在ActionListener中訪問另一個類時發生NullPointerException

Question

我試圖以一種標准方式向JButton添加ActionListener：在方法之外，我有private Actions listener; 在我已經放入的方法中

listener = new Actions(); // Create the action listener object 

    // Add action listeners to the necessary components
    isDatabaseDefault.addActionListener(listener);
    addEntry.addActionListener(listener);
    editEntry.addActionListener(listener);
    deleteEntry.addActionListener(listener);
    addDatabase.addActionListener(listener);
    editDatabase.addActionListener(listener);
    deleteDatabase.addActionListener(listener);

並且運行正常，沒有發現錯誤-這是ActionListener類：

package engines;

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

import javax.swing.JOptionPane;

import graphicalUI.Tabs;

public class Actions implements ActionListener, SoftwareProperties{
    // Create objects to access methods
    private DatabaseManagement database;
    private Tabs tabs;

    public Actions(){
        this.database = new DatabaseManagement();
        this.tabs = new Tabs();
    }

    // Method that is called when a button is clicked
    public void actionPerformed(ActionEvent e) {
        // Check the source of the action
        if(e.getActionCommand().equals("Make a new database")){
            System.out.println("Null pointer exception");
            String location = database.makeNewDatabase();
            if(location==null){
                JOptionPane.showMessageDialog(null, "Error: Your new database was not successfully created. Please try again if you like.", applicationName, JOptionPane.WARNING_MESSAGE);
                return;
            }
                        tabs.updateDatabaseMCombo();
            tabs.setDatabaseManagementContent(location, true);
        }
    }

}

當我按下按鈕時，盡管會輸出“ Null Pointer Exception”，並且出現了database.makeNewDatabase(); 運行，但是一旦它進入tabs類中的任何一個方法，我就會收到以下錯誤：

javax.swing.AbstractButton.fireActionPerformed（）上engine.Actions.actionPerformed（Actions.java:31）上圖形UI.Tabs.updateDatabaseMCombo（Tabs.java:148）上的線程“ AWT-EventQueue-0”中的java.lang.NullPointerException異常javax.swing.DefaultButtonModel.setPressed中的javax.swing.DefaultButtonModel.fireActionPerformed（未知源）處的javax.swing.DefaultButtonModel.setPressed（未知源）處的javax.swing.plaf.basic中的Unknown Source）。 java.awt.AWTEvent中的BasicButtonListener.mouseReleased（未知源）AWTEventMulticaster.mouseReleased（未知源）在java.awt.Component.processMouseEvent（未知源）在javax.swing.JComponent.processMouseEvent（java.awt.Component中。 java.awt.Container.dispatchEventImpl（未知源）java.awt.Component.dispatchEventImpl（java.awt.Component.dispatchEventImpl（未知源）java.awt.Component.dispatchEventImpl（未知源）未知源），位於java.awt.LightweightDispatcher.r java.awt.LightweightDispatcher.processMouseEvent（未知源）的etargetMouseEvent（未知源）java.awt.Container.dispatchatchEventImpl（java.awt.Container.dispatchEventImpl（未知源）的java.awt.LightweightDispatcher.dispatchEvent（未知源）未知源）java.awt.Component.dispatchEvent（未知源）java.awt.EventQueue.dispatchEventImpl（未知源）java.awt.EventQueue.access $ 000（未知源）java.awt.EventQueue $ 3.run（ java.awt.EventQueue $ 3.run中的Unknown Source）（java.security.AccessController.doPrivileged（本機方法）處的java.security.ProtectionDomain $ 1.doIntersectionPrivilege（本機方法）處的java.security.ProtectionDomain $ 1.doIntersectionPrivilege的未知源） （未知源）在java.awt.EventQueue $ 4.run（未知源）在java.awt.EventQueue $ 4.run（在java.security.AccessController.doPrivileged（本地方法）在java.security.ProtectionDomain $ 1。 java.awt.EventQueue.dispatchEve上的doIntersectionPrivilege（未知源） nt（未知源）（java.awt.EventDispatchThread.pumpOneEventForFilters（未知源） java.awt.EventDispatchThread.pumpEvents（源未知）（java.awt.EventDispatchThread.run中的未知源）

但是奇怪的是，當我在同一個類中運行這些方法時，它們可以完美地工作！ 這是updateDatabaseMCombo()方法：

public void updateDatabaseMCombo(){
        System.out.println("is this method running");
        int sIndex = selectDatabase.getSelectedIndex(); // Get the number value of the selected item
        String selectedItem = selectDatabase.getItemAt(sIndex); // Get the string of the selected item
        System.out.println(selectedItem);
        availableDBs4DM = db.getAvailableDatabases4DB(null); // Get a list of available databases to manage
        selectDatabase.removeAllItems(); // Remove all the current items in the combo
        // Loop through the array and manually add each item
        for(String item : availableDBs4DM)
            selectDatabase.addItem(item);
        // Select the item that was previously selected 
        int search = -1; // Initialise variable to hold the search results
        for(int s = 0; s < availableDBs4DM.length; s++){
            // If a match is found, update the search variable and stop searching
            if(availableDBs4DM[s].equals(selectedItem)){
                search = s;
                break;
            }
        }

        if(search != -1){
            // If the database that was previously selected is still in the JCombobox
            selectDatabase.setSelectedIndex(search);
        }else{
            // Select the default database
            db.setTranslationDefaultDB(selectDatabase);
        }
    }

那么誰能弄清楚為什么我會收到此錯誤？

順便說一句， selectDatabase已經被初始化為JComboBox<String>對象。

---

更新

---

好的，經過一些調試之后，我發現NullPointerException是因為我只在方法的外部聲明了變量，就像private JComboBox<String> selectDatabase; 我實際上是用另一種方法來初始化它，如下所示：

package test;
import java.awt.FlowLayout;

import javax.swing.DefaultComboBoxModel;
import javax.swing.JButton;
import javax.swing.JComboBox;
import javax.swing.JPanel;

import test2.Runner;


public class DBCombo {
    private JComboBox<String> combo = new JComboBox<String>();

    public JPanel makePanel(){
        JPanel panel = new JPanel(new FlowLayout());

        String[] options = {"Why", "will", "this", "not", "work"};
        combo.setModel(new DefaultComboBoxModel<String>(options));

        panel.add(combo);

        Runner main = new Runner();

        JButton doRead = new JButton("Read");
        doRead.addActionListener(main);

        panel.add(doRead);

        return panel;
    }

    public void getComboData(){
        System.out.println(combo.getItemCount());
    }
}

package test2;


import java.awt.Event;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.JFrame;

import test.DBCombo;


public class Runner implements ActionListener {
    public static void main(String[] args){
        JFrame frame = new JFrame("Test");

        DBCombo dbc = new DBCombo();

        frame.setContentPane(dbc.makePanel());
        frame.pack();
        frame.setVisible(true);
    }

    public void actionPerformed(ActionEvent e){
        new DBCombo().getComboData();
    }
}

有個主意嗎？

我現在認為這是因為動作偵聽器和方法位於不同的程序包中...

Answer 1

好的，不幸的是，這並不是我的初衷或想做的，但是至少我現在可以使它正常工作了！

事實證明，由於某種原因，我仍然不知道，除Tabs類本身之外的任何其他類updateDatabaseMCombo()稱為updateDatabaseMCombo()或setDatabaseManagementContent(String, boolean)生成空指針！ 所以，我只是硬着頭皮，將actionlistener放在Tabs類中...

哦，好吧，這並不是說我的最終用戶會有所不同。