使用 2D 对象数组 Java 的 NPE

Question

我在这个代码中得到了一个NPE。我试图启动每个索引，但似乎也不起作用。你能指出哪里出了问题吗？谢谢！

    package com.js.teachEnglish;
import java.awt.FlowLayout;
import java.awt.Font;
import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.io.BufferedReader;
import java.io.FileReader;
import javax.swing.ImageIcon;
import javax.swing.JButton;
import javax.swing.JLabel;
import javax.swing.JOptionPane;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.JTable;
public class Piggy_Panel extends JPanel {
    /**
     * 
     */
    private static final long serialVersionUID = 1L;
    int number_lines = getLines(Helper.piggybackResource.getPath());
    protected JLabel title_Label = new JLabel("Piggyback words");
    protected ImageIcon button_image = new ImageIcon(Helper.soundIconResource);
    protected JButton title_Button = new JButton(button_image);
    protected JPanel titlePanel = new JPanel();
    protected JScrollPane pane = new JScrollPane(this);
    protected JLabel[] label_group = new JLabel[number_lines];
    protected JButton[] button_group = new JButton[number_lines];
    protected String[] information = new String[number_lines];
    // Table information
    protected String[] col_titles = new String[] {"",""};
    protected Object[][] table_contents = new Object[number_lines][];
    protected JTable table;
    // constructor
    public Piggy_Panel() {
        setLayout(new GridLayout(number_lines+1,1));
        // scroll pane
        pane.setHorizontalScrollBarPolicy(JScrollPane.HORIZONTAL_SCROLLBAR_ALWAYS);
        pane.setVerticalScrollBarPolicy(JScrollPane.VERTICAL_SCROLLBAR_ALWAYS);
        // getting the information
        readInformation();
        /*
         * adding the title seperately
         * and also adding characteristics for the label
         *  */
        titlePanel = new JPanel();
        titlePanel.setLayout(new FlowLayout());
        // adding the characteristics for the label
        title_Label.setFont(new Font("Arial",Font.PLAIN,36));
        titlePanel.add(title_Label);
        titlePanel.add(title_Button);
        add(titlePanel);
        // calling the ini_labels_buttons() to initlialise the buttons and labels
        ini_labels_buttons();
        // fill the object[][] in
        fill_table();
        // initialising table and adding specific characteristics
        table = new JTable(table_contents, col_titles);
        table.setFillsViewportHeight(true);
        table.setShowHorizontalLines(false);
        table.setShowVerticalLines(false);
        add(table);

        // calling the ini_panel_group() to add everything
        title_Button.addActionListener(new buttonListener());

    }
    // will read the information from the file
    public String[] readInformation() {
        try {
            String line = null;
            BufferedReader br = new BufferedReader(new FileReader(Helper.piggybackResource.getPath()));
            int i = 0;
            while((line = br.readLine()) != null) {
                information[i] = line;
                i++;
            }
            return information;
        }catch(Exception ex) {
            ex.printStackTrace();
        }
        return null;
    }
    // will put out the sound
    // This will output the sound if a button is clicked
    class buttonListener implements ActionListener {
        public void actionPerformed(ActionEvent e) {
            JButton button = (JButton) e.getSource();
            String sayDynamicPath = Helper.sayDynamicResource.getPath() + " ";
            if(button.equals(title_Button)) {
                try {
                    Runtime.getRuntime().exec(sayDynamicPath +
                            title_Label.getText());
                }catch(Exception ex) {
                    JOptionPane.showMessageDialog(null,""+ex,"Unable to produce sound",
                            JOptionPane.ERROR_MESSAGE);
                    ex.printStackTrace();
                }
            }
        }
    }
    // Initialising the JLabels and JButtons and also adding the relevant functions
    // these include icons and information
    public void ini_labels_buttons() {
        // initialising the buttons first
        for (int i = 0; i < button_group.length; i++) {
            button_group[i] = new JButton(button_image);
        }
        // initialising the labels
        for (int i = 0; i < label_group.length; i++) {
            label_group[i] = new JLabel(information[i]);
        }
    }
    public void fill_table() {
        // fill in the first col
        for (int i = 0; i < table_contents.length; i++) {
            table_contents[i][0] = information[i]; // null pointer here
        }
        // fill in the second the col
        for(int j = 0;j < table_contents.length;j++) {
            table_contents[j][1] = button_group[j];
        }
    }
    // this method will get the number of lines and then send it back
    // to the label group
    public int getLines(String path) {
        int n = 0;
        try {
            BufferedReader br = new BufferedReader(new FileReader(path));
            @SuppressWarnings("unused")
            String line = "";
            while((line = br.readLine()) != null) {
                n++;
            }
        }catch(Exception e) {
            e.printStackTrace();
        }
        return n;
    }
}

Answer 1

在

NPE的堆栈跟踪中隐约可见会很有用。 ;)

这将创建对数组的引用数组，而不是数组的数组。

protected Object[][] table_contents = new Object[number_lines][];

这意味着

table_contents[i][0] = information[i]; 

应该抛出一个 NPE，因为 table_contents[i] 将被null .查看调试器将确认这一点。

最简单的解决方案是创建一个或多个数组

protected Object[][] table_contents = new Object[number_lines][2];

顺便说一句，我会使用一个循环而不是两个。