java - 组件绘制方法不绘制到 JPanel 的中间？

Question

好的，所以我有一个带有下面绘制方法的 JPanel，起初它工作得很好，但是当 JPanel 调整大小（在 JFrame 中）时，它不会将它绘制到框架的中心。

 @Override
    protected void paintComponent(Graphics g) {
        Graphics2D graphics = (Graphics2D) g;
        Dimension dimension = frame.getSize();
        Insets insets = getInsets();
        int w = (int) ((dimension.getWidth() - insets.left - insets.right) / 2);
        int h = (int) ((dimension.getHeight() - insets.top - insets.bottom) / 2);
        graphics.translate(w, h);
        graphics.drawString("Origin", 0, 0);
        double y = 0;
        for (double x = -25; x <= 25; x += .01) {
            y = -Math.pow(x, 2);
            int gx = (int) x;
            int gy = (int) y;
            System.out.println("Parabola Coordinate: " + x + ", " + y);
            g.drawRect(gx, gy, 0, 0);
        }
    }

Answer 1

改变你paintComponent的样子

@Override
protected void paintComponent(Graphics g) {
    // Create a copy of the graphics context...
    Graphics2D graphics = (Graphics2D) g.create();
    // Don't rely on the frame, rely on your own components size...
    //Dimension dimension = frame.getSize();
    Insets insets = getInsets();
    int w = (int) ((getWidth() - insets.left - insets.right) / 2);
    int h = (int) ((getHeight() - insets.top - insets.bottom) / 2);
    graphics.translate(w, h);
    graphics.drawString("Origin", 0, 0);
    double y = 0;
    for (double x = -25; x <= 25; x += .01) {
        y = -Math.pow(x, 2);
        int gx = (int) x;
        int gy = (int) y;
        System.out.println("Parabola Coordinate: " + x + ", " + y);
        // You were using the "un-translated" reference, don't know if that was deliberate
        graphics.drawRect(gx, gy, 0, 0);
    }
    // Dispose of the copy and safe resources...
    graphics.dispose();
}

确保您还使用了合适的布局管理器！

import java.awt.BorderLayout;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Insets;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;

public class BadPaint21 {

    public static void main(String[] args) {
        new BadPaint21();
    }

    public BadPaint21() {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                try {
                    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
                }

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

    public class TestPane extends JPanel {

        public TestPane() {
        }

        @Override
        public Dimension getPreferredSize() {
            return new Dimension(200, 200);
        }

        @Override
        protected void paintComponent(Graphics g) {
            Graphics2D graphics = (Graphics2D) g.create();
//            Dimension dimension = frame.getSize();
            Insets insets = getInsets();
            int w = (int) ((getWidth() - insets.left - insets.right) / 2);
            int h = (int) ((getHeight() - insets.top - insets.bottom) / 2);
            graphics.translate(w, h);
            graphics.drawString("Origin", 0, 0);
            double y = 0;
            for (double x = -25; x <= 25; x += .01) {
                y = -Math.pow(x, 2);
                int gx = (int) x;
                int gy = (int) y;
                System.out.println("Parabola Coordinate: " + x + ", " + y);
                graphics.drawRect(gx, gy, 0, 0);
            }
            graphics.dispose();
        }
    }
}

Answer 2

绘画区域的大小应该基于面板的大小而不是框架的大小：

//Dimension dimension = frame.getSize();
Dimension dimension = getSize();

此外，如果您想将一幅画居中，那么您需要知道这幅画的大小。因此，例如，高度范围为 -625 到 0。您需要考虑高度的最大尺寸 625，以便以 y 轴为中心，代码应为：

int h = (int) ((dimension.getHeight() - insets.top - insets.bottom - 625) / 2);

此外，由于计算的值都是负数，因此您需要进行额外的平移以确保坐标都是正数。所以翻译是：

graphics.translate(w, h + 625);

相同的基本逻辑也适用于水平平移。