JComboBox设置selectedItem不起作用

Question

I'm building a swing application in Java. I have a JComboBox that containts a list of Items. After that I wat to set selected Item of this JComboBox but I'm not able to do this.

This is the code that I use to add Item in JComboBox:

List<Stagione> listaStagioni = modelManager.getStagioneManager().getAllStagioni(null, null);
ComboFormat comboStagione = new ComboFormat();
comboStagione.setModel(new javax.swing.DefaultComboBoxModel(listaStagioni.toArray()));
comboStagione.addItem("");
comboStagione.setSelectedIndex(listaStagioni.size());

This is the class Stagione:

public class Stagione {
    private Integer id;
    private Integer anno;
    private String descrizione;

    public Integer getId() {
        return id;
    }
    public void setId(Integer id) {
        this.id = id;
    }
    public Integer getAnno() {
        return anno;
    }
    public void setAnno(Integer anno) {
        this.anno = anno;
    }
    public String getDescrizione() {
        return descrizione;
    }
    public void setDescrizione(String descrizione) {
        this.descrizione = descrizione;
    }

    public String toString() {
            return this.getDescrizione();
    }
}

In this code, I want to set from code an Item in JComboBox

comboCategoria.setSelectedItem("MAGLIE");

I don't have any error but the item is not selected. This is the item in my JComboBox

Answer 1

setSelectedItem internaly uses the equals method. You add Stagione objects, but use a String within setSelectItem.

The easiest solution in your case is to override equals in the Stagion object to handle String comparison.

Like :

 @Override
public boolean equals(Object obj) {
    if(obj==null){
        return false;
    }

    if(obj instanceof Stagione){
        return ((Stagione)obj).getId().equals(getId());
    }else if (obj instanceof String){
        return descrizione.equals(obj);
    }else {
        // Or return false...
        return super.equals(obj);
    }
}

PS : I'm not a big fan of instanceof use. Comaring String and Business Objects, well... Using Stagione object in setSelectedItem would be cleaner from an object point of view, but you'd have to implements equals anyway, to compare by IDs for example.

Answer 2

Another common way around this it to set the index instead of the item. Something like this;

String testValue = "MAGLIE";

for (int i=0; i<combobox.getModel().getSize(); i++)
{
    if (combobox.getItemAt(i).toString().equals(testValue))
    {
        combobox.setSelectedIndex(i);
        break;
    }
}

Answer 3

Not directly related to your question but the following won't work:

comboStagione.setSelectedIndex(listaStagioni.size());

Java indexes are 0 offset, so to select the last item is a list you use:

comboStagione.setSelectedIndex(listaStagioni.size() - 1);