Need help with paintComponent for a graph

Posted on 2006-11-26
Last Modified: 2010-03-31
I wrote a program that could take the values of a polynomial and calculate f(x). Im trying to get these values to graph,but when JFrame is called it just gives me a blank frame. How come none of my tick marks or my toString method isn't getting 'painted' onto the panel? I made a seperate class just like i was told to do on a previous question,but yet it just makes a blank panel. Any help would be greatly appreciated.

My code is as follows


import java.util.Scanner;
import java.text.DecimalFormat;
import javax.swing.*;
import java.awt.*;

public class Poly
      public static void main (String[] args)

//Sets up variables
double c4,c3,c2,c1,c0,x,interval,xvalue;
int ptnumber,counter;
String result;
//Sets up deciamal formatting so that only 3 significant digits are displayed
DecimalFormat fmt = new DecimalFormat ("0.##");

Scanner scan = new Scanner (;
//Asks the user to input 4 coefficients,a constant,number of values to calculate,and the starting point of x
System.out.print ("Enter the coefficient of x^4(0.0 if none:");
c4 = scan.nextDouble();

System.out.print ("Enter the coefficient of x^3(0.0 if none:");
c3 = scan.nextDouble();

System.out.print ("Enter the coefficient of x^2(0.0 if none:");
c2 = scan.nextDouble();

System.out.print ("Enter the coefficient of x^1(0.0 if none:");
c1 = scan.nextDouble();

System.out.print ("Enter the constant:");
c0 = scan.nextDouble();

System.out.print ("Enter the starting value of x:");
x = scan.nextDouble();

System.out.print ("Enter the number of points to be graphed(Integer):");
ptnumber = scan.nextInt();

System.out.print ("Enter the interval between points:");
interval = scan.nextDouble();

//Sets up an polynomial object with data in it
Polynomial pn = new Polynomial(c4, c3, c2, c1, c0);
//Calls the toString method to print out the formula with proper formatting
//Prints the lables for the table
System.out.println("x      | f(x)");
//Initiates Counter
counter = 0;
//Starts a while loop that loops according to how many values are to be printed
while (counter < ptnumber)
//Calls the evaluate method to calculate f(x) from x and then prints it so many times
System.out.println(fmt.format(x)            +"      | "+fmt.format(pn.evaluate(x)));
//Calculates the x intervals so that f(x) can be calculated appropriately
x = (x + interval);
//Adds one to variable counter for each loop

JFrame frame = new JFrame("Polynomial");
frame.setDefaultCloseOperation (JFrame.EXIT_ON_CLOSE);
frame.getContentPane().add(new PolynomialPanel());

import javax.swing.*;
import java.awt.*;
import java.text.DecimalFormat;

public class PolynomialPanel extends JPanel


public PolynomialPanel()

setPreferredSize (new Dimension(600,600));


 public void paintComponent(Graphics page, int xPixels, int yPixels)
      super.paintComponent (page);
    final int NUM = 11;
    final double INTERVAL = 0.5;
    final double XSTART = 0.0;

    final int XMARGIN = 70;
    final int XUSABLEPIXELS = xPixels - 2*XMARGIN;
    final int YMARGIN =40;
    final int YUSABLEPIXELS = yPixels - 2*YMARGIN;
       // Label the graph
    page.drawString("Graph of f(x) = " + toString(), XMARGIN + 20, YMARGIN);

    // Draw the x axis and put in hash marks
    page.drawLine(XMARGIN, yPixels - YMARGIN, xPixels - XMARGIN, yPixels - YMARGIN);
    for ( int i = 0; i < NUM; i++ )
        page.drawLine(XMARGIN + i * XINTERVALPIXELS, yPixels - YMARGIN, XMARGIN + i * XINTERVALPIXELS, yPixels - YMARGIN + 5);
      DecimalFormat twoPlaces = new DecimalFormat("0.00");

    // Label the x axis
    for (int i = 0; i < NUM; i++)
        page.drawString( twoPlaces.format(XSTART + i * INTERVAL), 3 * XMARGIN / 4 + i * XINTERVALPIXELS, yPixels - YMARGIN/2);

    // Draw the y axis
    page.drawLine(XMARGIN, YMARGIN, XMARGIN, yPixels - YMARGIN);

    // Label the y axis and put in hash marks
    final double YMIN = 0.0;
    final double YINTERVAL = 100.0;

    for (int i = 0; i < NUM; i++)
        page.drawString(twoPlaces.format(YMIN + i * YINTERVAL), XMARGIN / 4, yPixels - i * YINTERVALPIXELS - YMARGIN);
        page.drawLine(XMARGIN, yPixels - YMARGIN - i * YINTERVALPIXELS, XMARGIN - 5, yPixels - YMARGIN - i * YINTERVALPIXELS);

Question by:lusius188
  • 3
  • 2
LVL 86

Accepted Solution

CEHJ earned 250 total points
ID: 18015191
The method prototype needs to be not

>>public void paintComponent(Graphics page, int xPixels, int yPixels)


public void paintComponent(Graphics page)

or it won't be called

Author Comment

ID: 18015266
I changed it like CEHJ said to ,but then my variables XPixels and YPixels had to be declaired and initialized. I initialized them both to 0 just for a test trial and it worked!!! except for the fact that my tick marks are in a 1 by 1 square up at the top all bunched up and my toString () method is just printing garbage even though when it is called from any of my other classes it reports  a properly formatted string.,
LVL 86

Expert Comment

ID: 18015275
You need to check your geometry. Please post toString
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Author Comment

ID: 18015344
public String toString()

        String result = "";

           if (coeff4 != 0)
               result += coeff4+"x^4";
          if (coeff3 != 0)
            if (coeff3 > 0)
                result += "+";
               result += coeff3+"x^3";
          if (coeff2 != 0)
            if (coeff2 > 0)
                result += "+";
                result += coeff2+"x^2";
           if (coeff1 != 0)
            if (coeff1 > 0)
                result += "+";
               result += coeff1+"x";

         result += constant;

          return result;

LVL 86

Expert Comment

ID: 18015431
Ah that rings a bell ;-) I've already posted a working example of that
LVL 92

Assisted Solution

objects earned 250 total points
ID: 18017541
you're not calling that toString() method from your paintComponent() method, your calling the PolynomialPanel 's toString() method. You panel needs to call the method on the object that conatins your coeffs

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