Java - 一个值意外重置为0

Question

我正在制作一个衍生计算器,询问用户多项式的程度,然后是每个项的系数,部分原因是因为我是一个没有经验的程序员,无法解析输入3x^4/4 + sin(x).

这是我的课.

package beta;

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

import javax.swing.JOptionPane;

public class DerivativeCalculator 
{
    public DerivativeCalculator(String d, String v)
    {
        int degree = Integer.parseInt(d);

        double 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 = 0.0001;

        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 >= 0; d--)
        {
            equation = equation + String.valueOf(coeffList.get(d)) + "x^" + String.valueOf(d) + " + ";
        }
        return equation;
    }

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

    private int degree;
    private double value;
    private List<Double> coeffList;

}

现在这是我的测试课.

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;

        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");

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

}

运行此操作会创建一个显示的小程序窗口

5.0x^0+
0.0
0.0

这不是正确的衍生物.

我调试了我的程序,一切都按预期运行,直到视图切换到我的测试类并执行 g2.drawString(String.valueOf(myDerivCalc.calc()), 10, 100);

执行此行后,degree(多项式的次数)即使用户输入5也会重置为0.这样就会弄乱我班级中的所有for循环.

为什么会这样？有关修复此问题的建议吗？谢谢

Answer 1

您在构造函数中重新定义degree并value作为局部变量.它们使用相同的名称隐藏您的类变量.

不要重新声明它们.

代替

  int degree = <something>;
  double value = <something>;

你需要

  degree = <something>;
  value = <something>;