Question

我在从其他类访问变量的想法上遇到了一些麻烦。我在这里发了一篇帖子：Having access to a private variable from other classes in Java 我得到了一些有用的信息，并认为一个例子会更好地展示它，并提出一个单独的问题。我有一个可以输入数据的表单，它有一个List变量。我一开始并没有让它变为静态，但我想如果我需要从另一个类中获取总大小，那么我就不会创建该类的实例来使用该函数来获取getTotalContacts。我基本上想用我列表中的联系人总数更新我的状态栏。其中一位成员在上面的帖子中说使用原来的Foo成员来获取联系人，但我不确定在这种情况下它是如何工作的。任何想法将不胜感激。感谢。

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import java.util.List;
import java.util.ArrayList;

public class AddressBook
{
    public static void main(String[] args)
    {
        EventQueue.invokeLater(new Runnable()
        {
            public void run()
            {
                AddressBookFrame frame = new AddressBookFrame();
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

                JMenuBar menuBar = new JMenuBar();
                frame.setJMenuBar(menuBar);

                JMenu fileMenu = new JMenu("File");
                JMenuItem openItem = new JMenuItem("Open");
                JMenuItem saveItem = new JMenuItem("Save");
                JMenuItem saveAsItem = new JMenuItem("Save As");
                JMenuItem printItem = new JMenuItem("Print");
                JMenuItem exitItem = new JMenuItem("Exit");
                fileMenu.add(openItem);
                fileMenu.add(saveItem);
                fileMenu.add(saveAsItem);
                fileMenu.add(printItem);
                fileMenu.add(exitItem);
                menuBar.add(fileMenu);

                JMenu editMenu = new JMenu("Edit");
                JMenuItem newItem = new JMenuItem("New");
                JMenuItem editItem = new JMenuItem("Edit");
                JMenuItem deleteItem = new JMenuItem("Delete");
                JMenuItem findItem = new JMenuItem("Find");
                JMenuItem firstItem = new JMenuItem("First");
                JMenuItem previousItem = new JMenuItem("Previous");
                JMenuItem nextItem = new JMenuItem("Next");
                JMenuItem lastItem = new JMenuItem("Last");
                editMenu.add(newItem);
                editMenu.add(editItem);
                editMenu.add(deleteItem);
                editMenu.add(findItem);
                editMenu.add(firstItem);
                editMenu.add(previousItem);
                editMenu.add(nextItem);
                editMenu.add(lastItem);
                menuBar.add(editMenu);

                JMenu helpMenu = new JMenu("Help");
                JMenuItem documentationItem = new JMenuItem("Documentation");
                JMenuItem aboutItem = new JMenuItem("About");
                helpMenu.add(documentationItem);
                helpMenu.add(aboutItem);

                menuBar.add(helpMenu);

                frame.setVisible(true);

            }
        });
    }
}

class AddressBookFrame extends JFrame
{
    public AddressBookFrame() 
    {
        setLayout(new BorderLayout());
        setTitle("Address Book");
        setSize(DEFAULT_WIDTH, DEFAULT_HEIGHT);
        AddressBookToolBar toolBar = new AddressBookToolBar();
        add(toolBar, BorderLayout.NORTH);
        AddressBookStatusBar aStatusBar = new AddressBookStatusBar();
        add(aStatusBar, BorderLayout.SOUTH);
        AddressBookForm form = new AddressBookForm();
        add(form, BorderLayout.CENTER);
    }
    public static final int DEFAULT_WIDTH = 500;
    public static final int DEFAULT_HEIGHT = 500;

}

/* Create toolbar buttons and add buttons to toolbar */
class AddressBookToolBar extends JPanel
{
    public AddressBookToolBar()
    {
        setLayout(new FlowLayout(FlowLayout.LEFT));
        JToolBar bar = new JToolBar();
        JButton newButton = new JButton("New");
        JButton editButton = new JButton("Edit");
        JButton deleteButton = new JButton("Delete");
        JButton findButton = new JButton("Find");
        JButton firstButton = new JButton("First");
        JButton previousButton = new JButton("Previous");
        JButton nextButton = new JButton("Next");
        JButton lastButton = new JButton("Last");
        bar.add(newButton);
        bar.add(editButton);
        bar.add(deleteButton);
        bar.add(findButton);
        bar.add(firstButton);
        bar.add(previousButton);
        bar.add(nextButton);
        bar.add(lastButton);
        add(bar);
    }
}

/* Creates the status bar string */
class AddressBookStatusBar extends JPanel 
{
    public AddressBookStatusBar()
    {
        setLayout(new FlowLayout(FlowLayout.LEFT));
        this.statusBarString = new JLabel("Total: " + AddressBookForm.getTotalContacts());
        add(this.statusBarString);
    }

    public void updateLabel()
    {
        contactsLabel.setText(AddressBookForm.getTotalContacts().toString());
    }

    private JLabel statusBarString;
    private JLabel contactsLabel;
}

class AddressBookForm extends JPanel 
{
    public AddressBookForm()
    {   
        // create form panel
        this.setLayout(new GridLayout(2, 1));
        JPanel formPanel = new JPanel();
        formPanel.setLayout(new GridLayout(4, 2));
        firstName = new JTextField(20);
        lastName = new JTextField(20);
        telephone = new JTextField(20);
        email = new JTextField(20);
        JLabel firstNameLabel = new JLabel("First Name: ", JLabel.LEFT);
        formPanel.add(firstNameLabel);
        formPanel.add(firstName);
        JLabel lastNameLabel = new JLabel("Last Name: ", JLabel.LEFT); 
        formPanel.add(lastNameLabel);
        formPanel.add(lastName);
        JLabel telephoneLabel = new JLabel("Telephone: ", JLabel.LEFT);
        formPanel.add(telephoneLabel);
        formPanel.add(telephone);
        JLabel emailLabel = new JLabel("Email: ", JLabel.LEFT);
        formPanel.add(emailLabel);
        formPanel.add(email);
        add(formPanel);

        // create button panel
        JPanel buttonPanel = new JPanel();
        JButton insertButton = new JButton("Insert");
        JButton displayButton = new JButton("Display");

        ActionListener insertAction = new AddressBookListener();
        ActionListener displayAction = new AddressBookListener();
        insertButton.addActionListener(insertAction);
        displayButton.addActionListener(displayAction);
        buttonPanel.add(insertButton);
        buttonPanel.add(displayButton);
        add(buttonPanel);
    }

    public static int getTotalContacts()
    {
        return addressList.size();
    }

    //void addContact(Person contact);

    private JTextField firstName;
    private JTextField lastName;
    private JTextField telephone;
    private JTextField email;
    private JLabel contacts;

    private static List<Person> addressList = new ArrayList<Person>();

    private class AddressBookListener implements ActionListener
    {
        public void actionPerformed(ActionEvent e)
        {
            String buttonPressed = e.getActionCommand();
            System.out.println(buttonPressed);
            if (buttonPressed == "Insert") {
                Person aPerson = new Person(firstName.getText(), lastName.getText(), telephone.getText(), email.getText());
                addressList.add(aPerson);
            }
            else {
                for (Person p : addressList) {
                    System.out.println(p);
                }
            }
        }
    }

}

我的另一个问题是为什么我得到错误，“int不能解除引用contactsLabel.setText（AddressbookForm.getTotalContacts（）。toString（））; 谢谢！

Answer 1

请记住，通过使addressList静态，它属于AddressbookForm类而不是类的实例。这意味着所有AddressbookForm将共享相同的addressList。我不确定这是不是你想要的。

关于您的错误，AddressbookForm.getTotalContacts()会返回int。由于int是Java中的基本类型，因此它没有toString()方法。 int可以转换为String，如：

int a = 5;
String aString = a + "";

Answer 2

从我的观点来看，正确的解决方案是使用包装器Integer作为基本类型int。与Java中的所有类一样，整数扩展了Object，因此可以使用toString（）来覆盖它或使用它。

// http://java.sun.com/j2se/1.5.0/docs/api/java/lang/Integer.html
public static Integer getTotalContacts()
{
    return new Integer( addressList.size() );
}

在Java中使用getter / setter

2 个答案: