Fill JComboBox after changing another JComboBox

Question

I have a class

    public class Speciality {
    String specName;
    String eduProgram; }

and a TreeMap Map<String, ArrayList<Speciality>> treeMap = new TreeMap<String, ArrayList<Speciality>>(); where String is the name of faculty.

My 1st JComboBox comprises of names of faculties (so they're equal to keys in my Map) and in my BoxActionListener I need to fill my 2nd JComboBox with the array of specNames.

Here you can see what I actually need, but ofc it doesn't word (because I cast String[] to String)

boxSpeciality.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                for(int i = 0; i < treeMap.size(); i++){
                    Object key = treeMap.keySet().toArray(new Object[treeMap.size()])[i];
                    ArrayList<Speciality> value = treeMap.get(key);


                    if(key == boxFaculty.getSelectedItem()){
                        boxSpeciality.setModel(new DefaultComboBoxModel((String[]) value.get(i).specName));
                    }
                }
            }
        });

Answer 1

As I understand, your combobox boxFaculty shows the keys of treeMap which are faculty names and your combobox boxSpeciality should show the specialities of each faculty ( which is one of the values of the treeMap ).

If you want the boxSpeciality to be populated based on selection of boxFaculty, you should write an actionListener on boxFaculty.

Another thing to note is do not set a new DefaultComboboxModel everytime in the action listener. Set one when you defined the combobox and update its elements in the action listener.

You can do something like below:

DefaultComboBoxModel<Speciality> specialityModel = new DefaultComboBoxModel<Speciality>();
boxSpeciality.setModel( specialityModel );
boxFaculty.addActionListener( new ActionListener()
{
    @Override
    public void actionPerformed(ActionEvent e)
    {
        Object obj = boxFaculty.getSelectedItem();
        if ( obj != null )
        { 
           ArrayList<Speciality> specialities = treeMap.get( obj );
           specialityModel.removeAllElements();
           for ( Speciality speciality : specialities )
           {
              specialityModel.addElement( speciality );
           }
        }

    }

});

This will update the boxSpeciality box everytime you select an item in boxFaculty.