更新时的空指针JCombobox

Question

我一直在为此苦苦挣扎。我试图与老板的目的是更新一个空的JComboBox，只要它已被点击按钮创建，但我不断收到一个NullPointerException

 package gui; 

    import java.awt.BorderLayout; 
    import java.awt.EventQueue; 

    import javax.swing.JFrame; 
    import javax.swing.JPanel; 
    import javax.swing.border.EmptyBorder; 
    import javax.swing.JMenuBar; 
    import javax.swing.JToolBar; 
    import javax.swing.JTabbedPane; 
    import javax.swing.JDesktopPane; 
    import javax.swing.border.TitledBorder; 
    import javax.swing.border.LineBorder; 
    import java.awt.Color; 
    import javax.swing.JLabel; 
    import javax.swing.JTextField; 
    import javax.swing.JButton; 
    import java.awt.Insets; 
    import java.awt.Button; 
    import java.awt.Component; 
    import javax.swing.JLayeredPane; 
    import javax.swing.JTable; 
    import javax.swing.Box; 
    import javax.swing.JSplitPane; 
    import javax.swing.AbstractAction; 
    import java.awt.event.ActionEvent; 
    import javax.swing.Action; 
    import javax.swing.JInternalFrame; 
    import java.awt.event.*; 
    import controller.ForestOwnerCtr; 


    import model.OwnerCon; 
    import model.Owner; 
    import model.Tree; 

    import java.awt.event.ActionListener; 
    import java.util.ArrayList; 
    import javax.swing.JList; 
    import javax.swing.JComboBox; 
    import java.awt.Choice; 
    import java.awt.Label; 
    import javax.swing.DefaultComboBoxModel; 

    public class ForestGui extends JFrame { 

     private static ForestOwnerCtr FOCtr; 
     private JPanel contentPane; 
     private JTextField txtOwnerName; 
     private JTextField txtOwnerAdress; 
     private JTextField txtOwnerPhone; 
     private JTextField txtName; 
     private JTextField txtLocation; 
     private JTextField txtArea; 
     private JTextField textField; 
     private JTextField textField_3; 
     private JTextField textField_4; 
     private JTextField txtAge; 
     private JComboBox<Owner> owner_select; 
     private ArrayList<Owner> test; 

     /** 
     * Launch the application. 
     */ 
     public static void main(String[] args) { 
      EventQueue.invokeLater(new Runnable() { 
       public void run() { 
        try { 
         ForestGui frame = new ForestGui(); 
         frame.setVisible(true); 
        } catch (Exception e) { 
         e.printStackTrace(); 
        } 
       } 
      }); 
      FOCtr = new ForestOwnerCtr(); 
     } 

     /** 
     * Create the frame. 
     */ 
     public ForestGui() { 
      setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 
      setBounds(100, 100, 946, 509); 

      JMenuBar menuBar = new JMenuBar(); 
      setJMenuBar(menuBar); 

      JButton btnCreate = new JButton("Create"); 
      menuBar.add(btnCreate); 

      JButton btnSearch_1 = new JButton("Search"); 
      menuBar.add(btnSearch_1); 
      contentPane = new JPanel(); 
      contentPane.setBorder(new EmptyBorder(5, 5, 5, 5)); 
      setContentPane(contentPane); 
      contentPane.setLayout(null); 

      JPanel panel_search_forest = new JPanel(); 
      panel_search_forest.setBorder(new TitledBorder(new LineBorder(new Color(130, 135, 144)), "Seach forest", TitledBorder.LEADING, TitledBorder.TOP, null, null)); 
      panel_search_forest.setToolTipText("tst"); 
      panel_search_forest.setBounds(15, 11, 238, 238); 
      contentPane.add(panel_search_forest); 
      panel_search_forest.setLayout(null); 

      JLabel lblName_1 = new JLabel("Name"); 
      lblName_1.setBounds(10, 28, 46, 14); 
      panel_search_forest.add(lblName_1); 

      txtName = new JTextField(); 
      txtName.setColumns(10); 
      txtName.setBounds(92, 26, 119, 19); 
      panel_search_forest.add(txtName); 

      JLabel lblLocation = new JLabel("Location"); 
      lblLocation.setBounds(10, 56, 58, 14); 
      panel_search_forest.add(lblLocation); 

      JLabel lblArea = new JLabel("Area"); 
      lblArea.setBounds(10, 81, 46, 14); 
      panel_search_forest.add(lblArea); 

      txtLocation = new JTextField(); 
      txtLocation.setColumns(10); 
      txtLocation.setBounds(92, 54, 119, 19); 
      panel_search_forest.add(txtLocation); 

      txtArea = new JTextField(); 
      txtArea.setColumns(10); 
      txtArea.setBounds(92, 79, 119, 19); 
      panel_search_forest.add(txtArea); 

      JLabel lblAge = new JLabel("Age"); 
      lblAge.setBounds(10, 106, 46, 14); 
      panel_search_forest.add(lblAge); 

      txtAge = new JTextField(); 
      txtAge.setColumns(10); 
      txtAge.setBounds(92, 107, 119, 19); 
      panel_search_forest.add(txtAge); 

      JButton CreateForest = new JButton("New forest"); 
      CreateForest.addActionListener(new ActionListener() { 
       public void actionPerformed(ActionEvent arg0) { 
        createForest(); 
       } 
      }); 
      CreateForest.setBounds(109, 204, 119, 23); 
      panel_search_forest.add(CreateForest); 

      JLabel lblOwner = new JLabel("Owner"); 
      lblOwner.setBounds(10, 131, 46, 14); 
      panel_search_forest.add(lblOwner); 

      Choice choice = new Choice(); 
      choice.setBounds(92, 163, 119, 19); 
      panel_search_forest.add(choice); 

      JLabel lblTreeSorts = new JLabel("Tree sorts"); 
      lblTreeSorts.setBounds(10, 163, 76, 20); 
      panel_search_forest.add(lblTreeSorts); 


      JComboBox<Owner> owner_select = new JComboBox<Owner>(); 
      owner_select.setModel(new DefaultComboBoxModel(FOCtr.getOwners().toArray())); 
      owner_select.setBounds(90, 137, 121, 20); 

      panel_search_forest.add(owner_select); 




      JPanel searc_owner = new JPanel(); 
      searc_owner.setToolTipText("tst"); 
      searc_owner.setBorder(new TitledBorder(new LineBorder(new Color(130, 135, 144)), "Seach owner", TitledBorder.LEADING, TitledBorder.TOP, null, null)); 
      searc_owner.setBounds(263, 11, 250, 196); 
      contentPane.add(searc_owner); 
      searc_owner.setLayout(null); 

      JLabel lblName = new JLabel("Name"); 
      lblName.setBounds(10, 22, 46, 14); 
      searc_owner.add(lblName); 

      JLabel lblAdress = new JLabel("Adress"); 
      lblAdress.setBounds(10, 47, 46, 14); 
      searc_owner.add(lblAdress); 

      JLabel lblPhoneNumber = new JLabel("Phone number"); 
      lblPhoneNumber.setBounds(10, 77, 83, 14); 
      searc_owner.add(lblPhoneNumber); 

      txtOwnerName = new JTextField(); 
      txtOwnerName.setBounds(111, 20, 119, 17); 
      searc_owner.add(txtOwnerName); 
      txtOwnerName.setColumns(10); 

      txtOwnerAdress = new JTextField(); 
      txtOwnerAdress.setColumns(10); 
      txtOwnerAdress.setBounds(111, 45, 119, 17); 
      searc_owner.add(txtOwnerAdress); 

      txtOwnerPhone = new JTextField(); 
      txtOwnerPhone.setColumns(10); 
      txtOwnerPhone.setBounds(111, 75, 119, 17); 
      searc_owner.add(txtOwnerPhone); 

      JButton btnSearch = new JButton("New Owner"); 
      btnSearch.addActionListener(new ActionListener() { 
       public void actionPerformed(ActionEvent E) { 
        createOwner(); 
       } 
      }); 



      btnSearch.setBounds(141, 162, 89, 23); 
      searc_owner.add(btnSearch); 



      JPanel panel = new JPanel(); 
      panel.setLayout(null); 
      panel.setToolTipText("tst"); 
      panel.setBorder(new TitledBorder(new LineBorder(new Color(130, 135, 144)), "Seach forest", TitledBorder.LEADING, TitledBorder.TOP, null, null)); 
      panel.setBounds(585, 171, 238, 196); 
      contentPane.add(panel); 

      JLabel label = new JLabel("Name"); 
      label.setBounds(10, 28, 46, 14); 
      panel.add(label); 

      textField = new JTextField(); 
      textField.setColumns(10); 
      textField.setBounds(78, 25, 119, 17); 
      panel.add(textField); 

      JLabel label_1 = new JLabel("Location"); 
      label_1.setBounds(10, 56, 58, 14); 
      panel.add(label_1); 

      JLabel label_2 = new JLabel("Area"); 
      label_2.setBounds(10, 81, 46, 14); 
      panel.add(label_2); 

      textField_3 = new JTextField(); 
      textField_3.setColumns(10); 
      textField_3.setBounds(78, 53, 119, 17); 
      panel.add(textField_3); 

      textField_4 = new JTextField(); 
      textField_4.setColumns(10); 
      textField_4.setBounds(78, 78, 119, 17); 
      panel.add(textField_4); 

      JButton button_1 = new JButton("Search"); 
      button_1.setBounds(139, 162, 89, 23); 
      panel.add(button_1); 


      } 






    private void createForest(){ 

     String name = txtName.getText(); 
     String location = txtLocation.getText(); 
     int totalArea = Integer.parseInt(txtArea.getText()); 
     int age = Integer.parseInt(txtAge.getText()); 
     //Owner owner = 
     FOCtr.createForest(name, location, totalArea, age); 
     } 

    private void createOwner(){ 


     String newName = txtOwnerName.getText(); 
     String newAdress = txtOwnerAdress.getText(); 
     String newPhone = txtOwnerPhone.getText(); 
     Owner o = new Owner(newName, newAdress, newPhone); 
     test.add(o); 
     owner_select.addItem(o); 
     } 

    } 

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException 
    at gui.ForestGui.createOwner(ForestGui.java:285) 
    at gui.ForestGui.access$1(ForestGui.java:278) 
    at gui.ForestGui$3.actionPerformed(ForestGui.java:211) 
    at javax.swing.AbstractButton.fireActionPerformed(Unknown Source) 
    at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source) 
    at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source) 
    at javax.swing.DefaultButtonModel.setPressed(Unknown Source) 
    at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(Unknown Source) 
    at java.awt.Component.processMouseEvent(Unknown Source) 
    at javax.swing.JComponent.processMouseEvent(Unknown Source) 
    at java.awt.Component.processEvent(Unknown Source) 
    at java.awt.Container.processEvent(Unknown Source) 
    at java.awt.Component.dispatchEventImpl(Unknown Source) 
    at java.awt.Container.dispatchEventImpl(Unknown Source) 
    at java.awt.Component.dispatchEvent(Unknown Source) 
    at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source) 
    at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source) 
    at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source) 
    at java.awt.Container.dispatchEventImpl(Unknown Source) 
    at java.awt.Window.dispatchEventImpl(Unknown Source) 
    at java.awt.Component.dispatchEvent(Unknown Source) 
    at java.awt.EventQueue.dispatchEventImpl(Unknown Source) 
    at java.awt.EventQueue.access$200(Unknown Source) 
    at java.awt.EventQueue$3.run(Unknown Source) 
    at java.awt.EventQueue$3.run(Unknown Source) 
    at java.security.AccessController.doPrivileged(Native Method) 
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source) 
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source) 
    at java.awt.EventQueue$4.run(Unknown Source) 
    at java.awt.EventQueue$4.run(Unknown Source) 
    at java.security.AccessController.doPrivileged(Native Method) 
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source) 
    at java.awt.EventQueue.dispatchEvent(Unknown Source) 
    at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source) 
    at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source) 
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source) 
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source) 
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source) 
    at java.awt.EventDispatchThread.run(Unknown Source) 

Answer 1

I have been struggling with this for a time 

修复这不应该需要五分钟以上。

NPE是最容易解决的问题。打印堆栈跟踪，记下它发生的类别和行号，并通过设置它们等于非空引用来扫描该行以查找未初始化的引用。如果不明显，请使用IDE和调试器。

只要不重复相同的行为，并期待不同的结果的错误。除非是导致你的问题的竞争条件，否则这是疯狂的一个标志。

FWIW，你写的代码太多了。这就是我讨厌Swing的原因之一 - 代码太多！一点思考和分解将会有很长的路要走。

例如，你的班级没有理由延长JFrame。 JPanel将是更好的选择。你可以将你的课程嵌入JFrame，然后离开你。

textField_4？那是什么？怎么有人能说出这是为了什么？从现在开始的三个星期里，这个让人迷惑的人可能就是你。

Answer 2

你定义一个类变量为空：

private JComboBox<Owner> owner_select; 

然后创建一个局部变量，是不是在你的构造空：

JComboBox<Owner> owner_select = new JComboBox<Owner>(); 

在createOwner代码（）方法只能访问类变量。您不想创建局部变量，因此构造函数中的代码应该是：

owner_select = new JComboBox<Owner>();