第32行的Nullpointerexception（java）

Question

好的，所以，这次我试试这个。我使用了JMenuItem和JPopupMenu，我得到了这个NPE。

当我尝试运行它时，我收到此错误：

java.lang.NullPointerException
        at inv.init(inv.java:28)
        at sun.applet.AppletPanel.run(AppletPanel.java:424)
        at java.lang.Thread.run(Thread.java:619)

荡。

这是代码：

import java.awt.*;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;
import java.awt.event.MouseAdapter;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.*;
import javax.swing.border.*;
import javax.swing.event.*;

public class inv extends JApplet implements MouseListener
{
    public JList listbox;
    public JPopupMenu popup;
    public JMenuItem item;

    public void init()
    {
        ActionListener menuListener = new ActionListener()
        {
            public void actionPerformed(ActionEvent event)
            {
                String invAction = event.getActionCommand();
                System.out.println("Popup menu item [" + invAction + "] was pressed.");
            }
        };

        popup.add(item = new JMenuItem("Use"));
        item.setHorizontalTextPosition(JMenuItem.RIGHT);
        item.addActionListener(menuListener);
        popup.add(item = new JMenuItem("Drop"));
        item.setHorizontalTextPosition(JMenuItem.RIGHT);
        item.addActionListener(menuListener);
        popup.add(item = new JMenuItem("Cancel"));
        item.setHorizontalTextPosition(JMenuItem.RIGHT);
        item.addActionListener(menuListener);
        addMouseListener(new MousePopupListener());

        String listData[] =
        {
            "Item 1","Item 2","Item 3","Item 4"
        };

        listbox = new JList( listData );
        listbox.addMouseListener( new MouseAdapter()
        {
            public void mousePressed(MouseEvent e)
            {
                if ( SwingUtilities.isRightMouseButton(e) )
                {
                    System.out.println("Row: " + getRow(e.getPoint()));
                    listbox.setSelectedIndex(getRow(e.getPoint()));
                }
            }
        }
        );

        listbox.setSelectionMode(ListSelectionModel.SINGLE_SELECTION);
        add(listbox);
        listbox.setVisible(true);
        listbox.setFocusable(false);
    }

    class MousePopupListener extends MouseAdapter
    {
        public void mousePressed(MouseEvent e)
        {
            checkPopup(e);
        }

        public void mouseClicked(MouseEvent e)
        {
            checkPopup(e);
        }

        public void mouseReleased(MouseEvent e)
        {
            checkPopup(e);
        }

        private void checkPopup(MouseEvent e)
        {
            if (e.isPopupTrigger())
            {
                popup.show(inv.this, e.getX(), e.getY());
            }
        }
    }

    private int getRow(Point point)
    {
        return listbox.locationToIndex(point);
    }

    public void mouseEntered(MouseEvent e)
    {
    }

    public void mouseReleased(MouseEvent e)
    {
    }

    public void mousePressed(MouseEvent e)
    {
    }

    public void mouseClicked(MouseEvent e)
    {
    }

    public void mouseExited(MouseEvent e)
    {
    }
}

Answer 1

在创建对象之前，弹出窗口为空。

在向其添加项目之前创建JPopupMenu成员弹出窗口。

Answer 2

popup尚未初始化。

Answer 3

JPopupMenu popup = new JPopupMenu();