Select Item

int index = 3;
int size = jComboBox1.getModel().getSize();
if (index >= 0 && index < size) {
    jComboBox1.setSelectedIndex(index);
}

Clear Item Selection

jComboBox1.setSelectedIndex(-1);

Set Custom Key Selection Manager

jComboBox1.setKeySelectionManager(new JComboBox.KeySelectionManager() {
    @Override
    public int selectionForKey(char aKey, ComboBoxModel aModel) {
        int charValue = (int) aKey - 49;
        if (charValue >= 0 && charValue < 9 && charValue < jComboBox1.getItemCount()) {
            return charValue;
        } else {
            return -1;
        }
    }
});

Description:
    Returns the new Item selection index that is generated from the key that was pressed

Note:
    The JComboBox must have focus to use this new selection method

Reset:
    jComboBox1.setKeySelectionManager(null);

Selected Item Index

System.out.println("Selected Item Index: "+jComboBox1.getSelectedIndex());