我目前正在尝试为一项任务调试一些代码并且已经碰壁，我意识到这不仅仅是为了给我答案，因此我只是在寻找正确方向的帮助。给我的程序应该允许用户通过使用滑块来改变方程中的系数来绘制各种二次方程，但是当b和c值改变时它们总是保持为正。还值得一提的是，系数在图形屏幕的顶部正确显示，只是图形错误。
//********************************************************************
// 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，因为另一个只是驱动程序类。我测试了通过在重新绘制图形之后立即打印值（在第二个代码块的第124行之后）从滑块获得正确的值并且值是正确的但是当它们在Equation方法中指定时它们似乎改变了符号（第一个代码块的第024行）。
我一直试图弄清楚为什么会发生这种情况或者解决这个问题，但似乎无法正常工作，所以任何和所有的帮助都会非常感激。
谢谢
答案 0 :(得分：1)
在toString()方法中，您在某些条件下更改了b和c的符号。这是错误的：toString()不应该改变对象的状态。
使用局部变量重写toString()方法，例如：
int local_b = b;