java - Core Java 中的绘制形状示例未出现在框架中

Question

我在阅读 Core Java 时遇到了以下代码片段:

package draw;

import java.awt.*;
import java.awt.geom.*;
import javax.swing.*;
public class DrawTest 
{
    public static void main(String[] args) 
    {
        EventQueue.invokeLater(new Runnable()
{
    public void run()
    {
        JFrame frame = new DrawFrame();
    frame.setTitle("DrawTest");
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setVisible(true);
    }
});
}
}

class DrawFrame extends JFrame
{
    public DrawFrame()
    {
    add(new DrawComponent());
    pack();
    }
}

class DrawComponent extends JComponent
{
    private static final int DEFAULT_WIDTH = 400;
    private static final int DEFAULT_HEIGHT = 400;

    public void paintCompent(Graphics g)
    {
    Graphics2D g2 = (Graphics2D) g;

    // draw a rectangle
    double leftX = 100;
    double topY = 100;
    double width = 200;
    double height = 150;

    Rectangle2D rect = new Rectangle2D.Double(leftX, topY, width, height);
    g2.draw(rect);
    // draw the enclosed ellipse
    Ellipse2D ellipse = new Ellipse2D.Double();
    ellipse.setFrame(rect);
    g2.draw(ellipse);

    // draw a diagonal line
    g2.draw(new Line2D.Double(leftX, topY, leftX + width, topY + height));

    // draw a circle with the same center
    double centerX = rect.getCenterX();
    double centerY = rect.getCenterY();
    double radius = 150;
    Ellipse2D circle = new Ellipse2D.Double();
    circle.setFrameFromCenter(centerX, centerY, centerX + radius, centerY + radius);
    g2.draw(circle);
    }

    public Dimension getPreferredSize()
    {
        return new Dimension(DEFAULT_WIDTH, DEFAULT_HEIGHT);
    }
}

我在 Eclipse 上尝试了这段代码，它确实运行了，但是框架中没有出现矩形、椭圆、对角线和圆形，而是什么都没有出现。我对照书上的代码仔细检查了一遍，没有错别字。怎么了？

Answer 1

有一个拼写错误...

public void paintCompent(Graphics g) {

应该是

public void paintComponent(Graphics g) {

这就是为什么您应该使用@Override注释，因为当您尝试覆盖父层次结构中不存在的方法时，它会给您一个编译时错误。

在执行任何自定义绘画之前，您还应该调用 super.paintComponent(g);