(Java) JPanel没有打印出来

Question

我在大学课程中做了一个JPanel作业，但是我在最终打印出来的时候遇到了麻烦。面板本身看起来很棒，我唯一不能让它做的是显示应该打印出来的行。

它被设计用来输入一个杂货商品，用户输入名称、价格、类别和数量。当点击"添加到购物车"按钮时，它应该显示之前输入的所有信息。但目前还没有。

下面是我使用的两个类:

MyCart.java

package assignment2;
import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.ArrayList;
public class MyCart extends JFrame {
    private JPanel display;
    private JLabel itemLabel;
    private JTextField item;
    private JLabel catLabel;
    private JComboBox category;
    private JLabel quanLabel;
    private JTextField quantity;
    private JLabel priceLabel;
    private JTextField price;
    private JPanel btns;
    private JButton addBtn;
    private JButton exitBtn;
    private JPanel list;
    private JList myList;
    private ArrayList<Item> itemList;
    public MyCart() {
        itemList = new ArrayList();
        myList = new JList(itemList.toArray());
        this.setLayout(new GridLayout(2,1,5,5));
        display = new JPanel();
        display.setLayout(new GridLayout(5,2,5,5));
        itemLabel = new JLabel("  Item: ");
        item = new JTextField(10);
        catLabel = new JLabel("  Category: ");
        category = new JComboBox(new String[]{"Meat", "Fruit/Vegetable", "Dairy", "Grains", "Sweets", "Other"});
        quanLabel = new JLabel("  Quantity: ");
        quantity = new JTextField(10);
        priceLabel = new JLabel("  Price: ");
        price = new JTextField(10);
        display.add(itemLabel);
        display.add(item);
        display.add(catLabel);
        display.add(category);
        display.add(quanLabel);
        display.add(quantity);
        display.add(priceLabel);
        display.add(price);
        this.add(display);
        btns = new JPanel();
        btns.setLayout(new GridLayout(1,3,5,5));
        addBtn = new JButton("Add to Cart");
        exitBtn = new JButton("Exit");
        btns.add(addBtn);
        btns.add(exitBtn);
        this.add(btns); 
        list = new JPanel();
        list.setLayout(new FlowLayout());
        list.add(new JLabel("My Cart:"));
        JScrollPane myScrollPane = new JScrollPane(myList);
        list.add(myScrollPane);
        setLayout(new BorderLayout());
        this.add(display,BorderLayout.NORTH);
        this.add(btns,BorderLayout.CENTER);
        this.add(list,BorderLayout.SOUTH);
        //event listener object created
        ButtonListeners buttonListener = new ButtonListeners();
        addBtn.addActionListener(buttonListener);
        exitBtn.addActionListener(buttonListener);
    }
    public static void main(String[] args) {
        MyCart cart = new MyCart();
        cart.setTitle("Cart <Adam>");
        cart.setSize(400,350);
        cart.setLocationRelativeTo(null);
        cart.setVisible(true);
        cart.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        }
    //event listener class for adding
    class ButtonListeners implements ActionListener {
        @Override
        public void actionPerformed(ActionEvent e) {
            switch(e.getActionCommand()) {
                case "Add": String n = item.getText();
                            String c = category.getSelectedItem().toString();
                            double q = Double.valueOf(quantity.getText());
                            double p = Double.valueOf(price.getText());
                            Item myItem = new Item();
                            myItem.setName(n);
                            myItem.setCategory(c);
                            myItem.setQuantity(q);
                            myItem.setPrice(p);
                            itemList.add(myItem);
                            myList.setListData(itemList.toArray());
                            break;
                case "Exit": System.exit(0);
            }
        }
    }
    //exit listener
    class ExitListener implements ActionListener{
        @Override
        public void actionPerformed(ActionEvent e){
           System.exit(0);
        }
    }
}

Item.java

package assignment2;
public class Item {
    private String name;
    private String category;
    private double price;
    private double quantity;
    public Item(String name, String category, double price, double quantity) {
    }
    public Item() {
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public String getCategory() {
        return category;
    }
    public void setCategory(String category) {
        this.category = category;
    }
    public double getPrice() {
        return price;
    }
    public void setPrice(double price) {
        this.price = price;
    }
    public double getQuantity() {
        return quantity;
    }
    public void setQuantity(double quantity) {
        this.quantity = quantity;
    }
    public double calcAmount() {
        return quantity * price;
    }
    public String toString(){
        return(name + ", " + category + ", " + quantity + ", " + price + ", " + calcAmount()); 
    }
}

Answer 1

您忘记为ActionListener中检查case语句的JButton添加ActionCommand

每次调用此switch(e.getActionCommand())时，侦听器都会抓取已添加的按钮的ActionCommand，如果未添加则按钮的名称为ActionCommand。

解决方案:

addBtn = new JButton("Add to Cart");
addBtn.setActionCommand("Add"); //will call the Add case actionListener

Answer 2

Rod已经为您提供了实际问题的解决方案，但是您没有正确使用JList和ListModel。

对模型中数据的更改应通过ListModel完成。所以你的ArrayList就不需要跟踪item了。ListModel将为您做这些。

所以你的基本代码应该是:

DefaultListModel<Item> model = new DefaultListModel<Item>();
JList list = new JList( model );

然后你想添加项目到JList你实际上更新了ListModel:

model.addElement( myItem );