（Java）JPanel无法打印出来

Question

I'm doing a JPanel assignment in my college course and I'm having trouble with the final print out. The panel itself looks great and everything, the only thing I can't get it to do is display the line that is supposed to be printed out.

It's designed to take the input of a grocery item, with the name, price, category and quantity all being input by the user. When the "Add to Cart" button is clicked, it should display all the information previously typed in. But as of right now it does not.

Here's the two classes I'm using:

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

You forgot to add the ActionCommand for the JButton that checks the case statement in your ActionListener

Everytime you call this switch(e.getActionCommand()) the Listener is grabbing the ActionCommand of the button that was added, if it is not added then the name of the button is the ActionCommand .

solution:

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

Answer 2

Rod has given you the solution to your actual problem, however you are not using the JList and the ListModel properly.

Changes to the data in the model should be done via the ListModel . So there is not need for your ArrayList to keep track of the Items. The ListModel will do this for you.

So your basic code should be:

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

Then then you want to add items to the JList you actually update the ListModel:

model.addElement( myItem );