I'm new and I REALLY need help

Question

Hi, my name is Chris, I'm new to java.sun.com and I'm a beginner programmer seeking a degree in Comp Sci. I've been writing some code and I've stumbled on some problems that I can't troubleshoot, (run-time?) error seems to be way over my head, the program should work, i don't know why not? Can someone please please give me a clue as to what's going on?
Here's the program;
I have to greate a Coins class that will define the different coins (dime, nickelsk, etc.) that will be used in the main .java file. The main .java file has to randomly generate some "change" and sort it out in the least amount of coins possible.
The problem:
There are no compilation errors; however, when I run the program I receive this run-time error:
Exception occurred during event dispatching:
java.lang.NullPointerException
     at cs130project3.Project3.paint(Project3.java:48)
     at sun.awt.RepaintArea.paint(RepaintArea.java:298)
     at sun.awt.windows.WComponentPeer.handleEvent(WComponentPeer.java:196)
     at java.awt.Component.dispatchEventImpl(Component.java:2663)
     at java.awt.Container.dispatchEventImpl(Container.java:1213)
     at java.awt.Component.dispatchEvent(Component.java:2497)
     at java.awt.EventQueue.dispatchEvent(EventQueue.java:339)
     at java.awt.EventDispatchThread.pumpOneEventForHierarchy(EventDispatchThread.java:131)
     at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:98)
     at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93)
     at java.awt.EventDispatchThread.run(EventDispatchThread.java:85)
What does this mean?
Some things I've tried are sticking a g.drawOval() into the paint method, it works just fine, so i assumed there must a problem passing the g object into the coins.draw(g) method, is there a file placement problem or an object passing problem? I'm using Borland JBuilder 7 and i'm assuming that it places the files in the right location, its all in a package called cs130Project3.
Here is the code:
Coins class:
import java.awt.*;
public class Coins {
private final int DIAMETER = 30;
private int coinsAmount = 0;
private String coinsType = "";
private int coinsPosX = 0;
private int coinsPosY = 0;
public Coins(int coinAmount, String coinType, int coinPosX, int coinPosY) {
coinsAmount = coinAmount;
coinsType = coinType;
coinsPosX = coinPosX;
coinsPosY = coinPosY;
public void draw(Graphics screen){
for(int i = 0; i < coinsAmount; i++)
screen.drawOval(coinsPosX,coinsPosY,DIAMETER,DIAMETER);
coinsPosX += 5;
coinsPosY += 5;
if(i == (coinsAmount - 1))
screen.drawString(coinsType,(coinsPosX+10),(coinsPosY+10));
Here is the main class:
import java.awt.*;
import java.util.Random;
import java.applet.Applet;
public class Project3 extends Applet {
private Coins Pennies, Nickels, Dimes, Quarters;
public void init() {
int randomChange = 0;
int numPenny = 0, numNickel = 0, numDime = 0, numQuarter = 0;
String Penny = "1c", Nickel = "5c", Dime = "10c", Quarter = "25c";
Random generator = new Random();
randomChange = generator.nextInt(100);
while(randomChange > 0)
while(randomChange >=1)
while(randomChange >= 5)
while(randomChange >= 10)
while(randomChange >= 25)
numQuarter++;
randomChange -= 25;
if(randomChange >= 10)
numDime++;
randomChange -= 10;
if(randomChange >= 5)
numNickel++;
randomChange -= 5;
if(randomChange >= 1)
numPenny++;
randomChange -= 1;
} // End while loop
Coins Pennies = new Coins(numPenny,Penny,10,10);
Coins Nickels = new Coins(numNickel,Nickel,100,10);
Coins Dimes = new Coins(numDime,Dime,150,10);
Coins Quarters = new Coins(numQuarter,Quarter,200, 10);
public void paint (Graphics g)
Pennies.draw(g);
Nickels.draw(g);
Dimes.draw(g);
Quarters.draw(g);
help me!!
Thank you,
Chris

Answer 1

Chris, I can't even compile your stuff because you didn't post all your classes (Coin is missing), but the runtime error is telling you a lot:
java.lang.NullPointerException
at cs130project3.Project3.paint(Project3.java:48)It says you're dereferencing an object that's null at line 48 of your Project3 class. Turn on line numbering in your text editor, go to line 48, and see what you've missed. One of your objects wasn't initialized. I'll bet you declared a reference type in a class but never initialized it properly in a constructor. The JVM will set those references to null unless you provide a value. - MOD