java - 将项目从不同的类添加到 JComboBox

Question

我一直在尝试在将新项目添加到后端数据库时更新 JComboBox。

在实际代码中，有一个单独的类来处理添加对话框，当添加新项目时，它会更新数据库，然后应该通过调用在 main 中接受 String 的方法将相同的项目添加到下拉菜单中图形用户界面类。 (尝试遵循模型- View - Controller )。

下面是一个触发错误的最小示例，尽管在实际应用程序中它会默默地失败。

我有一种预感，它与对象实例有关。另外，为了填充列表，我使用 addItem() 迭代列表，以确保该列表正常工作并且 ComboBox 是可变的。

感谢汤姆的帮助

import java.awt.*;
import javax.swing.*;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;
public class TestComboBox extends JPanel implements ActionListener{

JComboBox moduleList = new JComboBox(new DefaultComboBoxModel());
TestComboBox testComboBox;
JFrame frame;

public void actionPerformed(ActionEvent e){
    if("additem".equals(e.getActionCommand())){
        addItem("Item");
    }
    if("additemfail".equals(e.getActionCommand())){
        testComboBox.addItemFail("Item Fail");
    }
}

public void addItem(String item){
    moduleList.addItem(item);
}

public void addItemFail(String item){
    testComboBox = new TestComboBox();
    moduleList.addItem(item);
}


protected JPanel createPanel(){
    JPanel panel = new JPanel(false);

    String[] getModuleList = {"MODULE 1", "MODULE 2"};
    moduleList = new JComboBox(new DefaultComboBoxModel(getModuleList));
    panel.add(moduleList);

    JButton additem = new JButton("Add Item");
    additem.setActionCommand("additem");
    additem.addActionListener(this);
    panel.add(additem);

    JButton additemfail = new JButton("Add Item Fail");
    additemfail.setActionCommand("additemfail");
    additemfail.addActionListener(this);
    panel.add(additemfail);

    return panel;
}

public void createAndShowGui(){
    testComboBox = new TestComboBox();
    frame = new JFrame("JComboTest");
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

    frame.add(testComboBox.createPanel());
    frame.setSize(450, 150);
    frame.setVisible(true);
}

public static void main(String[] args){
    TestComboBox t = new TestComboBox();
    t.createAndShowGui();
}
}

抛出异常

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerExceptionat TestComboBox.actionPerformed(TestComboBox.java:16)    at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2012)    at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2335)    at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:404)    at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)    at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:253)    at java.awt.Component.processMouseEvent(Component.java:6268)    at javax.swing.JComponent.processMouseEvent(JComponent.java:3267)    at java.awt.Component.processEvent(Component.java:6033)    at java.awt.Container.processEvent(Container.java:2045)    at java.awt.Component.dispatchEventImpl(Component.java:4629)    at java.awt.Container.dispatchEventImpl(Container.java:2103)    at java.awt.Component.dispatchEvent(Component.java:4455)    at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4633)    at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4297)    at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4227)    at java.awt.Container.dispatchEventImpl(Container.java:2089)    at java.awt.Window.dispatchEventImpl(Window.java:2517)    at java.awt.Component.dispatchEvent(Component.java:4455)    at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:649)    at java.awt.EventQueue.access$000(EventQueue.java:96)    at java.awt.EventQueue$1.run(EventQueue.java:608)    at java.awt.EventQueue$1.run(EventQueue.java:606)    at java.security.AccessController.doPrivileged(Native Method)    at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:105)    at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:116)    at java.awt.EventQueue$2.run(EventQueue.java:622)    at java.awt.EventQueue$2.run(EventQueue.java:620)    at java.security.AccessController.doPrivileged(Native Method)    at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:105)    at java.awt.EventQueue.dispatchEvent(EventQueue.java:619)    at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:275)    at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:200)    at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:190)    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:185)    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:177)    at java.awt.EventDispatchThread.run(EventDispatchThread.java:138)

Answer 1

您应该简单地在 actionPerformed 方法中调用 addItemFail("Item Fail");，而不是 testComboBox.addItemFail("Item Fail");。

编辑:并且您不应该在 addItemFail 方法中再次重新创建同一类的对象(再次这样做有什么意义？)