Java Swing - 如何按值设置所选索引JComboBox

import java.awt.GridLayout;

import javax.swing.JButton;
import javax.swing.JComboBox;
import javax.swing.JFrame;

public class Main {
  JFrame frame = new JFrame("Test");
  JComboBox comboBox = new JComboBox();

  public Main() {
    comboBox.addItem("One");
    comboBox.addItem("Two");
    comboBox.addItem("Three");
    JButton button = new JButton("Show Selected");
    button.addActionListener(e ->System.out.println("Selected item: " + comboBox.getSelectedItem()));
    JButton button1 = new JButton("Append Items");
    button1.addActionListener(e ->appendCbItem());
    JButton button2 = new JButton("Reduce Items");
    button2.addActionListener(e ->reduceCbItem());
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setLayout(new GridLayout(4, 1));
    frame.add(comboBox);
    frame.add(button);
    frame.add(button1);
    frame.add(button2);
    frame.pack();
    frame.setVisible(true);
    selectFirstItem();
  }
  public void appendCbItem() {
    comboBox.addItem("Four");
    comboBox.addItem("Five");
    comboBox.addItem("Six");
    comboBox.setSelectedItem("Six");
    requestCbFocus();
  }

  public void reduceCbItem() {
    comboBox.removeItem("Four");
    comboBox.removeItem("Five");
    comboBox.removeItem("Six");
    selectFirstItem();
  }

  public void selectFirstItem() {
    comboBox.setSelectedIndex(0);
    requestCbFocus();
  }

  public void requestCbFocus() {
    comboBox.requestFocus();
    comboBox.requestFocusInWindow();
  }

  public static void main(String[] args) {
    new Main();
  }
}