程序运行但在IDE中显示错误

Question

我正在使用Eclipse Helios和JDK 1.5来运行处理JAVA图形的程序，目标是编写一个绘制函数图的抽象类。这个问题我很奇怪！

我在我的机器上运行它编译并输出所需的结果，但它仍然显示程序和整个项目中的错误（文件上的红色X），我尝试清理项目，重新编译并重新运行它再次，但它仍然在发生。我也尝试在另外两台机器上运行它，只有轴显示而不是其他曲线。我无法找到解释。关于什么是错的任何建议？

import java.awt.*;
import javax.swing.*;
public class Thirteen extends JFrame {
    public Thirteen() {
        setLayout(new GridLayout(2, 2));

        add(new DrawParabola());
        add(new DrawSineWave());
        add(new DrawCosineWave());
        add(new DrawTangent());
    }

    public static void main(String[] args) {
        Thirteen frame = new Thirteen();
        frame.setTitle("Exercise15_13");
        frame.setSize(800, 400);
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
        frame.setDefaultCloseOperation(EXIT_ON_CLOSE);
    }
}

abstract class AbstractDrawFunction extends JPanel {
    /** Polygon to hold the points */
    private Polygon p = new Polygon();

    protected AbstractDrawFunction() {
        drawFunction();
    }

    /** Return the y-coordinate */
    abstract double f(double x);

    /** Obtain points for x-coordinates 100, 101, ..., 300 */
    public void drawFunction() {
        for (int x = -100; x <= 100; x++) {
            p.addPoint(x + 200, 200 - (int)f(x));
        }
    }

    /** Implement paintComponent to draw axes, labels, and
    * connecting points
    */
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);

        // Draw x axis
        g.drawLine(30, 100, 370, 100);
        g.drawLine(370, 100, 360, 105);
        g.drawLine(370, 100, 360, 95);
        g.drawString("X", 370, 80);

        // Draw y axis
        g.drawLine(200, 30, 200, 160);
        g.drawLine(200, 30, 195, 40);
        g.drawLine(200, 30, 205, 40);
        g.drawString("Y", 220, 30);
    }
}

class DrawParabola extends AbstractDrawFunction {
    protected double f(double x) {
        double scaleFactor = 0.1;
        return scaleFactor * x * x;
    }
}

class DrawSineWave extends AbstractDrawFunction {
    protected double f(double x) {
        return 50 * Math.sin((x / 100.0) * 2 * Math.PI);
    }
}

class DrawCosineWave extends AbstractDrawFunction {
    protected double f(double x) {
        return 50 * Math.cos((x / 100.0) * 2 * Math.PI);
    }
}

class DrawTangent extends AbstractDrawFunction {
    protected double f(double x) {
        return 50 * Math.tan((x / 100.0) * 2 * Math.PI);
    }
}

class DrawCosineAnd5SineWave extends AbstractDrawFunction {
    protected double f(double x) {
        return (50 * Math.cos((x / 100.0) * 2 * Math.PI))
                + 5 * (50 * Math.sin((x / 100.0) * 2 * Math.PI));
    }
}

class Draw5CosineAndSineWave extends AbstractDrawFunction {
    protected double f(double x) {
        return 5 * (50 * Math.cos((x / 100.0) * 2 * Math.PI))
                + (50 * Math.sin((x / 100.0) * 2 * Math.PI));
    }
}

class DrawLogAndX2 extends AbstractDrawFunction {
    protected double f(double x) {
        double scaleFactor = 0.1;
        return Math.log(x) + scaleFactor * x * x;
    }
}