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所以这是first question的后续行动。我从porfiriopartida那里得到了建议，并将逻辑与用户界面分开，并将逻辑放入一个名为Simulator的单独文件中。这就是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计划：
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，它会显示“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？
答案 0 :(得分：2)
您的代码包括：
JTextComponent tf = null;
String text = tf.getText();
您无法在空值对象上调用方法。这会导致NullPointerException。
也许您想从现有的JTextField获取文字：
String text=tf.getText();
使用JTextField创建新的JTextComponent tf = null;引用时，会使用局部变量隐藏实例变量，从而无法访问。
编辑：在构造函数中看到Reimus的答案，其他（同样重要的）错误。
答案 1 :(得分：2)
您在textField的构造函数中隐藏了Simulator。取代
JTextField tf = textField;
与
tf = textField;
答案 2 :(得分：0)
当你使用它时：
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();
你的tf引用设置为null，它会在以下后面抛出异常：
String text = tf.getText(); //null.getText() !!
此处，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。
private JTextComponent tf; public Simulator（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，因为代码不能按原样编译。
...
public void print(LinkedList<FoodItem> fi, String s) {
LinkedListNode<FoodItem> temp = fi.list;
我认为没有保证LinkedList对象始终是LinkedListNode的实例;很可能你应该使用强制转换或接收LinkedListNode作为参数而不是LinkedList。