（Java）JPanel無法打印出來

Question

我在大學課程中正在做一個JPanel作業，但在最終打印時遇到了麻煩。 面板本身看起來很棒，並且所有事情，我唯一無法做的就是顯示應該打印的行。

它旨在接收雜貨項目的輸入，而名稱，價格，類別和數量均由用戶輸入。 單擊“添加到購物車”按鈕時，它應該顯示以前輸入的所有信息。但是，到目前為止，它還沒有。

這是我正在使用的兩個類：

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

您忘記為檢查ActionListener的case語句的JButton添加ActionCommand了

每次調用此switch(e.getActionCommand()) ，偵聽器都會抓住已添加按鈕的ActionCommand ，如果未添加，則按鈕的名稱為ActionCommand 。

解：

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

Answer 2

Rod為您提供了解決實際問題的解決方案，但是您沒有正確使用JList和ListModel 。

對模型中數據的更改應通過ListModel 。 因此，您的ArrayList不需要跟蹤項目。 ListModel將為您執行此操作。

因此，您的基本代碼應為：

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

然后，您想將項目添加到JList中，實際更新ListModel：

model.addElement( myItem );