如何从另一个类中获取 ArrayList 以显示在 JComboBox 中？答案

【问题标题】：How to get ArrayList from another class to show up inside a JComboBox?如何从另一个类中获取 ArrayList 以显示在 JComboBox 中？
【发布时间】：2015-12-13 09:45:52
【问题描述】：

我正在尝试构建一个程序，我无法从另一个类中获取我的 ArrayList 以显示在 JComboBox 中。我的程序看起来像这样，但在“Melissa”所在的位置之前写的，我想要一个 JComboBox 有一个用户列表，并能够从中选择得到结果：

更新这是我的错误：

 ----jGRASP exec: javac -g UserHistory.java

UserHistory.java:20: error: cannot find symbol
        String[] userListItems = users.toArray(new String[0]);
                                 ^
  symbol:   variable users
  location: class UserHistory
Note: UserHistory.java uses unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.
1 error

 ----jGRASP wedge2: exit code for process is 1.
 ----jGRASP: operation complete.

这是我不完整的代码，它给了我一个错误，因为它无法找到我的数组列表：

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

public class UserHistory extends JPanel {
    private JLabel jcomp1;
    private JComboBox userList;
    private JLabel jcomp3;
    private JTextField selectedUser;
    private JLabel jcomp5;
    private JTextField pointsEarned;
    private JLabel jcomp7;
    private JList choresCompleted;

    public UserHistory() {
        //construct preComponents
        String[] userListItems = users.toArray(new String[0]);
        String[] choresCompletedItems = {"Item 1", "Item 2", "Item 3"};

        //construct components
        jcomp1 = new JLabel ("User History");
        userList = new JComboBox(userListItems);
        jcomp3 = new JLabel ("Below are the results of : ");
        selectedUser = new JTextField (5);
        jcomp5 = new JLabel ("Total points earned: ");
        pointsEarned = new JTextField (5);
        jcomp7 = new JLabel ("List of chores completed: ");
        choresCompleted = new JList (choresCompletedItems);

        //set components properties
        userList.setToolTipText ("Select a user");

        //adjust size and set layout
        setPreferredSize (new Dimension (465, 343));
        setLayout (null);

        //add components
        add (jcomp1);
        add (userList);
        add (jcomp3);
        add (selectedUser);
        add (jcomp5);
        add (pointsEarned);
        add (jcomp7);
        add (choresCompleted);

        //set component bounds (only needed by Absolute Positioning)
        jcomp1.setBounds (120, 20, 70, 25);
        userList.setBounds (210, 20, 100, 25);
        jcomp3.setBounds (95, 70, 155, 25);
        selectedUser.setBounds (245, 70, 100, 25);
        jcomp5.setBounds (125, 105, 140, 25);
        pointsEarned.setBounds (245, 105, 100, 25);
        jcomp7.setBounds (95, 140, 160, 25);
        choresCompleted.setBounds (245, 145, 100, 75);
    }


    public static void main (String[] args) {
        JFrame frame = new JFrame ("UserHistory");
        frame.setDefaultCloseOperation (JFrame.EXIT_ON_CLOSE);
        frame.getContentPane().add (new UserHistory());
        frame.pack();
        frame.setVisible (true);
    }
}

这是另一个包含 ArrayList 的面板的代码：

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

public class ManageUsersGUI extends JPanel {
    public ArrayList<User> users = new ArrayList<>();

    private JLabel addNewUserLabel;
    private JTextField addNewUserTextField;
    private JLabel deleteUsersLabel;
    private JButton addButton;
    private JButton deleteButton;
    private JPanel namePanel;


    public ManageUsersGUI() {
        //construct components
        addNewUserLabel = new JLabel ("Add new User here:");
        addNewUserTextField = new JTextField (0);
        deleteUsersLabel = new JLabel ("Select which User(s) you would like to delete:");
        addButton = new JButton ("Add");
        deleteButton = new JButton ("Delete");
        namePanel = new JPanel();
        namePanel.setLayout(new BoxLayout(namePanel, BoxLayout.Y_AXIS));

        //set components properties
        addNewUserTextField.setToolTipText ("Enter name and click on Add button.");
        addButton.setToolTipText ("Click here to Add new user.");
        deleteButton.setToolTipText ("Click here to delete User(s) selected.");

        //adjust size and set layout
        setPreferredSize (new Dimension (580, 485));
        setLayout (null);

        //add components
        add (addNewUserLabel);
        add (addNewUserTextField);
        add (deleteUsersLabel);
        add (namePanel);
        add (addButton);
        add (deleteButton);

        //set component bounds (only needed by Absolute Positioning)
        addNewUserLabel.setBounds (85, 130, 120, 25);
        addNewUserTextField.setBounds (235, 130, 125, 25);
        deleteUsersLabel.setBounds (135, 225, 281, 25);
        addButton.setBounds (385, 130, 100, 25);
        namePanel.setBounds(225, 270, 140, 0);
        deleteButton.setBounds (230, 335, 100, 25);

        addButton.addActionListener(new AddButtonListener());

        deleteButton.addActionListener(new DeleteButtonListener());
    }

    public ArrayList<User> getUser() {
               return users;
            }        

    private class AddButtonListener implements ActionListener {
        public void actionPerformed(ActionEvent e) {
            String text = addNewUserTextField.getText();
            users.add(new User(text));

            // Display the changes.
            JOptionPane.showMessageDialog(null, text + " has been added.");

            JCheckBox nameCheckBox = new JCheckBox();
            nameCheckBox.setText(addNewUserTextField.getText());
            namePanel.add(nameCheckBox);
            namePanel.setBounds(225, 270, 140, namePanel.getHeight() + 25);
            deleteButton.setBounds(230, deleteButton.getY() + 25, 100, 25);
            JFrame frame = (JFrame) getRootPane().getParent();
            frame.setSize(frame.getWidth(), frame.getHeight() + 25);
            frame.pack(); 

        }
    }

    private class DeleteButtonListener implements ActionListener {
         @Override
         public void actionPerformed(ActionEvent e) {
            for(Component component : namePanel.getComponents()) {
               if(component instanceof JCheckBox) {
                  if(((JCheckBox)component).isSelected())
                     namePanel.remove(component);
               }
            }
            namePanel.revalidate();
            namePanel.repaint();
         }   
    }


    public static void main (String[] args) {
        JFrame frame = new JFrame ("AddUsersPanel1");
        frame.setTitle("Manage Users");
        frame.setDefaultCloseOperation (JFrame.EXIT_ON_CLOSE);
        frame.getContentPane().add (new ManageUsersGUI());
        frame.pack();
        frame.setVisible (true);
    }
}

这是我用来制作 ArrayList 的 User 类：

public class User {

   private String userName;
   private int points = 0;

   public User(String userName) {
      this.userName = userName;
   }

   public User() {
      userName = "";
   }   

   public void setUserName(String userName) {
      this.userName = userName;   
   }

   public String getUserName() {
      return userName;
   }

   public void addPoints(int chorePoints) {
      points += chorePoints;
   }

  // public String toString() {
     // return userName + "\n";
 //  }

}

【问题讨论】：

你的问题是......？
UserHistory 如何与ManageUsersGUI 绑定？它们都有自己的main方法，它们是独立的程序吗？
@LittleSanti 我的问题是我不知道如何使我的 userList JcomboBox 从位于另一个类中的 ArrayList 中获取名称。
@WalterM 它们是不同的窗口，我稍后会将它们组合成一个完整的程序。我正在尝试制作一个跟踪用户家务和分数的家务评分程序。 ManageUsersGUI 将用户添加到程序中，您还可以从 JCheckbox 中删除用户。 UserHistory 告诉您您选择查看的每个用户的历史记录。还有一个主页，可让您单击用户历史记录或管理用户并将您重定向到每个框架。

标签： java arraylist jpanel jcombobox

【解决方案1】：

首先你最好从历史主目录中调用用户管理模块

    JFrame frame = new JFrame ("UserHistory");
    frame.setDefaultCloseOperation (JFrame.EXIT_ON_CLOSE);
    frame.getContentPane().add (new UserHistory());
    frame.pack();
    frame.setVisible (true);

    manageUsers();

这将启动两个 JPanel，以便您可以同时管理用户和观看历史记录。

另外，

// in ManageUsersGUI
public static void manageUsers() {
    JFrame frame = new JFrame ("AddUsersPanel1");
    frame.setTitle("Manage Users");
    frame.setDefaultCloseOperation (JFrame.EXIT_ON_CLOSE);
    frame.getContentPane().add (new ManageUsersGUI());
    frame.pack();
    frame.setVisible (true);
}

还有一件事，一旦你初始化了用户列表，你需要像下面这样附加一个监听器：

    userList = new JComboBox(userListItems);

    userList.addPopupMenuListener(new PopupMenuListener() {
        @Override
        public void popupMenuWillBecomeVisible(PopupMenuEvent e) {
            userList.removeAllItems();
            for (User user: users) {
                userList.addItem(user.getUserName());
            }   
        }

        @Override
        public void popupMenuWillBecomeInvisible(PopupMenuEvent e) {
        }

        @Override
        public void popupMenuCanceled(PopupMenuEvent e) {
        }
    });

这两个更正你是否能够在组合框中列出用户名，并且用户列表的动态变化将反映在框中的列表中。

干杯。

【讨论】：

问题是我无法初始化我的 userList，因为我不知道如何将 ArrayList 从我的 ManageUsersGUI 解释到 JComboBox 内的 UserHistory 类中。我已经添加了上面得到的错误。
可能你有点糊涂了。不要担心初始化，因为它是在“popupMenuWillBecomeVisible”方法中完成的。只需按照我上传的代码进行操作即可。事实上，我在家里的电脑上有一个完整的运行版本。现在我在工作地点。
我完全按照您的编码并将其放入我的代码中，但它仍然给我同样的错误。
----jGRASP exec: javac -g UserHistory.java UserHistory.java:19: error: cannot find symbol String[] userListItems = users.toArray(new String[0]); ^ symbol: variable users location: class UserHistory UserHistory.java:29: error: cannot find symbol for (User user: users) { ^ symbol: variable users Note: UserHistory.java uses unchecked or unsafe operations. Note: Recompile with -Xlint:unchecked for details. 2 errors ----jGRASP wedge2: exit code for process is 1. ----jGRASP: operation complete.

【解决方案2】：

让我在这里上传整个源代码

User.java -- 源代码如下（我不知道他们为什么开箱即用。）

打包用户任务列表；

公共类用户{

私有字符串用户名；私有 int 点 = 0;

公共用户（字符串用户名）{ this.userName = 用户名； }

公共用户（）{ 用户名 = ""; }

public void setUserName(String userName) { this.userName = 用户名;
}

公共字符串 getUserName() { 返回用户名； }

public void addPoints(int chorePoints) { 点 += 杂务点； } }

ManageUsersGUI.java -- 源代码如下（我不知道为什么它们是开箱即用的。）

打包用户任务列表；

导入 javax.swing.; 导入 java.awt.; 导入 java.awt.event.*；导入 java.util.ArrayList；导入 javax.swing.JCheckBox;

公共类 ManageUsersGUI 扩展 JPanel { public static ArrayList users = new ArrayList();

private JLabel addNewUserLabel;
private JTextField addNewUserTextField;
private JLabel deleteUsersLabel;
private JButton addButton;
private JButton deleteButton;
private JPanel namePanel;


public ManageUsersGUI() {
    //construct components
    addNewUserLabel = new JLabel ("Add new User here:");
    addNewUserTextField = new JTextField (0);
    deleteUsersLabel = new JLabel ("Select which User(s) you would like to delete:");
    addButton = new JButton ("Add");
    deleteButton = new JButton ("Delete");
    namePanel = new JPanel();
    namePanel.setLayout(new BoxLayout(namePanel, BoxLayout.Y_AXIS));

    //set components properties
    addNewUserTextField.setToolTipText ("Enter name and click on Add button.");
    addButton.setToolTipText ("Click here to Add new user.");
    deleteButton.setToolTipText ("Click here to delete User(s) selected.");

    //adjust size and set layout
    setPreferredSize (new Dimension (580, 485));
    setLayout (null);

    //add components
    add (addNewUserLabel);
    add (addNewUserTextField);
    add (deleteUsersLabel);
    add (namePanel);
    add (addButton);
    add (deleteButton);

    //set component bounds (only needed by Absolute Positioning)
    addNewUserLabel.setBounds (85, 130, 120, 25);
    addNewUserTextField.setBounds (235, 130, 125, 25);
    deleteUsersLabel.setBounds (135, 225, 281, 25);
    addButton.setBounds (385, 130, 100, 25);
    namePanel.setBounds(225, 270, 140, 0);
    deleteButton.setBounds (230, 335, 100, 25);

    addButton.addActionListener(new AddButtonListener());

    deleteButton.addActionListener(new DeleteButtonListener());
}

public ArrayList<User> getUser() {
           return users;
        }        

private class AddButtonListener implements ActionListener {
    public void actionPerformed(ActionEvent e) {
        String text = addNewUserTextField.getText();
        users.add(new User(text));

        // Display the changes.
        JOptionPane.showMessageDialog(null, text + " has been added.");

        JCheckBox nameCheckBox = new JCheckBox();
        nameCheckBox.setText(addNewUserTextField.getText());
        namePanel.add(nameCheckBox);
        namePanel.setBounds(225, 270, 140, namePanel.getHeight() + 25);
        deleteButton.setBounds(230, deleteButton.getY() + 25, 100, 25);
        JFrame frame = (JFrame) getRootPane().getParent();
        frame.setSize(frame.getWidth(), frame.getHeight() + 25);
        frame.pack(); 

    }
}

private class DeleteButtonListener implements ActionListener {
     @Override
     public void actionPerformed(ActionEvent e) {
        for(Component component : namePanel.getComponents()) {
           if(component instanceof JCheckBox) {
              if(((JCheckBox)component).isSelected())
                 namePanel.remove(component);
           }
        }
        namePanel.revalidate();
        namePanel.repaint();
     }   
}


public static void main (String[] args) {
    JFrame frame = new JFrame ("AddUsersPanel1");
    frame.setTitle("Manage Users");
    frame.setDefaultCloseOperation (JFrame.EXIT_ON_CLOSE);
    frame.getContentPane().add (new ManageUsersGUI());
    frame.pack();
    frame.setVisible (true);
}

// in ManageUsersGUI
public static void manageUsers() {
    JFrame frame = new JFrame ("AddUsersPanel1");
    frame.setTitle("Manage Users");
    frame.setDefaultCloseOperation (JFrame.EXIT_ON_CLOSE);
    frame.getContentPane().add (new ManageUsersGUI());
    frame.pack();
    frame.setVisible (true);
}

}

UserHistory.java -- 源代码如下（我不知道他们为什么开箱即用，第三次也是最后一次。）

导入 java.awt.; 导入 javax.swing.; 导入 javax.swing.event.PopupMenuEvent；导入 javax.swing.event.PopupMenuListener；导入 userchorelist.ManageUsersGUI；导入静态 userchorelist.ManageUsersGUI.manageUsers；导入静态 userchorelist.ManageUsersGUI.users；导入userchorelist.User;

公共类 UserHistory 扩展 JPanel { 私人 JLabel jcomp1；私人 JComboBox 用户列表；私人 JLabel jcomp3；私人 JTextField selectedUser; 私人 JLabel jcomp5；私人 JTextField 积分；私人 JLabel jcomp7； private JList choresCompleted;

public UserHistory() {
    //construct preComponents
    String[] userListItems = new String[users.size()];

    String[] choresCompletedItems = {"Item 1", "Item 2", "Item 3"};

    //construct components
    jcomp1 = new JLabel ("User History");
    userList = new JComboBox(userListItems);

    userList.addPopupMenuListener(new PopupMenuListener() {
        @Override
        public void popupMenuWillBecomeVisible(PopupMenuEvent e) {
            userList.removeAllItems();
            for (User user: users) {
                userList.addItem(user.getUserName());
            }   
        }

        @Override
        public void popupMenuWillBecomeInvisible(PopupMenuEvent e) {
        }

        @Override
        public void popupMenuCanceled(PopupMenuEvent e) {
        }
    });

    jcomp3 = new JLabel ("Below are the results of : ");
    selectedUser = new JTextField (5);
    jcomp5 = new JLabel ("Total points earned: ");
    pointsEarned = new JTextField (5);
    jcomp7 = new JLabel ("List of chores completed: ");
    choresCompleted = new JList (choresCompletedItems);

    //set components properties
    userList.setToolTipText ("Select a user");

    //adjust size and set layout
    setPreferredSize (new Dimension (465, 343));
    setLayout (null);

    //add components
    add (jcomp1);
    add (userList);
    add (jcomp3);
    add (selectedUser);
    add (jcomp5);
    add (pointsEarned);
    add (jcomp7);
    add (choresCompleted);

    //set component bounds (only needed by Absolute Positioning)
    jcomp1.setBounds (120, 20, 70, 25);
    userList.setBounds (210, 20, 100, 25);
    jcomp3.setBounds (95, 70, 155, 25);
    selectedUser.setBounds (245, 70, 100, 25);
    jcomp5.setBounds (125, 105, 140, 25);
    pointsEarned.setBounds (245, 105, 100, 25);
    jcomp7.setBounds (95, 140, 160, 25);
    choresCompleted.setBounds (245, 145, 100, 75);
}


public static void main (String[] args) {
    JFrame frame = new JFrame ("UserHistory");
    frame.setDefaultCloseOperation (JFrame.EXIT_ON_CLOSE);
    frame.getContentPane().add (new UserHistory());
    frame.pack();
    frame.setVisible (true);

    manageUsers();
}

}

【讨论】：

有没有办法在不使用包的情况下做到这一点？
我试着用包做这件事，它给了我更多的错误，它仍然找不到用户的符号

【解决方案3】：

我建议您在UserHistory 和ManageUsersGUI 两个类中添加接收用户列表作为显式参数：

class UserHistory
{
    private final List<User> users;

    public UserHistory(List<User> users)
    {
        super();
        this.users=users;
        ...
    }
}

class ManageUsersGUI
{
    private final List<User> users;

    public UserHistory(List<User> users)
    {
        super();
        this.users=users;
        ...
    }
}

然后，您可以创建一个唯一的用户列表作为外部类，比如UserManagement，并将其设为单例：

public class UserManagement
{
    private static final UserManagement INSTANCE=new UserManagement();

    private final List<User> list=new ArrayList<User>();

    public static UserManagement getInstance()
    {
        return INSTANCE;
    }

    private UserManagement()
    {
    }

    public List<User> getUsers()
    {
        return users;
    }
}

然后，在main 方法中实例化这两个类时，您必须提供用户列表（以及将来您可能拥有的实例）：

public static void main (String[] args) {
    ...
    UserManagement.getInstance().getUsers().add("winkum");
    UserManagement.getInstance().getUsers().add("blinkum");
    UserManagement.getInstance().getUsers().add("nod");
    frame.getContentPane().add (new UserHistory(UserManagement.getInstance().getUsers()));
    ...
}

【讨论】：

当我尝试添加显式参数时，它给了我一个错误“变量用户可能没有被初始化”这两个类。
@pyuntae 在每个类中声明只有一个构造函数，并在其中初始化实例变量users，作为构造函数中的第一件事，正如我发布的那样：@ 987654329@（作为“用户”参数的名称）。