为什么我的 JPanel 背景图像不显示

Question

我正在尝试将背景图像添加到 Jpanel。下面是我正在使用的代码

    private void createUIComponents() {
        JPanel panel1 = new BgPanel();
    } 

class BgPanel extends JPanel {
        Image bg = new ImageIcon("image.png").getImage();
        @Override
        public void paintComponent(Graphics g) {
            g.drawImage(bg, 0, 0, getWidth(), getHeight(), this);
        }
}

我正在使用 IntelliJ，因此大部分 GUI 代码都是隐藏的。自定义创建的 JPanel 不应该是一个 BgPanel 并因此显示图像吗？我哪里做错了？

尝试编译时出现以下情况：

Exception in thread "main" java.lang.NullPointerException
    at testGUI.$$$setupUI$$$(testGUI.java)
    at testGUI.<init>(testGUI.java:4)
    at testGUI.main(testGUI.java:11)

完整代码：

import javax.swing.*;
import java.awt.*;
public class testGUI {
    private JButton button1;
    private JPanel panel1;
    public static void main(String[] args) {
        JFrame frame = new JFrame("testGUI");
        frame.setContentPane(new testGUI().panel1);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.pack();
        frame.setVisible(true);
    }
    private void createUIComponents() {
        JPanel panel1 = new BgPanel();
    }
    class BgPanel extends JPanel {
        Image bg = new ImageIcon("imagelink.png").getImage();
        @Override
        public void paintComponent(Graphics g) {
            g.drawImage(bg, 0, 0, getWidth(), getHeight(), this);
        }
    }
}

Answer 1

试试这个：

 class BgPanel extends JPanel {
    private final BufferedImage image;
             public BgPanel() throws IOException {
      image = ImageIO.read(new File("path-to-image"));
         }
    @Override
            public void paintComponent(Graphics g) {
                g.drawImage(image, 0, 0, getWidth(), getHeight(), this);
            }
        }

希望它会起作用完整代码：

import javax.swing.*;
import java.awt.*;
import java.awt.image.BufferedImage;
import java.io.File;
import java.io.IOException;
import javax.imageio.ImageIO;
public class testGUI {
    private JButton button1;
    private JPanel panel1;
    public static void main(String[] args) throws IOException {
        JFrame frame = new JFrame("testGUI");
        frame.getContentPane().add(new BgPanel());
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.pack();
        frame.setVisible(true);
    }
 static class BgPanel extends JPanel {
private final BufferedImage image;
         public BgPanel() throws IOException {
             setPreferredSize(new Dimension(400,800));
  image = ImageIO.read(new File("C:\Users\Sourav\Desktop\fb\IMG_20171209_181249.jpg"));
     }
@Override
        public void paintComponent(Graphics g) {
            g.drawImage(image, 0, 0, getWidth(), getHeight(), this);
        }
    }
}