Java：线程“ AWT-EventQueue-0”中的异常java.lang.NullPointerException

Question

public class MainFrame extends JFrame {
    public DefaultTableModel historyModel;
    public DefaultTableModel dataModel;
    public JTable historyTable;
    public JTable dataTable;
    public JTableHeader header;

private void createUI() {
        browsersTabbedPane = new BrowsersTabbedPane();
        add(browsersTabbedPane.getTabPane(), CENTER);
        addDefaultBrowserComponentForTabbedPane();
        createActions();
        add(createToolBar(), BorderLayout.NORTH);
        add(createStatusBar(), BorderLayout.SOUTH);
        add(createDataView(dataModel,dataTable, header), BorderLayout.PAGE_END);
        add(createHistoryView(historyModel,historyTable), BorderLayout.LINE_END);
        add(createBigButton(browsersTabbedPane.getActiveBrowser(),dataModel,historyModel,dataTable,historyTable), BorderLayout.WEST);

        createMenuBar();
    }

private JPanel createDataView(DefaultTableModel dataModel, JTable dataTable, JTableHeader header){
        JPanel panel = new JPanel();

        Object data[][] = {{"001","vinod","Bihar","India","Biology","65","First"},
                           {"002","Raju","ABC","Kanada","Geography","58","second"},
                           {"003","Aman","Delhi","India","computer","98","Dictontion"},
                           {"004","Ranjan","Bangloor","India","chemestry","90","Dictontion"},
                           {"004","Ranjan","Bangloor","India","chemestry","90","Dictontion"}};
        Object col[] = {"Roll","Name","State","country","Math","Marks","Grade"};
        dataModel = new DefaultTableModel(data,col);
        dataTable = new JTable(dataModel);

        dataTable.setPreferredScrollableViewportSize(new Dimension(1500,100));
        header = dataTable.getTableHeader();
        header.setBackground(Color.yellow);
        JScrollPane pane = new JScrollPane(dataTable);
        dataTable.setAutoResizeMode(JTable.WIDTH);
        dataTable.doLayout();
        panel.add(pane);
        return panel;

    }

    private JPanel createBigButton(Browser browser, DefaultTableModel dataModel, DefaultTableModel historyModel, JTable dataTable, JTable historyTable){
        JPanel panel = new JPanel();
        JButton bigbutton = new JButton("Big Button");
        bigbutton.addActionListener(new BigButtonListener(browser,dataModel,historyModel,dataTable, historyTable));
        panel.add(bigbutton);
        return panel;
    }

   class BigButtonListener implements ActionListener{
        Image screenshot;
        Browser bigbrowser;

        BigButtonListener(Browser browser, DefaultTableModel dataModel, DefaultTableModel historyModel, JTable dataTable, JTable historyTable) {
            screenshot = browser.toImage(true);
            bigbrowser = browser;
            historyModel = historyModel;
            dataModel = dataModel;
            dataTable = dataTable;
            historyTable = historyTable;
            //table1.addRow
        }

        @Override
        public void actionPerformed(ActionEvent e) {
            // save current image           
            //historyModel.insertRow(2,new Object[]{"Sushil"}); 
            System.out.println(historyTable.getRowCount());

        }   
    }  
}

The problem occurs when the Big button is clicked. It's supposed to output the total row count of the historyTable (which is public).

however this is the stacktrace output

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at com.my.demo.MainFrame$BigButtonListener.actionPerformed(MainFrame.java:356)
    at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
    at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source)
    at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source)
    at javax.swing.DefaultButtonModel.setPressed(Unknown Source)
    at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(Unknown Source)
    at java.awt.Component.processMouseEvent(Unknown Source)
    at javax.swing.JComponent.processMouseEvent(Unknown Source)
    at java.awt.Component.processEvent(Unknown Source)
    at java.awt.Container.processEvent(Unknown Source)
    at java.awt.Component.dispatchEventImpl(Unknown Source)
    at java.awt.Container.dispatchEventImpl(Unknown Source)
    at java.awt.Component.dispatchEvent(Unknown Source)
    at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source)
    at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source)
    at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source)
    at java.awt.Container.dispatchEventImpl(Unknown Source)
    at java.awt.Window.dispatchEventImpl(Unknown Source)
    at java.awt.Component.dispatchEvent(Unknown Source)
    at java.awt.EventQueue.dispatchEvent(Unknown Source)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.run(Unknown Source)

Answer 1

您永远不会将historyTable初始化为非null值。

Answer 2

您正在使用本地类变量historyTable进行自身设置，这又将其自身设置为null，因为原始值是null，因此不允许检索所需的数据