Thread: Array List Help - Can't find where problem is!!

Hi, I'm having the following error with the below code, and I have no idea how to fix it. Any suggestions would be greatly appreciated.

Exception in thread "main" java.lang.NullPointerException
at Experiment.runExperimentOnce(Experiment.java:29)
at Experiment.runExperimentManyTimes(Experiment.java: 37)
at PT5.main(PT5.java:24)

import java.awt.Color;
import java.util.ArrayList;
 
public class Experiment {
private Aviary av;
private int[] results = new int[11];
 
public Experiment (Aviary x) {
x = av;
}
 
public void printResults() {
for(int i=0; i<=11; i++){
System.out.println(results[i]);
}	
}
 
public int howManyRed(ArrayList<Bird> x) {
int count = 0;
for(Bird i : x) {
if (i.getMyColor()==Color.red) {
count++;
}
}
return count;
}
 
public void runExperimentOnce() {
results[howManyRed(av.see10Birds())]++;
}
 
public void runExperimentManyTimes(int w) {
int x = w;
int avg = 0;
results = new int[11];
for(int i = 0; i<=x; i++) {
runExperimentOnce();
avg = avg + results[i];
}
System.out.println(1.0 * avg + " red birds out of 10");
for(int i = 0; i<=x; i++) {
int maxindex = results[0];
if (results[i]> maxindex){
maxindex = results[i];
}
}
}
 
}

My guess is that your Aviary class contains an ArrayList<Bird> that has not been initialized.

OR

The Aviary av variable is null.


Most likely the latter. Those are the only two suspects that I would consider to throw a null pointer in this situation.

Also, please put code tags around your code so it is more readable.

public Experiment (Aviary x) {
x = av;
}

I think you have this switched around? Java takes the value on the right and assigns it to the one on the left.

Hi, I switched around the variables in my constructor, setting av = x;, instead of x = av;.

I'm not sure if that makes a difference but now I am getting an array out of bounds error, which I understand means its trying to access an array position that doesn't exist.

Here is the error.


Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 11
at Experiment.runExperimentManyTimes(Experiment.java: 38)
at PT5.main(PT5.java:24)


I don't see what the issue is, because the code should accept the length of the array of the array as a parameter and test it up to that many times.

Any additional help would be great, thanks!

edit: nvm, I didn't look closely at the code. Editing and coming up with the new solution now

edit2: here's the new solution:

You shouldn't be summing up the average while running the experiment x number of times, but rather at the end. Same thing goes for the finding which bird count came up the most often.

// this goes after you've run the experiment x number of times
for(int i = 0; i <= 10; ++i)
{
    // TODO: sum up the total number of red birds seen
    // TODO: find the max index in results
}
// TODO: using the sum, calculate the average