    try {
        InitialContext ctx = new InitialContext();
        dataSource = (DataSource) ctx.lookup("jdbc/NemkeDB");
        conn = dataSource.getConnection();
        
            String insert = "insert into cart() values()";
            PreparedStatement ps = conn.prepareStatement(insert, RETURN_GENERATED_KEYS);
            ps.executeUpdate();
            rs = ps.getGeneratedKeys();
            if (rs.next()) {
                autoIncKeyFromApi = rs.getInt(1);
            } catch(Exception e){
            System.err.println(e);
           }

In CartDAO i want to have that check, something like this:

if(cart == null){
'insert into cart()values()'
}else{ // if cart exist i want to return that cart id}

i don't know how to do that with GeneratedKeys.

Below is the class where i'm passing the id from cart in ArticleDAO

public class ArticleDAO {

private static DataSource dataSource;
private static ArrayList<Article> articleList = new ArrayList<>();

public static void insertArticle(String title, double price) {
    Connection conn = null;
    Statement st = null;
    ResultSet rs = null;
    try {
        InitialContext ctx = new InitialContext();
        dataSource = (DataSource) ctx.lookup("jdbc/NemkeDB");
        conn = dataSource.getConnection();
        st = conn.createStatement();
        CartDAO.insertCart(); // Here is where i call(insert) Cart
        String insert = "insert into article(title,price,cart_id) values(?,?,?)";
        PreparedStatement ps = conn.prepareStatement(insert);
        ps.setString(1, title); 
        ps.setDouble(2, price);;
        ps.setInt(3, CartDAO.getAutoIncKey()); // Here i pass that id
        ps.executeUpdate();
        rs = st.executeQuery("select * from artical");
        while (rs.next()) {
            String articalTitle = rs.getString("title");
            double articalPrice = rs.getDouble("price");
            Artical artical = new Artical(articalTitle, articalPrice);
            articalList.add(artical);
        }
    } catch (Exception e) {
        System.err.println(e);
    }

My question is what is the proper way to make that check? Maybe with session? And how to return id if cart exist? I know it's a long question but i think it's easy for anyone with any working experience with java. Thanks everyone who invested time to read and reply this problem.

来源：https://stackoverflow.com/questions/62853215/problem-with-logic-of-book-store-application-in-java-ee