java - NullPointerException，杀死了我的程序

Question

我正在尝试创建我的小方框，它会在从组合框中选择时显示颜色。但是当我尝试运行该程序时，我不断收到 NullPointerException 错误。我看不出有什么问题。

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class ThreeColorsFrame extends JFrame
{
    private static final int FRAME_WIDTH = 300;
    private static final int FRAME_HEIGHT = 400;

    private JComboBox box;
    private JLabel picture;

    private static String[] filename = { "Red", "Blue", "Green" };
    private Icon[] pics = { new ImageIcon(getClass().getResource(filename[0])),
                    new ImageIcon(getClass().getResource(filename[1])),
                    new ImageIcon(getClass().getResource(filename[2])) };

    public ThreeColorsFrame()
    {
        super("ThreeColorsFrame");
        setLayout(new FlowLayout());

        box = new JComboBox(filename);

        box.addItemListener(new ItemListener()
        {
            public void itemStateChanged(ItemEvent event)
            {
                if (event.getStateChange() == ItemEvent.SELECTED)
                    picture.setIcon(pics[box.getSelectedIndex()]);
            }
        });

        add(box);
        picture = new JLabel(pics[0]);
        add(picture);

    }

}

Exception in thread "main" java.lang.NullPointerException
    at javax.swing.ImageIcon.<init>(Unknown Source)
    at ThreeColorsFrame.<init>(ThreeColorsFrame.java:33)
    at ThreeColorsViewer.main(ThreeColorsViewer.java:36)

Answer 1

你的问题是你还没有初始化picture。你有

private JLabel picture;

但这之前从未设置过:

 picture.setIcon(...);

在构造函数中调用，尽管是在条件内。

你需要初始化它，例如

picture = new JLabel(...); // whatever