JFrame不显示图片

You declare a instance field called steve in you WindowPractice class, this is fine, but in your main method, you declare ANOTHER variable called steve which you are using the reference to the loaded image...

public static void main(String[] args) {
    new WindowPractice();
    ImageIcon steve = new ImageIcon("C:/Users/shane/Dropbox/issue459.jpg");
    JLabel imageLabel = new JLabel(steve);
}

这意味着类实例变量永远不会被初始化并且保持null.

This means that the class instance variable is never initialised and remains null.

问题 #2

虽然没有直接关系，但您永远不会从您的 paint 方法中调用 super.paint.这是一个很大的不，不.您有义务维护油漆链.绘制方法很复杂，而且非常非常重要.

While not directly related, you never call super.paint from your paint method. This is a BIG NO, NO. You are obligated to maintain the paint chain. The paint methods are complex and very, very important.

问题 #3

您不应该覆盖顶级容器(例如 JFrame)，也不应该覆盖它的任何 paint 方法.这有很多原因，但在前两个中，大多数顶级容器实际上包含许多组件(JRootPane，其中包含玻璃窗格、内容窗格、图层窗格和菜单栏)这可能会影响您的绘画工作，并且通常它们不是双重缓冲的，这意味着您的绘画更新会闪烁并且看起来很糟糕；)

You should never override a top level container (such as JFrame) nor you should you override any of it's paint methods. There are lots of reasons for this, but among the top two are, most top level containers actually contain a number of components (JRootPane, which houses the glass pane, content pane, layer pane and menu bar) which can sit over your painting efforts and, generally, they're not double buffered, meaning you paint updates will flicker and look horrible ;)

您还应该避免使用 paint，相反，您应该考虑在可用的地方使用 paintComponent.

You should also avoid using paint, instead, you should look towards using paintComponent where it's available.

问题 #4

ImageIcon 不是加载图像的最佳选择.我不使用它们的主要原因是您不知道正在加载的图像何时真正可用(实际上有方法，但坦率地说，ImageIO 更简单).这是 1999 年的一个很棒的功能，当时拨号速度在 14.4k 左右，但现在……

ImageIcon is not you best choice for loading images. The main reason I don't use them is that you have no idea of when the image being loaded will actually become available (actually there are ways, but to be frank, ImageIO is just simpler). This was a great feature back in 1999 when dial-up speeds where around the 14.4k mark, but now days...

ImageIO 支持更广泛的图片格式，支持图片的读写，并保证当方法返回(成功)时，图片像素数据可供您的应用使用.

ImageIO supports a wider range of picture formats, supports reading and writing of images and guarantees that when the method returns (successfully), the image pixel data is available to your application.

示例

这是一种更好的(恕我直言)方法...

Here's a better (IMHO) approach...

public class BetterDrawing {

    public static void main(String[] args) {
        new BetterDrawing();
    }

    public BetterDrawing() {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                try {
                    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
                }

                JFrame frame = new JFrame();
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.setLayout(new BorderLayout());
                frame.add(new PaintPane());
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    public class PaintPane extends JPanel {

        private BufferedImage background;

        public PaintPane() {
            try {
                background = ImageIO.read(new File("/path/to/image"));
                // Use this instead to load embedded resources instead
                //background = ImageIO.read(getClass().getResource("/path/to/image"));
            } catch (IOException ex) {
                ex.printStackTrace();
            }
        }

        @Override
        public Dimension getPreferredSize() {
            return background == null ? super.getPreferredSize() : new Dimension(background.getWidth(), background.getHeight());
        }

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);

            if (background != null) {

                int x = (getWidth() - background.getWidth()) / 2;
                int y = (getHeight() - background.getHeight()) / 2;

                g.drawImage(background, x, y, this);

            }

        }
    }
}

阅读

• 执行自定义绘画
• 使用图片

了解更多信息.

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