Java 在组件中心之间绘制一条线

当用户单击一个标签，在另一个标签上拖动并释放时，我试图在两个 JLabel 的中心之间绘制一条线。无论窗口大小如何，都应该有效。

但线条不在中心，如何解决？

以下示例有效，但线条似乎被 JFrame 的边界偏移，因此它们不是中心。

我不想尝试从点计算中删除 JFrame 边框，因为真实的界面比给出的示例更复杂，并且 JFrame 中包含更多组件。

我认为点计算将与我正在使用的 JPanel 相关，因此我不会遇到 JFrame 边界问题，但情况似乎并非如此。

预先感谢您的任何帮助。

import java.awt.Color;
import java.awt.Component;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.GridLayout;
import java.awt.Point;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;

import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;

public class test extends JFrame implements MouseListener {

    private static JPanel panel = new JPanel();
    private static test window = new test();

    public test() { 
        panel.setLayout(new GridLayout(2, 2));

        JLabel l1 = new JLabel();
        JLabel l2 = new JLabel();
        JLabel l3 = new JLabel();
        JLabel l4 = new JLabel();

        l1.setOpaque(true);
        l2.setOpaque(true);
        l3.setOpaque(true);
        l4.setOpaque(true);

        l1.setBackground(Color.RED);
        l2.setBackground(Color.BLUE);
        l3.setBackground(Color.GREEN);
        l4.setBackground(Color.ORANGE);

        l1.setName("l1");
        l2.setName("l2");
        l3.setName("l3");
        l4.setName("l4");

        panel.add(l1);
        panel.add(l2);
        panel.add(l3);
        panel.add(l4);

        panel.addMouseListener(this);

        this.add(panel);    
    }

    public static void drawArcs(int x1, int y1, int x2, int y2) {
        Graphics g = window.getGraphics();
        Graphics2D g2 = (Graphics2D) g;
        g2.drawLine(x1,  y1,  x2,  y2);
    }

    private static int x1 = 0;
    private static int y1 = 0;
    public void mousePressed(MouseEvent e) {
        Component square1 = panel.getComponentAt(new Point(e.getX(), e.getY()));
        System.out.println( square1.getName() );    
        x1 = square1.getX() + square1.getWidth() / 2;
        y1 = square1.getY() + square1.getHeight() / 2;
    }

    public void mouseReleased(MouseEvent e) {
        Component square2 = panel.getComponentAt(new Point(e.getX(), e.getY()));
        System.out.println( square2.getName() );    
        int x2 = square2.getX() + square2.getWidth() / 2;
        int y2 = square2.getY() + square2.getHeight() / 2;
        drawArcs(x1, y1, x2, y2);
    }

    @Override
    public void mouseClicked(MouseEvent arg0) {}
    @Override
    public void mouseEntered(MouseEvent arg0) {}
    @Override
    public void mouseExited(MouseEvent arg0) {}

    public static void main(String[] args) {
        window.setVisible(true);
        window.setSize(400, 400);
        window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    }


}

所以，基本问题是，组件的位置是相对于panel，它被框架的装饰抵消，但您正在使用框架的现有Graphics上下文来绘制线条，因此线条未对齐。

除了不使用getGraphics，永远，您可以通过使用框架来达到预期的结果glassPane， 例如

import java.awt.Color;
import java.awt.Component;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.GridLayout;
import java.awt.Point;
import java.awt.Rectangle;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;
import java.awt.geom.Line2D;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;

public class Test extends JFrame implements MouseListener {

    private JPanel panel = new JPanel();

    public Test() {
        ConnectTheDots dots = new ConnectTheDots();
        setGlassPane(dots);
        dots.setVisible(true);
        panel.setLayout(new GridLayout(2, 2));

        panel.add(createLabel(Color.RED));
        panel.add(createLabel(Color.BLUE));
        panel.add(createLabel(Color.GREEN));
        panel.add(createLabel(Color.ORANGE));

        panel.addMouseListener(this);

        this.add(panel);
    }

    private Component pressComponent;
    private Component releaseComponent;

    public void mousePressed(MouseEvent e) {
        pressComponent = panel.getComponentAt(new Point(e.getX(), e.getY()));
    }

    public void mouseReleased(MouseEvent e) {
        releaseComponent = panel.getComponentAt(new Point(e.getX(), e.getY()));
        joinTheDots();
    }

    @Override
    public void mouseClicked(MouseEvent arg0) {
    }

    @Override
    public void mouseEntered(MouseEvent arg0) {
    }

    @Override
    public void mouseExited(MouseEvent arg0) {
    }

    protected void joinTheDots() {

        Rectangle bounds = pressComponent.getBounds();
        Point startPoint = centerOf(bounds);
        bounds = releaseComponent.getBounds();
        Point endPoint = centerOf(bounds);

        ((ConnectTheDots) getGlassPane()).drawLine(startPoint, endPoint);

    }

    protected Point centerOf(Rectangle bounds) {

        return new Point(
                        bounds.x + (bounds.width / 2),
                        bounds.y + (bounds.height / 2));

    }

    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                try {
                    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
                    ex.printStackTrace();
                }

                Test frame = new Test();
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    protected JLabel createLabel(Color background) {
        JLabel label = new JLabel() {
            @Override
            public Dimension getPreferredSize() {
                return new Dimension(100, 100);
            }
        };
        label.setOpaque(true);
        label.setBackground(background);
        return label;
    }

    public class ConnectTheDots extends JPanel {

        private Point startPoint;
        private Point endPoint;

        public ConnectTheDots() {
            setOpaque(false);
        }

        public void drawLine(Point startPoint, Point endPoint) {
            this.startPoint = startPoint;
            this.endPoint = endPoint;
            repaint();
        }

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            if (startPoint != null && endPoint != null) {
                Graphics2D g2d = (Graphics2D) g.create();
                Line2D line = new Line2D.Double(startPoint, endPoint);
                g2d.setColor(Color.BLACK);
                g2d.draw(line);
                g2d.dispose();
            }
        }

    }

}

现在，只有当内容覆盖整个可见区域时，这才有效。contentPane，虽然您可能会乱七八糟地将位置信息从一个组件上下文转换为另一个组件上下文，但更简单的解决方案是使用JXLayer.

我会避免覆盖的原因paint在这种情况下，Swing 组件是否可以在不需要绘制父组件的情况下进行更新，这可能会清除父组件上次绘制时绘制的内容...

看一眼如何使用根窗格 https://docs.oracle.com/javase/tutorial/uiswing/components/rootpane.html更多细节