Pick object id attribute selected in Jcombobox

Question

I have a table in BD with Columns Id, Board N° and Drilling on board.

I’d like to show on JComboBox the concatenation of "Plate N° + Drilling".

Example:

Plate 10 - 113,00 mm

But when a combobox item is selected by the user, capture the corresponding object ID Placa. I’m doing it in Netbeans and Mysql.

Follow the method you search in the bank (DAO):

public ArrayList<Placa> ObterTabelaPlaca() {
    conectar();
    ArrayList<Placa> placa = new ArrayList<Placa>();
    Statement stmt = null;
    ResultSet rs = null;
    String sql = "SELECT * FROM cadastroplaca ORDER BY CAST(furoPL AS DECIMAL(5,2))";
    try {
        stmt = con.createStatement();
        rs = stmt.executeQuery(sql);
        while (rs.next()) {
            Placa pl = new Placa();
            pl.setId(rs.getInt("id"));
            pl.setCodPlaca(rs.getInt("codigoPL"));
            pl.setFuracao(rs.getDouble("furoPL"));
            placa.add(pl);
        }
    } catch (Exception e) {
        System.out.println("Erro " + e.getMessage());

    }
    return placa;
}

The Board class(Model):

public class Placa {
    int codPlaca, qtdMoldePlaca, id;
    double furacao;

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }
    String notaPlaca;

    public int getCodPlaca() {
        return codPlaca;
    }

    public void setCodPlaca(int codPlaca) {
        this.codPlaca = codPlaca;
    }

    public int getQtdMoldePlaca() {
        return qtdMoldePlaca;
    }

    public void setQtdMoldePlaca(int qtdMoldePlaca) {
        this.qtdMoldePlaca = qtdMoldePlaca;
    }

    public double getFuracao() {
        return furacao;
    }

    public void setFuracao(double furacao) {
        this.furacao = furacao;
    }

    public String getNotaPlaca() {
        return notaPlaca;
    }

    public void setNotaPlaca(String notaPlaca) {
        this.notaPlaca = notaPlaca;
    }


}

The method that fills the component (From the controller):

 public void MostraComboPlaca() throws SQLException{
        CadastroDAO dao = new CadastroDAO();        
        ArrayList<Placa> listaplaca = dao.ObterTabelaPlaca();
        DecimalFormat formato = new DecimalFormat("#0.00");
        placaCilindrico.setModel(new ComboModelTipo(listaplaca));
        for(int i=0;i<listaplaca.size();i++){ 
        placaCombo.addItem(String.valueOf(formato.format(listaplaca.get(i).getFuracao()).replace('.', ','))+" - PL "+listaplaca.get(i).getCodPlaca());
        }
 }

I cannot complete the reasoning of the Control part, to get the id of the selected item.

---

I store the details of the table in one ArrayList with the DAO above.

placa.add(pl); 

And I populate the Combobox (Hurricane Values) using the Model below:

public void MostraComboPlaca() throws SQLException{
    CadastroDAO dao = new CadastroDAO();        
    ArrayList<Placa> listaplaca = dao.ObterTabelaPlaca();
    placaCombo.setModel(new ComboModelTipo(listaplaca));

 }

I use this class as Comboboxmodel:

public class ComboModelTipo extends AbstractListModel implements ComboBoxModel {

private ArrayList<Placa> lista;
private Placa selected;

public ComboModelTipo(ArrayList<Placa> lista) {
    this.lista = lista;
}

@Override
public int getSize() {
    return this.lista.size();
}

@Override
public Object getElementAt(int index) {
    return this.lista.get(index).getFuracao();
}

@Override
public void setSelectedItem(Object anItem) {
    this.selected = (Placa) anItem;
}

@Override
public Object getSelectedItem() {
    return this.selected;
}

public Integer getIdObjetoSelecionado() {
return selected.getId();
}
}

After these steps my Combobox is populated, but when I click on some value, the error occurs below:

Answer 1

The cause of the exception

The method getElementAt() must return the selected object in the list, not its attribute, as you are doing. Change the method below as follows:

@Override
public Object getElementAt(int index) {
    return this.lista.get(index);
}

---

Changing the display of items in Combomodel

To display the information in the combo without affecting the object type is using a ListCellRenderer:

public class PlacaComboRenderer extends DefaultListCellRenderer{
    @Override
    public Component getListCellRendererComponent(
            JList<?> list, Object value, int index, boolean isSelected, boolean cellHasFocus) {
        super.getListCellRendererComponent(list, value, index, isSelected, cellHasFocus);

        if (value instanceof Placa) {
            Placa placa = (Placa) value;
            DecimalFormat formato = new DecimalFormat("#0.00");
            String text = String.valueOf(formato.format(placa.getFuracao()).replace('.', ','))+" - PL "+ placa.getCodPlaca();
            setText(text);
        } else if(value == null) {
            setText("Selecione uma placa:");
        }
        return this;
    }
}

In this case, I chose to use the class DefaultListCellRenderer because it not only implements the interface ListCellRenderer as well as other features to avoid having to implement a complete Listcellrenderer from scratch.

To apply, just do this:

 public void MostraComboPlaca() throws SQLException{
        CadastroDAO dao = new CadastroDAO();        
        ArrayList<Placa> listaplaca = dao.ObterTabelaPlaca();
        DecimalFormat formato = new DecimalFormat("#0.00");
        placaCilindrico.setModel(new ComboModelTipo(listaplaca));
        //precisa ser exatamente após o ComboBoxModel ser aplicado
        placaCombo.setRenderer(new PlacaComboRenderer());
}

And to capture items from this JComboBox, just grab the selected item, do the casting for your object Placa and call the method that returns the ID:

Placa placaSelecionada = (Placa)seuCombo.getSelectedItem();
int id = placaSelecionada.getId();

No need to create a ComboBoxModel(in case the getIdObjetoSelecionado()) to redeem the id of the selected object, once you already know that the getSelectedItem return a type object Placa, the above solution already covers this.