DefaultComboBoxModel model = new DefaultComboBoxModel( yourStringArray );
comboBox.setModel( model );

public class ComboPanel extends JPanel {

    JComboBox jcbo;
    // this is constructor
    public ComboPanel(ArrayList<String> items) {
        jcbo = new JComboBox();
        // getting exiting combo box model
        DefaultComboBoxModel model = (DefaultComboBoxModel) jcbo.getModel();
        // removing old data
        model.removeAllElements();

        for (String item : items) {
            model.addElement(item);
        }

        // setting model with new data
        jcbo.setModel(model);
        // adding combobox to panel
        this.add(jcbo);
    }
}

JComboBox op=new JComboBox(new String[] {"d","e","f"});
op.removeAllItems();
String[] new_entries=new String[] {"a","b","c"}
for (String s : new_entries) {
     op.insertItemAt(s, op.getItemCount());
}
op.setSelectedIndex(0);