Java Swing 应用程序 JList null 异常

Question

这个问题已经有答案了:

What is a NullPointerException, and how do I fix it? (12 个回答)

已关闭 7 年前。

我想制作一个java swing应用程序，用于搜索一些数据(通过JDBC从分层数据库中提取)。为了测试目的，我做了这样的:-

1 级:1、2、32 级:{1.1, 1.2, 1.3}, {2.1, 2.2, 2.3}, {3.1, 3.2, 3.3}3 级:{1.1.1, 1.1.2},{1.2.1, 1.2.2}, ..., {3.3.1, 3.3.2}

用户界面是这样的:-Level-1 将显示在 JComboBox 中。Level-2 也是一个 JComboBox，但通过 ComboBoxModel 进行填充取决于 level-1 selectedIndex。Level-3 我想在依赖于 level-1 和 level-2 的 JList 中显示，如 level-3 中那样。

level-1 和 level-2 工作正常。但是当我尝试启动 level3values 属性时，level-3 给出了“NullPointerException”。

这是代码:

import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.Iterator;

public class TestSwing extends JPanel implements Runnable, ActionListener {

    private JComboBox combo1;
    private JComboBox combo2 = new JComboBox();
    private ComboBoxModel[] models ;
    private JList<String> list;
    private DefaultListModel<String>[][] level3values;
    //TODO:add displaying level3, copy method
    public TestSwing(String[] level1candidates, String[][] level2candidates, String[][][] level3candidates){

        this.models = new ComboBoxModel[level1candidates.length];
        this.combo1 = new JComboBox(level1candidates);
        for (int i=0; i<level1candidates.length; i++ )
            models[i] = new DefaultComboBoxModel(level2candidates[i]);
        for (int i=0; i<level1candidates.length; i++) {
            for (int j=0; j<level2candidates[i].length; j++) {
                level3values[i][j] = new DefaultListModel<String>();
                for (int k=0; k<level3candidates[i][j].length; k++) {
                    this.level3values[i][j].addElement(level3candidates[i][j][k]);
                } ;//TODO:DEBUG-how to initiate list model
            }
        }
        combo2.setModel(models[0]);
        this.list = new JList<String>(new String[]{});
        this.add(combo1);
        this.add(combo2);
        this.add(list);
        this.combo1.addActionListener(this);
        this.combo2.addActionListener(this);
    }
    @Override
    public void actionPerformed(ActionEvent e) {

        int i = combo1.getSelectedIndex();
        combo2.setModel(models[i]);
        int j = combo2.getSelectedIndex();
        list.setModel(level3values[i][j]);
    }

    @Override
    public void run() {

        JFrame f = new JFrame("CPBG-Store_Material-Finder");
        f.setSize(400,400);
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        JPanel controlPane = new JPanel();
        GridLayout gridLayout = new GridLayout(3,2, 10, 10);
        controlPane.setLayout(gridLayout);
        controlPane.add(new JLabel("Level-1"));
        controlPane.add(this.combo1);
        controlPane.add(new JLabel("Level-2"));
        controlPane.add(this.combo2);
        controlPane.add(new JLabel("Level-3"));
        //controlPane.add(new JLabel("Level-4"));
        controlPane.add(this.list);
        f.add(controlPane);
        //f.pack();
        //f.setLocationRelativeTo(null);
        f.setVisible(true);
    }

    public static void main(String[] args) {
        String[] level1candidates = new String[]{"1", "2", "3"};
        String[][] level2candidates = {new String[]{"1.1", "1.2", "1.3"},
                new String[]{"2.1", "2.2", "2.3"}, new String[]{"3.1", "3.2", "3.3"}};
        String[][][] level3candidates = {
                {new String[]{"1.1.1", "1.1.2"},new String[]{"1.2.1", "1.2.2"}, new String[]{"1.3.1","1.3.2"}},
                {new String[]{"2.1.1", "2.1.2"},new String[]{"2.2.1", "2.2.2"}, new String[]{"2.3.1","2.3.2"}},
                {new String[]{"3.1.1", "3.1.2"},new String[]{"3.2.1", "3.2.2"}, new String[]{"3.3.1","3.3.2"}},
                };
        //creating and showing this application's GUI.
        EventQueue.invokeLater(new TestSwing(level1candidates, level2candidates, level3candidates));
    }
}

预先感谢您帮助调试问题。

特别是，comboboxmodel[] 工作正常，但 defaultlistmodel[][] 给出空指针异常，有什么特殊原因吗？

这是两个级别的工作代码:

import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.sql.Connection;
import java.sql.SQLException;

public class MaterialFinder extends JPanel implements Runnable, ActionListener {

    private JComboBox combo1;
    private JComboBox combo2 = new JComboBox();
    private ComboBoxModel[] models ;

    public MaterialFinder(String[] level1candidates, String[][] level2candidates, String[][][] level3candidates){

        this.models = new ComboBoxModel[level1candidates.length];
        this.combo1 = new JComboBox(level1candidates);
        for (int i=0; i<level1candidates.length; i++ )
            models[i] = new DefaultComboBoxModel(level2candidates[i]);
        combo2.setModel(models[0]);
        this.add(combo1);
        this.add(combo2);
        this.combo1.addActionListener(this);
        this.combo2.addActionListener(this);
    }
    @Override
    public void actionPerformed(ActionEvent e) {

        int i = combo1.getSelectedIndex();
        combo2.setModel(models[i]);
        int j = combo2.getSelectedIndex();

    }

    @Override
    public void run() {

        JFrame f = new JFrame("CPBG-Store_Material-Finder");
        f.setSize(400,400);
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        f.add(this);
        //f.pack();
        f.setLocationRelativeTo(null);
        f.setVisible(true);
    }

    public static void main(String[] args) {

        JDBCUtilities jdbcUtilities = new JDBCUtilities();
        jdbcUtilities.dbName = "cpbg";
        jdbcUtilities.userName = "root";
        jdbcUtilities.password = "root";
        jdbcUtilities.serverName = "localhost";
        Connection connection = null;
        try {
            Class.forName("com.mysql.jdbc.Driver");
            System.out.println("Driver Loaded");
            connection = jdbcUtilities.getConnection();
            //TODO:copy SQL code here
        } catch (SQLException se) {

            jdbcUtilities.printSQLException(se);
        } catch (Exception e) {

            e.printStackTrace(System.err);
        } finally {

            jdbcUtilities.closeConnection(connection);
        }
        //TODO:remove the following dummyLevelCandidate
        String[] level1candidates = new String[]{"1", "2", "3"};
        String[][] level2candidates = {new String[]{"1.1", "1.2", "1.3"},
                new String[]{"2.1", "2.2", "2.3"}, new String[]{"3.1", "3.2", "3.3"}};
        String[][][] level3candidates = {
                {new String[]{"1.1.1", "1.1.2"},new String[]{"1.2.1", "1.2.2"}, new String[]{"1.3.1","1.3.2"}},
                {new String[]{"2.1.1", "2.1.2"},new String[]{"2.2.1", "2.2.2"}, new String[]{"2.3.1","2.3.2"}},
                {new String[]{"3.1.1", "3.1.2"},new String[]{"3.2.1", "3.2.2"}, new String[]{"3.3.1","3.3.2"}},
                };
        //creating and showing this application's GUI.
        EventQueue.invokeLater(new MaterialFinder(level1candidates, level2candidates, level3candidates));
    }
}

Answer 1

问题出在这一行:

level3values[i][j] = new DefaultListModel<String>();

因为level3values是null。因此，您应该使用正确的尺寸来正确初始化它，就像您的数据一样。因此，在用于初始化模型的嵌套 for 循环之前，您应该初始化它:

this.level3values = new DefaultListModel[level2candidates.length][level2candidates.length];

** level2candidates.length 只是一个示例，展示了如何解决问题。