如果我使用JTable，setLookAndFeel（）给出NullPointerException

Question

我有一个带有两个面板的JFrame 。 我正在一个扩展JFrame的类中设置UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName()) 。所以它适用于两个面板如果我没有JTable 。 但是，如果我将JTable添加到任何面板，那么转到该面板会给我NullPointerException如下所示。

那么这里有什么问题，我是否需要为JTable设置任何东西

编辑：我发现问题不是JTable，而是tableCellRenderer。 我正在添加一个tableRenderer，如下所示

public class Frame extends JFrame
{
public void initialize()
{
    try
    {
        UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
    }
    catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException e)
    {
        e.printStackTrace();
    }

    JPanel panel1 = new JPanel();
    panel1.add(new JLabel("Panel1"));
    JPanel panel2 = new JPanel();
    JTable table = new JTable(2,2);
    table.getColumnModel().getColumn(1).setCellRenderer(new CheckBoxRenderer()); // Adding this renderer creates me problem
    JScrollPane jScrollPane = new JScrollPane(table);
    jScrollPane.setSize(100, 100);
    panel2.add(jScrollPane);
    JTabbedPane tabbedPane = new JTabbedPane();
    tabbedPane.addTab("Panel1", panel1);
    tabbedPane.addTab("Panel2", panel2);
    add(tabbedPane);
    setSize(400, 400);
    setVisible(true);
}

class CheckBoxRenderer implements TableCellRenderer
{
    JCheckBox   check   = new JCheckBox();

    public Component getTableCellRendererComponent(JTable table, Object obj, boolean isSelected, boolean hasFocus, int row, int column)
    {
        if (row != 0 && obj instanceof Boolean)
        {
            check.setSelected((Boolean) obj);
            return check;
        }
        return null;
    }
}

public static void main(String[] args)
{
    Frame frame = new Frame();
    frame.initialize();
}

 }

所以这给了我NullPointerException。 如果我删除setCellRenderer行，它工作正常。 有人可以帮我吗

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at javax.swing.plaf.synth.SynthTableUI.paintCell(SynthTableUI.java:685)
at javax.swing.plaf.synth.SynthTableUI.paintCells(SynthTableUI.java:581)
at javax.swing.plaf.synth.SynthTableUI.paint(SynthTableUI.java:365)
at javax.swing.plaf.synth.SynthTableUI.update(SynthTableUI.java:276)
at javax.swing.JComponent.paintComponent(JComponent.java:779)
at javax.swing.JComponent.paint(JComponent.java:1055)
at javax.swing.JComponent.paintChildren(JComponent.java:888)
at javax.swing.JComponent.paint(JComponent.java:1064)
at javax.swing.JViewport.paint(JViewport.java:731)
at javax.swing.JComponent.paintChildren(JComponent.java:888)
at javax.swing.JComponent.paint(JComponent.java:1064)
at javax.swing.JComponent.paintChildren(JComponent.java:888)
at javax.swing.JComponent.paint(JComponent.java:1064)
at javax.swing.JComponent.paintChildren(JComponent.java:888)
at javax.swing.JComponent.paint(JComponent.java:1064)
at javax.swing.JComponent.paintChildren(JComponent.java:888)
at javax.swing.JComponent.paint(JComponent.java:1064)
at javax.swing.JComponent.paintToOffscreen(JComponent.java:5232)
at javax.swing.BufferStrategyPaintManager.paint(BufferStrategyPaintManager.java:295)
at javax.swing.RepaintManager.paint(RepaintManager.java:1249)
at javax.swing.JComponent._paintImmediately(JComponent.java:5180)
at javax.swing.JComponent.paintImmediately(JComponent.java:4991)
at javax.swing.RepaintManager$3.run(RepaintManager.java:808)
at javax.swing.RepaintManager$3.run(RepaintManager.java:796)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
at javax.swing.RepaintManager.paintDirtyRegions(RepaintManager.java:796)
at javax.swing.RepaintManager.paintDirtyRegions(RepaintManager.java:769)
at javax.swing.RepaintManager.prePaintDirtyRegions(RepaintManager.java:718)
at javax.swing.RepaintManager.access$1100(RepaintManager.java:62)
at javax.swing.RepaintManager$ProcessingRunnable.run(RepaintManager.java:1677)
at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:312)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:745)
at java.awt.EventQueue.access$300(EventQueue.java:103)
at java.awt.EventQueue$3.run(EventQueue.java:706)
at java.awt.EventQueue$3.run(EventQueue.java:704)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:715)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:242)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:161)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:150)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:146)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:138)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:91)

EDIT2：在谷歌上搜索我在javax.swing.plaf.synth.SynthContext.getPainter找到了这个NullPointerException 。

如果我做UIManager.setLookAndFeel("com.sun.java.swing.plaf.motif.MotifLookAndFeel"); 哪个工作正常。 但我使用的是Ubuntu操作系统，所以我需要GTK外观。 所以我做了UIManager.setLookAndFeel("ccom.sun.java.swing.plaf.gtk.GTKLookAndFeel"); 但这又给了我一个例外。 我无法弄清楚这里有什么问题。 我从getSystemLookAndFeelClassName()的实际实现中了解了这些包

Answer 1

您的问题遗漏了几个有用的信息，如：JVM版本，操作系统......

我在过去遇到过类似的问题，它与该特定JVM版本的BUG有关，所以您是否更新到最新的JVM版本？

看看这篇文章 。

我假设您的JTable没有自定义任何东西，并且没有事件处理程序...

Answer 2

这段代码现在适用于我

class CheckBoxRenderer implements TableCellRenderer
{
    JCheckBox   check   = new JCheckBox();

    @Override
    public Component getTableCellRendererComponent(JTable table, Object obj, boolean isSelected, boolean hasFocus, int row, int column)
    {
        if (row != 0 && obj instanceof Boolean)
        {
            check.setSelected((Boolean) obj);
        }
//      return null; this line gives NullPointerException
        return check;
    }
}

最后，我想出了问题所在。 问题是上面的行return null 。 返回null是给出NullPointerException 。 因为如果我们看到了实施

private void paintCell(SynthContext context, Graphics g, Rectangle cellRect, int row, int column)的SynthTableUI.class它有一行

Color b = component.getBackground(); 但因为我返回null ，它试图获得null的background 。 这就是实际问题所在。 感谢你的帮助