java - 变量在 if 语句之外未更新

Question

有人可以解释一下，为什么单击“borrowBook”或“returnBook”按钮后变量“bookAvailable1”和“bookAvailable2”没有更新。我需要制作一个虚拟图书馆，并且在用户借阅或归还一本书后我不会更新库存。我该怎么办？提前致谢。这是我的代码:

import java.awt.BorderLayout;
import java.awt.TextField;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.BoxLayout;
import javax.swing.JButton;
import javax.swing.JCheckBox;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JOptionPane;
import javax.swing.JPanel;
import javax.swing.JTextField;


public class MyFrame extends JFrame {

    JPanel panelChB, panelText;
    private JLabel title, bookTitle, bookTitle1;
    private JButton borrowBook, returnBook;
    private JCheckBox ch1, ch2;
    int bookAvailable1 = 3;
    int bookAvailable2 = 3;
    boolean haveBook1 = false;
    boolean haveBook2 = false;

    public MyFrame() {

        setSize(400, 150);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setLocationRelativeTo(null);
        setTitle("Books");
        setResizable(false);
        setLayout(new BorderLayout());
        setVisible(true);

        panelChB = new JPanel();
        panelChB.setLayout(new BoxLayout(panelChB, BoxLayout.Y_AXIS));

        ch1 = new JCheckBox();
        ch2 = new JCheckBox();
        borrowBook = new JButton("BorrowBook");
        borrowBook.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                if (ch1.isSelected() || ch2.isSelected()) {
                    if (ch1.isSelected() && bookAvailable1 != 0) {
                        String infoMessage = "You borrowed \"Walk\"";
                        JOptionPane.showMessageDialog(null, infoMessage,
                                "Message: ", JOptionPane.INFORMATION_MESSAGE);
                        bookAvailable1--;
                        haveBook1 = true;

                    }
                    if (ch2.isSelected() && bookAvailable2 != 0) {
                        String infoMessage = "You borrowed \"Red and Black\"";
                        JOptionPane.showMessageDialog(null, infoMessage,
                                "Message: ", JOptionPane.INFORMATION_MESSAGE);
                        bookAvailable2--;
                        haveBook2 = true;
                    }
                } else {
                    String infoMessage = "No book selected";
                    JOptionPane.showMessageDialog(null, infoMessage,
                            "Message: ", JOptionPane.INFORMATION_MESSAGE);
                }
            }

        });
        returnBook = new JButton("Return");
        returnBook.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {

                if (ch1.isSelected() || ch2.isSelected()) {
                    if (ch1.isSelected() && haveBook1 == true) {
                        String infoMessage = "Book \"Walk\" returned";
                        JOptionPane.showMessageDialog(null, infoMessage,
                                "Message: ", JOptionPane.INFORMATION_MESSAGE);
                        bookAvailable1++;
                        haveBook1 = false;

                    }
                    if (ch2.isSelected() && haveBook2 == true) {

                        String infoMessage = "Book \"Red and Black\" returned";
                        JOptionPane.showMessageDialog(null, infoMessage,
                                "Message: ", JOptionPane.INFORMATION_MESSAGE);
                        bookAvailable2++;
                        haveBook2 = false;
                    }
                } else {
                    String infoMessage = "No book selected";
                    JOptionPane.showMessageDialog(null, infoMessage,
                            "Message: ", JOptionPane.INFORMATION_MESSAGE);
                }

            }

        });


        //System.out.println(available1);

        panelChB.add(ch1);
        panelChB.add(ch2);
        panelChB.add(borrowBook);
        panelChB.add(returnBook);


        //System.out.println(available1);
        panelText = new JPanel();
        panelText.setLayout(new BoxLayout(panelText, BoxLayout.Y_AXIS));
        bookTitle = new JLabel("Walk - stock  " + bookAvailable1);
        bookTitle1 = new JLabel("Red and Black - stock  " + bookAvailable2);
        panelText.add(bookTitle);
        panelText.add(bookTitle1);

        add(panelChB, BorderLayout.WEST);
        add(panelText, BorderLayout.CENTER);

    }

    public static void main(String[] args) {
        JFrame jf = new MyFrame();
    }
}

Answer 1

变量确实更新，但这不会影响屏幕上的标签。当 bookAvailable1--; 执行时，标签已经初始化，并且不会再次自动更改。

替换这个:

bookTitle = new JLabel("Walk - stock  " + bookAvailable1);
bookTitle1 = new JLabel("Red and Black - stock  " + bookAvailable2);

与:

bookTitle = new JLabel();
bookTitle1 = new JLabel();
updateStockDisplay();

并添加一个新方法:

private void updateStockDisplay() {
    bookTitle.setText("Walk - stock  " + bookAvailable1);
    bookTitle1.setText("Red and Black - stock  " + bookAvailable2);
}

现在，在借用/返回按钮的每个 ActionListener 末尾添加对 updateStockDisplay(); 的调用，以便每当变量发生更改时都会更新标签。