Interconnection of tables in Mysql in Java program

Question

Good afternoon ladies and gentlemen, I am developing a project for study with 2 tables and these tables need to be connected in a certain value. In my package DAO, have 2 classes Stockpile and Stockpiling and they correspond to a product table (containing code, description, make and observing) and other with the product-specific information previously registered (cod_gra, cod_est, cod_bar, value, quantity and size). I need the values of code_stock (column of the second table) and code (column of the first table) assume the same value.

my method:

public List<EstoqueGrade> read() {
        Connection con = ConnectionFactory.abrirConexao();
        PreparedStatement stmt = null;
        ResultSet rs = null;

        List<EstoqueGrade> estoqueGrade = new ArrayList<>();
        try {
            stmt = con.prepareStatement("SELECT * FROM estoque_grade");
            rs = stmt.executeQuery();

            while (rs.next()) {
                EstoqueGrade eg = new EstoqueGrade();
                eg.setCod_gra(rs.getString("cod_gra"));
                eg.setCod_est(rs.getString("cod_est"));
                eg.setCod_bar(rs.getString("cod_bar"));
                eg.setTamanho(rs.getString("tamanho"));
                eg.setValor(rs.getString("valor"));
                eg.setQtd(rs.getString("qtd"));
                estoqueGrade.add(eg);
            }
        } catch (SQLException ex) {
            JOptionPane.showMessageDialog(null, "Erro ao ler dados: \n" + ex);
        } finally {
            ConnectionFactory.fecharConexao(con, stmt, rs);
        }
        return estoqueGrade;
    }

    public ArrayList retornarEstoqueGrade() {
        ArrayList estoqueGrade = new ArrayList();
        Connection con = ConnectionFactory.abrirConexao();
        PreparedStatement stmt = null;
        ResultSet rs = null;

        try {
            stmt = con.prepareStatement("SELECT * FROM estoque_grade");
            rs = stmt.executeQuery();
                        
            if (rs.next()) {
                do {
                    estoqueGrade.add(new Object[]{
                        rs.getString("cod_gra"),
                        rs.getString("cod_est"),
                        rs.getString("cod_bar"),
                        rs.getString("tamanho"),
                        rs.getString("valor"),
                        rs.getString("qtd")
                    });
                } while (rs.next());
            }
        } catch (SQLException ex) {
            JOptionPane.showMessageDialog(null, "Erro ao mostrar dados: \n" + ex);
        }
        return estoqueGrade;
    } 

I did not pass the method of the other table, but it is the same thing with the different variables.

I can’t use foreign key for this case, I have to work only with data manipulation.

If you need more information, I’ll let you know, but I don’t think that’ll be necessary. Remember that I just need to solve this, the registration is working for both (separately).

PS: I started learning Object Orientation, Java and Mysql less than 2 months ago, sorry if I’m asking obvious questions...