Java JPanel getGraphics（）

Question

由于Java仅支持single inheritance ，因此我希望直接在属于Panel类成员的JPanel实例上进行paint 。 我从成员那里获取一个Graphics实例，然后在其上绘制任何所需的Graphics 。

我如何不能继承自JComponent或JPanel并仍然使用getGraphics() this进行绘画而又不重写public void paintComponent(Graphics g) ？

private class Panel
{
      private JPanel panel;
      private Grahics g;

      public Panel()
      {
           panel = new JPanel();
      }

      public void draw()
      {
           g = panel.getGraphics();
           g.setColor(Color.CYAN);
           g.draw(Some Component);
           panel.repaint();
      }
}

该面板被添加到一个JFrame ，该JFrame在调用panel.draw()之前是可见的。 这种方法不适用于我，尽管我已经知道如何通过继承自JPanel并覆盖public void paintComponent(Graphics g)来绘制自定义组件，但我不想继承自JPanel 。

Answer 1

这是一些非常简单的示例，这些示例显示了如何在paintComponent外部进行绘制。

绘制实际上发生在java.awt.image.BufferedImage ，并且只要在事件调度线程上，我们就可以在任何地方进行绘制。 （有关使用Swing进行多线程的讨论，请参见此处和此处 。）

然后，我覆盖了paintComponent ，但仅是将图像绘制到面板上。 （我还在角落画了一个小样本。）

这样，绘图实际上是永久的，并且Swing可以根据需要重新绘画面板而不会给我们造成问题。 如果需要，我们还可以执行类似将图像轻松保存到文件中的操作。

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

/**
 * Holding left-click draws, and
 * right-clicking cycles the color.
 */
class PaintAnyTime {
    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            @Override
            public void run() {
                new PaintAnyTime();
            }
        });
    }

    Color[]    colors = {Color.red, Color.blue, Color.black};
    int  currentColor = 0;
    BufferedImage img = new BufferedImage(256, 256, BufferedImage.TYPE_INT_ARGB);
    Graphics2D  imgG2 = img.createGraphics();

    JFrame frame = new JFrame("Paint Any Time");
    JPanel panel = new JPanel() {
        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            // Creating a copy of the Graphics
            // so any reconfiguration we do on
            // it doesn't interfere with what
            // Swing is doing.
            Graphics2D g2 = (Graphics2D) g.create();
            // Drawing the image.
            int w = img.getWidth();
            int h = img.getHeight();
            g2.drawImage(img, 0, 0, w, h, null);
            // Drawing a swatch.
            Color color = colors[currentColor];
            g2.setColor(color);
            g2.fillRect(0, 0, 16, 16);
            g2.setColor(Color.black);
            g2.drawRect(-1, -1, 17, 17);
            // At the end, we dispose the
            // Graphics copy we've created
            g2.dispose();
        }
        @Override
        public Dimension getPreferredSize() {
            return new Dimension(img.getWidth(), img.getHeight());
        }
    };

    MouseAdapter drawer = new MouseAdapter() {
        boolean rButtonDown;
        Point prev;

        @Override
        public void mousePressed(MouseEvent e) {
            if (SwingUtilities.isLeftMouseButton(e)) {
                prev = e.getPoint();
            }
            if (SwingUtilities.isRightMouseButton(e) && !rButtonDown) {
                // (This just behaves a little better
                // than using the mouseClicked event.)
                rButtonDown  = true;
                currentColor = (currentColor + 1) % colors.length;
                panel.repaint();
            }
        }

        @Override
        public void mouseDragged(MouseEvent e) {
            if (prev != null) {
                Point  next = e.getPoint();
                Color color = colors[currentColor];
                // We can safely paint to the
                // image any time we want to.
                imgG2.setColor(color);
                imgG2.drawLine(prev.x, prev.y, next.x, next.y);
                // We just need to repaint the
                // panel to make sure the
                // changes are visible
                // immediately.
                panel.repaint();
                prev = next;
            }
        }

        @Override
        public void mouseReleased(MouseEvent e) {
            if (SwingUtilities.isLeftMouseButton(e)) {
                prev = null;
            }
            if (SwingUtilities.isRightMouseButton(e)) {
                rButtonDown = false;
            }
        }
    };

    PaintAnyTime() {
        // RenderingHints let you specify
        // options such as antialiasing.
        imgG2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
                            RenderingHints.VALUE_ANTIALIAS_ON);
        imgG2.setStroke(new BasicStroke(3));
        //
        panel.setBackground(Color.white);
        panel.addMouseListener(drawer);
        panel.addMouseMotionListener(drawer);
        Cursor cursor =
            Cursor.getPredefinedCursor(Cursor.CROSSHAIR_CURSOR);
        panel.setCursor(cursor);
        frame.setContentPane(panel);
        frame.pack();
        frame.setResizable(false);
        frame.setLocationRelativeTo(null);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);
    }
}

也可以使用ImageIcon设置JLabel ，尽管我个人不喜欢这种方法。 我认为JLabel和ImageIcon不需要它们的规范来查看将图像传递给构造函数后对图像所做的更改。

这种方式也不允许我们做画色板之类的事情。 （对于稍微复杂的绘制程序，例如在MSPaint级别上，我们希望有一种方法来选择一个区域并在其周围绘制一个边界框。这是我们希望能够直接在其上进行绘制的另一个地方面板，除了绘制图像。）

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

/**
 * Holding left-click draws, and
 * right-clicking cycles the color.
 */
class PaintAnyTime {
    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            @Override
            public void run() {
                new PaintAnyTime();
            }
        });
    }

    Color[]    colors = {Color.red, Color.blue, Color.black};
    int  currentColor = 0;
    BufferedImage img = new BufferedImage(256, 256, BufferedImage.TYPE_INT_ARGB);
    Graphics2D  imgG2 = img.createGraphics();

    JFrame frame = new JFrame("Paint Any Time");
    JLabel label = new JLabel(new ImageIcon(img));

    MouseAdapter drawer = new MouseAdapter() {
        boolean rButtonDown;
        Point prev;

        @Override
        public void mousePressed(MouseEvent e) {
            if (SwingUtilities.isLeftMouseButton(e)) {
                prev = e.getPoint();
            }
            if (SwingUtilities.isRightMouseButton(e) && !rButtonDown) {
                // (This just behaves a little better
                // than using the mouseClicked event.)
                rButtonDown  = true;
                currentColor = (currentColor + 1) % colors.length;
                label.repaint();
            }
        }

        @Override
        public void mouseDragged(MouseEvent e) {
            if (prev != null) {
                Point  next = e.getPoint();
                Color color = colors[currentColor];
                // We can safely paint to the
                // image any time we want to.
                imgG2.setColor(color);
                imgG2.drawLine(prev.x, prev.y, next.x, next.y);
                // We just need to repaint the
                // label to make sure the
                // changes are visible
                // immediately.
                label.repaint();
                prev = next;
            }
        }

        @Override
        public void mouseReleased(MouseEvent e) {
            if (SwingUtilities.isLeftMouseButton(e)) {
                prev = null;
            }
            if (SwingUtilities.isRightMouseButton(e)) {
                rButtonDown = false;
            }
        }
    };

    PaintAnyTime() {
        // RenderingHints let you specify
        // options such as antialiasing.
        imgG2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
                            RenderingHints.VALUE_ANTIALIAS_ON);
        imgG2.setStroke(new BasicStroke(3));
        //
        label.setPreferredSize(new Dimension(img.getWidth(), img.getHeight()));
        label.setBackground(Color.white);
        label.setOpaque(true);
        label.addMouseListener(drawer);
        label.addMouseMotionListener(drawer);
        Cursor cursor =
            Cursor.getPredefinedCursor(Cursor.CROSSHAIR_CURSOR);
        label.setCursor(cursor);
        frame.add(label, BorderLayout.CENTER);
        frame.pack();
        frame.setResizable(false);
        frame.setLocationRelativeTo(null);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);
    }
}

Answer 2

class SomeComponent extends JComponent {

    private Graphics2D g2d;

    public void paintComponent(Graphics g) {
        g2d = (Graphics2D) g.create();
        g2d.setColor(Color.BLACK);
        g2d.scale(scale, scale);
        g2d.drawOval(0, 0, importance, importance);

    }

    public Graphics2D getG2d() {
        return g2d;
    }
    public void setG2d(Graphics2D g2d) {
        this.g2d = g2d;
    }
}

然后您可以执行以下操作以在面板中获取SomeComponent实例并对其进行修改

Graphics2D x= v.getPanel().get(i).getG2d;
x.setColor(Color.BLUE);
v.getPanel().get(i).setG2d(x);
v.getPanel().repaint();
v.getPanel().revalidate();

V是扩展JFrame并在其中包含面板的类，而i是SomeComponent的实例