Java - JApplet 不绘制图形 - 可能与像素和缩放有关

Question

我正在创建一个导数计算器，用户在其中输入多项式的次数，然后输入每个项的系数。计算器在小程序窗口上显示生成的导数以及原始函数的图形。

这是图形类。

package beta;

import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.geom.Line2D;
import java.awt.geom.Point2D;
import java.util.List;

import javax.swing.JApplet;

public class GraphingCalc extends JApplet
{
    public void drawAxes(Graphics g)
    {
        Graphics2D g2 = (Graphics2D)g;

        Line2D.Double yaxis = new Line2D.Double(200, 400, 200, 0);
        Line2D.Double xaxis = new Line2D.Double(0, 200, 400, 200);
        g2.draw(yaxis);
        g2.draw(xaxis);

        for (int i = 0; i<=20; i++)
        {
            Line2D.Double ytick = new Line2D.Double(197, 400 - i * 20, 203, 400 - i * 20);
            Line2D.Double xtick = new Line2D.Double(400 - i * 20, 203, 400 - i * 20, 197);
            g2.draw(ytick);
            g2.draw(xtick);
        }
    }

    public void drawFunction(Graphics g, List<Double> l)
    {
        Graphics2D g2 = (Graphics2D)g;

        double x1 = 0;
        double y1 = 0;
        double x2 = 0;
        double y2 = 0;
        int size = l.size();

        for (double x = -10; x <= 10; x += 0.2)
        {
            x1 = x;
            for (int d = size-1; d>=0; d--)
            {
                y1 += l.get(d) * Math.pow(x1, d);
            }
            Point2D.Double first = new Point2D.Double(20 * x1 + 200, -20 * y1 + 200);

            x2 = x1 + 0.2;
            for (int d = size-1; d>=0; d--)
            {
                y2 += l.get(d) * Math.pow(x2, d);
            }
            Point2D.Double second = new Point2D.Double(20 * x2 + 200, -20 * y2 + 200);

            Line2D.Double line = new Line2D.Double(first, second);
            g2.draw(line);
        }
    }
}

这里是导数计算类。唯一相关的部分是当我从用户coeffList获得系数列表时

package beta;

import java.math.RoundingMode;
import java.text.DecimalFormat;
import java.util.ArrayList;
import java.util.List;

public class DerivativeCalculator 
{
    public DerivativeCalculator()
    {
        String d = JOptionPane.showInputDialog("Enter the degree of your polynomial: ");
        String v = JOptionPane.showInputDialog("Enter the x value "
                + "at which you want to take the derivative: ");

        degree = Integer.parseInt(d);
        value = Double.parseDouble(v);

        coeffList = new ArrayList<Double>();
        for (int i = 0; i <= degree; i++)
        {
            String console = JOptionPane.showInputDialog("Enter the coefficient of the "
                    + "x^" + i + " term.");
            Double coeff = Double.parseDouble(console);

            coeffList.add(coeff);
        }

    }
    public double calc()
    {
        double dx = 0.00001;

        double x1 = value;
        double y1 = 0;
        for (int d = degree; d >= 0; d--)
        {
            y1 += coeffList.get(d) * Math.pow(x1, d);
        }

        double x2 = x1 + dx;
        double y2 = 0;
        for (int d = degree; d >= 0; d--)
        {
            y2 += coeffList.get(d) * Math.pow(x2, d);
        }

        double slope = (y2 - y1)/ (x2 - x1);

        DecimalFormat round = new DecimalFormat("##.##");
        round.setRoundingMode(RoundingMode.DOWN);

        return Double.valueOf(round.format(slope));
    }

    public String getEquation()
    {
        String equation = "";
        for (int d = degree; d >= 1; d--)
        {
            equation = equation + String.valueOf(coeffList.get(d)) + "x^" + String.valueOf(d) + " + ";
        }
        equation = equation + String.valueOf(coeffList.get(0)) + "x^" + String.valueOf(0);
        return equation;
    }

    public String getValue()
    {
        return String.valueOf(value);
    }

    public List<Double> getCoeff()
    {
        return coeffList;
    }
    private int degree;
    private double value;
    private List<Double> coeffList;

}

最后，这是一个包含前两个类的测试类。

package beta;
import java.awt.Graphics;
import java.awt.Graphics2D;

import javax.swing.JApplet;
import javax.swing.JOptionPane;

public class DerivativeCalculatorTest extends JApplet
{
    public void paint(Graphics g)
    {
        Graphics2D g2 = (Graphics2D)g;
        setSize(800,800);

        DerivativeCalculator myDerivCalc = new DerivativeCalculator();
        g2.drawString(String.valueOf(myDerivCalc.calc()), 10, 100);
        g2.drawString(myDerivCalc.getEquation(), 10, 40);
        g2.drawString(myDerivCalc.getValue(), 10, 70);

        GraphingCalc myGrapher = new GraphingCalc();
        myGrapher.drawAxes(g2);
        myGrapher.drawFunction(g2, myDerivCalc.getCoeff());
    }

}

小程序运行，正确显示所有导数信息，但函数图未正确绘制。例如，当我输入 x + 5 时，小程序绘制了一堆单独的直线，但它们以抛物线的形状聚集在一起。

我立即怀疑这与我绘制图表的方式有关。我实际上做了一堆长度为 0.2 的短线。

for (double x = -10; x <= 10; x += 0.2)
            {
                x1 = x;
                for (int d = size-1; d>=0; d--)
                {
                    y1 += l.get(d) * Math.pow(x1, d);
                }
                Point2D.Double first = new Point2D.Double(20 * x1 + 200, -20 * y1 + 200);

                x2 = x1 + 0.2;
                for (int d = size-1; d>=0; d--)
                {
                    y2 += l.get(d) * Math.pow(x2, d);
                }
                Point2D.Double second = new Point2D.Double(20 * x2 + 200, -20 * y2 + 200);

                Line2D.Double line = new Line2D.Double(first, second);
                g2.draw(line);
            }

有什么问题？有什么建议吗？

Answer 1

几点：

您的GraphingCalc 类不应该extends JApplet 并且对于绘制线条，您不需要创建一些Line2D 对象然后绘制它们。您可以简单地调用drawLine 方法：

package beta;

import java.awt.Graphics;
import java.awt.Graphics2D;
import java.util.List;

public class GraphingCalc
{
    public void drawAxes(Graphics g)
    {
        Graphics2D g2 = (Graphics2D)g;

        g2.drawLine(200, 400, 200, 0);
        g2.drawLine(0, 200, 400, 200);

        for (int i = 0; i<=20; i++)
        {
            g2.drawLine(197, 400 - i * 20, 203, 400 - i * 20);
            g2.drawLine(400 - i * 20, 203, 400 - i * 20, 197);
        }
    }

    public void drawFunction(Graphics g, List<Double> l)
    {
        Graphics2D g2 = (Graphics2D)g;

        double x1 = 0;
        double y1 = 0;
        double x2 = 0;
        double y2 = 0;
        int size = l.size();

        for (double x = -10; x <= 10; x += 0.2)
        {
            x1 = x;
            for (int d = size-1; d>=0; d--)
            {
                y1 += l.get(d) * Math.pow(x1, d);
            }

            x2 = x1 + 0.2;
            for (int d = size-1; d>=0; d--)
            {
                y2 += l.get(d) * Math.pow(x2, d);
            }

            g2.drawLine((int)x1, (int)y1, (int)x2, (int)y2);
        }
    }
}

这样您的程序会同时显示轴和刻度。同样对于 1 次多项式，它会画一条线，但不知何故，我可以说它没有正确绘制多项式。也许您想更正一些计算。如您所见，具有这些系数的简单一阶多项式不应如下所示：

祝你好运。