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请参阅此链接以获取更新且格式更好的信息！- 问题的版本。

所以这是第一个问题的后续。我接受了 porfiriopartida 的建议，并将逻辑与用户界面分开，并将逻辑放入一个名为Simulator. 下面是GUI程序现在的样子：

import java.util.*;
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class GUI extends JPanel implements ActionListener {
    LinkedList<FoodItem> foodItemList = new LinkedList<FoodItem>();
    CategoryList<String> categoryList = new CategoryList<String>();
    Scanner in = new Scanner(System.in);
    protected JButton[] buttons = new JButton[6];
    String[] buttonNames = {"Add Item", "Remove Item", "Search for Item", "Update Item", "Check if List is Empty", "Print Items"};
    String[] buttonCommands = {"add", "remove", "search", "update", "isEmpty", "print"};
    protected JTextField textField;
    public GUI() {
        textField = new JTextField(20);
        textField.addActionListener(this);
        add(textField);
        for (int i = 0; i < 6; i++) {
            buttons[i] = new JButton(buttonNames[i]);
            buttons[i].setActionCommand(buttonCommands[i]);
            buttons[i].setVerticalTextPosition(AbstractButton.CENTER);
            buttons[i].setHorizontalTextPosition(AbstractButton.LEADING);
            buttons[i].addActionListener(this);
            add(buttons[i]);
        }
    }
    public void actionPerformed(ActionEvent e) {
        String command = e.getActionCommand();
        Simulator helper = new Simulator(textField);
        if (command.equals(buttonCommands[0]))
            helper.add(foodItemList);
        else if (command.equals(buttonCommands[1]))
            helper.remove(foodItemList);
        else if (command.equals(buttonCommands[2]))
            helper.search(foodItemList);
        else if (command.equals(buttonCommands[3]))
            helper.update(foodItemList);
        else if (command.equals(buttonCommands[4]))
            helper.isEmpty(foodItemList);
        else
            helper.print(foodItemList);
    }
    public static void createAndShowGUI() {
        JFrame frame = new JFrame("Vendor Interface");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        GUI newContentPane = new GUI();
        newContentPane.setOpaque(true);
        frame.setContentPane(newContentPane);
        frame.pack();
        frame.setVisible(true);
    }
}

这是Simulator程序：

import javax.swing.JTextField;
import javax.swing.text.JTextComponent;

public class Simulator {
    private JTextComponent tf;
    public Simulator(JTextField textField) {
        this.tf = textField;
    }
    public void add(LinkedList<FoodItem> fi)    {
        System.out.println("Please enter the name of the product you want to add.");
        String text = tf.getText();
        try {
            if (fi.containsName(text))
                throw new DuplicateProductException();
            else
                fi.add(enterFoodItem(text));
        }
        catch(DuplicateProductException ex) {
            System.out.println("The product has already been added to the list.");
            System.out.println("Would you like to update or remove this item? (U for update, R for remove, N for neither)");
            String response = tf.getText();
            if (response.equals("U"))
                update(fi);
            if (response.equals("R"))
                remove(fi);
        }
    }
    public void remove(LinkedList<FoodItem> fi, String s)   {
        if (fi.size() == 1)
            fi.list = null;
        else
            fi.oneBeforeContainingName(s).setLink(fi.oneBeforeContainingName(s).getLink().getLink());
        System.out.println("The product has been removed.");
    }

    public void remove(LinkedList<FoodItem> fi) {
        System.out.println("Please enter the name of the product you want to remove.");
        String text = tf.getText();
        try {
            if (fi.containsName(text))
                remove(fi, text);
            else
                throw new NonexistentProductException();
        }
        catch(NonexistentProductException ex)   {
            System.out.println("The product does not exist in the list.");
            System.out.println("Would you like to add this product? (Y/N)");
            String response = tf.getText();
            if (response.equals("Y"))
                add(fi);
        }
    }
    public void search(LinkedList<FoodItem> fi) {
        System.out.println("Please enter the name of the product you want to search for.");
        String text = tf.getText();
        if (fi.containsName(text))  {
            System.out.println("The product is found.");
            System.out.println("Would you like to print it or update it? (P for print, U for update, N for neither)");
            String response = tf.getText();
            if (response.equals("P"))
                print(fi, text);
            if (response.equals("U"))
                update(fi, text);
        }
        else
            System.out.println("The product does not exist in the list.");
    }
    public void update(LinkedList<FoodItem> fi, String s){
        fi.nodeContainingName(s).setElement(enterFoodItem(s));
    }
    public void update(LinkedList<FoodItem> fi) {
        System.out.println("Please enter the name of the product you want to update.");
        String text = tf.getText();
        try {
            if (fi.containsName(text))
                update(fi, text);
            else
                throw new NonexistentProductException();
        }
        catch(NonexistentProductException ex)   {
            System.out.println("The product does not exist in the list.");
            System.out.println("Would you like to add this product? (Y/N)");
            String response = tf.getText();
            if (response.equals("Y"))
                add(fi);
        }
    }
    public void isEmpty(LinkedList<FoodItem> fi)    {
        if (fi.isEmpty())
            System.out.println("The list is empty.");
        else
            System.out.println("The list is not empty.");
    }
    public void print(LinkedList<FoodItem> fi, String s){
        LinkedListNode<FoodItem> temp = fi.list;
        while (!temp.getElement().name.equals(s))
            temp = temp.getLink();
        System.out.println(temp.getElement().toString());
    }
    public void print(LinkedList<FoodItem> fi)  {
        if (fi.isEmpty())
            System.out.println("The list is empty.");
        else
            System.out.println("Here is the list:" + fi.toString());
    }
    public FoodItem enterFoodItem(String input) {
        String[] requiredEntry = {"price", "quantity", "description", "size", "special order", "category"};
        double p = 0.0;
        int q = 0;
        String d = "";
        double s = 0.0;
        String so = "";
        String c = "";
        for (int i = 0; i < 6; i++) {
            System.out.println("Please enter the " + requiredEntry[i] + " of the product.");
            if (i == 0)
                p = Double.parseDouble(tf.getText());
            if (i == 1)
                q = Integer.parseInt(tf.getText());
            if (i == 2)
                for (int j = 0; j < 2; j++)
                    d = tf.getText();
            if (i == 3)
                s = Double.parseDouble(tf.getText());
            if (i == 4)
                for (int j = 0; j < 2; j++)
                    so = tf.getText();;
            if (i == 5)
                c = tf.getText();
        }
        return new FoodItem(input,p,q,d,s,so,c);
    }

    public class DuplicateProductException extends Exception    {
        public DuplicateProductException() {;}
    }
    public class NonexistentProductException extends Exception  {
        public NonexistentProductException() {;}
    }
}

当我运行程序时，在我按下Add Item按钮后，它会说“请输入您要添加的产品的名称。” （在控制台上），但即使在我可以在文本框中输入任何内容之前，它也会立即显示此错误：

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at Simulator.add(Simulator.java:12)
    at GUI.actionPerformed(GUI.java:31)
    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)

这次发生了什么，我该如何解决这个问题？

4

3 回答 3

2

您的代码包括：

JTextComponent tf = null;
String text = tf.getText();

您不能在空值对象上调用方法。这会导致NullPointerException.

也许您想从现有的文本中获取文本JTextField：

String text=tf.getText();

当你用 来创建一个新的JTextField引用时JTextComponent tf = null;，你会用局部变量来隐藏实例变量，使其无法访问。

编辑：请参阅 Reimus 对构造函数中另一个（同样重要的）错误的回答。

于 2013-10-02T00:16:38.197 回答
2

你textField在Simulator. 代替

JTextField tf = textField;

和

tf = textField;
于 2013-10-02T00:20:42.427 回答
0

当你使用这个时：

public void add(LinkedList<FoodItem> fi) {
    System.out.println("Please enter the name of the product you want to add.");
    JTextComponent tf = null; // tf points to NULL, you might remove this line.
    String text = tf.getText();

您的 tf 引用设置为 null，它会在以下情况下立即引发异常：

String text = tf.getText(); //null.getText() !!

同样在这里，您将文本字段设置为构造函数中的局部变量，但全局变量设置为 null。

私有 JTextComponent tf; 公共模拟器（JTextField textField）{ JTextField tf = textField; 将其更改为：

  private JTextComponent tf;
  public Simulator(JTextField textField) {
   //you don't need "this" but that way you make sure you refeer to the class 
    this.tf = textField; variable.
  }

我不知道这是否足够，因为代码无法按原样编译。

  • 缺少 CategoryList 类。
  • 缺少 FoodItem 类。
  • 缺少 LinkedListNode 类。

...

public void print(LinkedList<FoodItem> fi, String s) {
    LinkedListNode<FoodItem> temp = fi.list;

我认为不能保证 LinkedList 对象始终是 LinkedListNode 的实例；您很可能应该使用 cast 或接收 LinkedListNode 作为参数而不是 LinkedList。

于 2013-10-02T00:28:05.273 回答