java - JComboBox getSelectedItem

Question

刚接触 java，我不明白为什么我的 Action 监听器不能在 jcombobox 上工作。我想我已经按照网上的其他示例来 getSelectedItem，但没有发生任何事情。仅供引用，我的项目是一个单位转换器(希望使用 MVC，但这不是我的首要任务)。非常感谢任何帮助。谢谢，西蒙。

    import java.awt.event.ActionEvent;
    import java.awt.event.ActionListener;

    import javax.swing.*;


    public class UnitConverterView extends JFrame{

    //variables and components
    private static final long serialVersionUID = -4673040337179571462L;
    private JComboBox<String> unitCategory;

    private JTextField fromValue = new JTextField(7);
    private JComboBox<String> convertFrom;
    private JLabel equalsLabel = new JLabel(" = ");

    private JTextField toValue = new JTextField(7);
    private JComboBox<String> convertTo;


    //constructor
    UnitConverterView(){
    //set up the view and components

        JPanel unitPanel = new JPanel();

        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        this.setSize(600,300);

        String[] categories = {"Length","Weight","Speed","Temperature"};
        unitCategory = new JComboBox<>(categories);

        String[] tofromValues = {" "};
        convertFrom = new JComboBox<>(tofromValues);
        convertTo = new JComboBox<>(tofromValues);


        unitPanel.add(unitCategory);

        unitPanel.add(fromValue);
        unitPanel.add(convertFrom);
        unitPanel.add(equalsLabel);
        unitPanel.add(toValue);
        unitPanel.add(convertTo);

        this.add(unitPanel);

    }

    //get value to convert from
    public int getMeasurement() {
        return Integer.parseInt(fromValue.getText());
    }

    //listen for unitCategory to be selected
    void addUnitCategoryListener(ActionListener listenForUnitCategory) {
        unitCategory.addActionListener(listenForUnitCategory);
    }

class UnitCatListener implements ActionListener { 

        public void actionPerformed(ActionEvent e) {

            /*String unitSelected = (String) unitCategory.getSelectedItem();
            if (e.getSource() == unitCategory) {
                String unitName = (String) unitCategory.getSelectedItem();
                System.out.println("UnitName = " + unitName);
                changeText(unitName);
            }*/

            JComboBox cb = (JComboBox)e.getSource();
            String unitName = (String) cb.getSelectedItem();
            System.out.println("UnitName = " + unitName);

        }

        void changeText(String name) {
            toValue.setText(name);
        }

    }



}

Answer 1

您已经声明了一个方法 addUnitCategoryListener() 用于将监听器注册到组合框，但您从未调用过此方法。这就是监听器从未注册的原因。

在构造函数末尾添加以下行，然后就可以了:

addUnitCategoryListener(new UnitCatListener());