java - 调试二次方程绘图器 (Java)

Question

我目前正在尝试调试作业的一些代码，但遇到了困难，我意识到这不仅仅是为了给我答案，因此我只是在正确的方向上寻求一些帮助。提供给我的程序应该允许用户通过使用 slider 更改方程中的系数来绘制各种二次方程，但是当 b 和 c 值更改时，它们始终保持正值。还值得一提的是，系数在图形屏幕顶部显示正确，只是图形是错误的。

//********************************************************************
//   Equation.java       Authors: Lewis/Loftus
//
//   Solution to Programming Project 10.9 
//********************************************************************

public class Equation
{
   private int a, b, c;
   public final String SQUARED = "\u00B2";

   //------------------------------------------------------------------
   //   Sets up the default coeficients of this equation:
   //   a*x^2 + b*x + c
   //------------------------------------------------------------------
   public Equation()
   {
      a = b = c = 0;
   }

   //------------------------------------------------------------------
   //   Sets up the coeficients of this equation as specified.
   //------------------------------------------------------------------
   public Equation (int aValue, int bValue, int cValue)
   {
      a = aValue;
      b = bValue;
      c = cValue;
   }

   //------------------------------------------------------------------
   //   Computes the current value of this equation.
   //------------------------------------------------------------------
   public double computeValue(double x)
   {
      return a*x*x + b*x + c;
   }

   //------------------------------------------------------------------
   //   Returns a string representation of this equation.
   //------------------------------------------------------------------
   public String toString()
   {
      StringBuffer equation = new StringBuffer();

      if (a==0 && b==0 && c==0)
         equation.append("0");
      else
      {
         if (a != 0)
         {
            if (a==-1)
               equation.append("-");
            else
               if (a!=1)
                  equation.append(a);
            equation.append( "x" + SQUARED);
         }
         if (b != 0)
         {
            if (b < 0)
            {
               if (a==0)
                  equation.append("-");
               else
                  equation.append(" - ");
                  b = -b;
            }
            else
               equation.append(" + ");
            if (b!=1 && b!= -1)
               equation.append(b);
            equation.append("x");
         }
         if (c != 0)
         {
            if (c < 0)
            {
               if (a!=0 || b!=0)
                  equation.append(" - ");
               else
                  equation.append("-");
                  c = -c;
            }
            else
               equation.append(" + ");
            equation.append(c);
         }
      }

      // erase leading +
      if (equation.length() > 2)
      {
         char ch = equation.charAt(1);
         if (ch=='+')
         {
           equation.deleteCharAt(0);
           equation.deleteCharAt(0);
         }
      }
      return equation.toString();
   }
}


//********************************************************************
//  EquationGraphPanel.java       Authors: Lewis/Loftus
//
//  Solution to Programming Project 10.9 
//********************************************************************

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.event.*;
import javax.swing.border.LineBorder;

public class EquationGraphPanel extends JPanel
{
   private JSlider aSlider;
   private JSlider bSlider;
   private JSlider cSlider;
   private EquationViewportPanel display;
   private JLabel equationLabel;

   private final String EQUATION_SUBSTRING = "Graph of equation y = ";

   private final int MIN_VALUE = -5;
   private final int MAX_VALUE = 5;
   private final int INIT_VALUE = 0;
   private final int TICK_SPACING = 5;
   private final int MINOR_TICK_SPACING = 1;

   //-----------------------------------------------------------------
   //  Sets up the panel.
   //-----------------------------------------------------------------
   public EquationGraphPanel()
   {
      setLayout(new BorderLayout());

      add(getSliderPanel(), BorderLayout.WEST);

      display = new EquationViewportPanel();
      add(display, BorderLayout.CENTER);

      equationLabel = new JLabel(EQUATION_SUBSTRING + 0);
      equationLabel.setHorizontalAlignment(SwingConstants.CENTER);
      equationLabel.setFont(new Font("Dialog", Font.BOLD, 16));
      add(equationLabel, BorderLayout.NORTH);
   }

   //-----------------------------------------------------------------
   //  Creates the sliders and slider panel.
   //-------------------------------------------------------------
   private JPanel getSliderPanel()
   {
      SliderMouseListener changed = new SliderMouseListener();

      aSlider = new JSlider(JSlider.HORIZONTAL, MIN_VALUE, MAX_VALUE, INIT_VALUE);
      aSlider.setPaintTicks(true);
      aSlider.setPaintLabels(true);
      aSlider.setMajorTickSpacing(TICK_SPACING);
      aSlider.setMinorTickSpacing(MINOR_TICK_SPACING);
      aSlider.setSnapToTicks(true);
      aSlider.addMouseListener(changed);

      bSlider = new JSlider(JSlider.HORIZONTAL, MIN_VALUE, MAX_VALUE, INIT_VALUE);
      bSlider.setPaintTicks(true);
      bSlider.setPaintLabels(true);
      bSlider.setMajorTickSpacing(TICK_SPACING);
      bSlider.setMinorTickSpacing(MINOR_TICK_SPACING);
      bSlider.setSnapToTicks(true);
      bSlider.addMouseListener(changed);

      cSlider = new JSlider(JSlider.HORIZONTAL, MIN_VALUE, MAX_VALUE, INIT_VALUE);
      cSlider.setPaintTicks(true);
      cSlider.setPaintLabels(true);
      cSlider.setMajorTickSpacing(TICK_SPACING);
      cSlider.setMinorTickSpacing(MINOR_TICK_SPACING);
      cSlider.setSnapToTicks(true);
      cSlider.addMouseListener(changed);

      JPanel aPanel = new JPanel();
      aPanel.setLayout(new BoxLayout(aPanel, BoxLayout.Y_AXIS));
      aPanel.setBorder(new LineBorder(Color.black));
      aPanel.add(new JLabel("value of 'a'"));
      aPanel.add(aSlider);

      JPanel bPanel = new JPanel();
      bPanel.setLayout(new BoxLayout(bPanel, BoxLayout.Y_AXIS));
      bPanel.setBorder(new LineBorder(Color.black));
      bPanel.add(new JLabel("value of 'b'"));
      bPanel.add(bSlider);

      JPanel cPanel = new JPanel();
      cPanel.setLayout(new BoxLayout(cPanel, BoxLayout.Y_AXIS));
      cPanel.setBorder(new LineBorder(Color.black));
      cPanel.add(new JLabel("value of 'c'"));
      cPanel.add(cSlider);

      JPanel sliderPanel = new JPanel();
      sliderPanel.setLayout(new BoxLayout(sliderPanel, BoxLayout.Y_AXIS));
      JLabel title = new JLabel("ax\u00B2 + bx + c");
      title.setFont(new Font("Dialog", Font.BOLD, 16));
      title.setHorizontalAlignment(SwingConstants.CENTER);
      sliderPanel.add(title);
      sliderPanel.add(new JLabel("Select values for each coefficient:"));
      sliderPanel.add(Box.createVerticalGlue());
      sliderPanel.add(aPanel);
      sliderPanel.add(Box.createVerticalGlue());
      sliderPanel.add(bPanel);
      sliderPanel.add(Box.createVerticalGlue());
      sliderPanel.add(cPanel);

      return sliderPanel;
   }

   //-----------------------------------------------------------------
   //  Update and equation label.
   //-------------------------------------------------------------
   private void updateEquationLabel (Equation equation)
   {
      equationLabel.setText (EQUATION_SUBSTRING + equation.toString());
   }

   //********************************************************************
   //  Represents the mouse listener class for the sliders.
   //********************************************************************
   private class SliderMouseListener extends MouseAdapter
   {
      //-----------------------------------------------------------------
      //  Redraws the graph.
      //-----------------------------------------------------------------
      public void mouseReleased(MouseEvent event)
      {
         Equation equation = new Equation(aSlider.getValue(), bSlider.getValue(), cSlider.getValue());
         display.setEquation(equation);
         updateEquationLabel(equation);
         repaint();
      }
   }
}


//********************************************************************
//  EquationViewportPanel.java       Authors: Lewis/Loftus
//
//  Solution to Programming Project 10.9 
//********************************************************************

import java.awt.*;
import javax.swing.JPanel;
import javax.swing.border.LineBorder;

public class EquationViewportPanel extends JPanel
{
   private final int X_MIN = -10, Y_MIN = -10, X_MAX = 10, Y_MAX = 10;
   private final int REAL_WORLD_WIDTH = X_MAX - X_MIN;
   private final int REAL_WORLD_HEIGHT = Y_MAX - Y_MIN;
   private final double TICK_LENGTH = 0.2;

   private Equation equation;

   //-----------------------------------------------------------------
   //  Constructor: Sets up this panel.
   //-----------------------------------------------------------------
   public EquationViewportPanel()
   {
      equation = new Equation();
      setBorder (new LineBorder(Color.black, 4));
      setPreferredSize(new Dimension(500, 500));
   }

   //-----------------------------------------------------------------
   //  Sets the equation.
   //-----------------------------------------------------------------
   void setEquation (Equation newEquation)
   {
      equation = newEquation;
   }

   //-----------------------------------------------------------------
   //  Converts world X coordinate to screen X coordinate.
   //-----------------------------------------------------------------
   private int convertX (double x)
   {
      double offset = x - X_MIN;
      double result = offset * getSize().width / REAL_WORLD_WIDTH;
      return (int)Math.round(result);
   }

   //-----------------------------------------------------------------
   //  Converts world Y coordinate to screen Y coordinate.
   //-----------------------------------------------------------------
   private int convertY (double y)
   {
      double offset = Y_MAX - y;
      double result = offset * getSize().height / REAL_WORLD_HEIGHT;
      return (int)Math.round(result);
   }

   //-----------------------------------------------------------------
   //  Draws a line in world coordinates on the screen.
   //-----------------------------------------------------------------
   private void drawScreenLine(Graphics page, double xMin, double yMin,
      double xMax, double yMax)
   {
      page.drawLine(convertX(xMin), convertY(yMin), convertX(xMax), convertY(yMax));
   }

   //-----------------------------------------------------------------
   //  Draws a point in world coordinates on the screen.
   //-----------------------------------------------------------------
   private void drawScreenPoint(Graphics page, double x, double y)
   {
      page.drawLine(convertX(x), convertY(y), convertX(x), convertY(y));
   }

   //-----------------------------------------------------------------
   //  Draws the graph axes and equation.
   //-----------------------------------------------------------------
   public void paintComponent(Graphics page)
   {
      page.setColor(Color.white);
      page.fillRect(0,0,getSize().width, getSize().height);

      // draw the x and y axis
      page.setColor(Color.pink);
      drawScreenLine(page, X_MIN, 0, X_MAX, 0);
      drawScreenLine(page, 0, Y_MIN, 0, Y_MAX);

      // draw tick marks
      for (int x=X_MIN; x<X_MAX; x++)
         drawScreenLine(page, x, -TICK_LENGTH, x, TICK_LENGTH);
      for (int y=Y_MIN; y<Y_MAX; y++)
         drawScreenLine(page, -TICK_LENGTH, y, TICK_LENGTH, y);

      // draw the graph of the equation
      page.setColor(Color.black);
      double x = X_MIN;
      double y;
      double stepSize = (double)(X_MAX - X_MIN) / getSize().width;
      int screenX = getSize().width;
      for (int i = 0; i <= screenX; i++)
      {
         y = equation.computeValue(x);
         drawScreenPoint(page, x, y);
         x += stepSize;
      }
   }
}

这三个程序是相关的，另一个只是驱动程序类。我测试了通过在重新绘制图形后(在第二个代码块的第 124 行之后)立即打印值来从 slider 获得正确的值，并且这些值是正确的，但在方程方法中分配它们时它们似乎改变了符号(第一个代码块的第 024 行)。

我一直在试图找出为什么会发生这种情况或解决办法，但似乎无法让任何东西发挥作用，因此我们将不胜感激。

谢谢

Answer 1

在您的 toString() 方法中，您在某些条件下更改 b 和 c 的符号。这是错误的:toString() 不应更改对象的状态。

使用局部变量重写toString()方法，例如:

int local_b = b;