Question

My program (included below) keeps coming up with this error that the list size is zero, and that’s messing up everything else. I’m guessing that means I didn’t add the elements in correctly, but I can’t figure out what’s wrong with it. I’m technologically challenged, so I’m sorry that this isn’t formatted right. Thanks for your help!

import java.util.Scanner;
import java.util.ArrayList;
public class ListStat {
    public static void main (String[] args) {
        ListStat r = new ListStat();
        System.out.println("A default list was created." + r.toString());
        ListStat r1 = new ListStat(30);
        System.out.println("Another list was created." + r1.toString());
        ListStat r2 = new ListStat(40);
        System.out.println("Another list was created." + r2.toString());
        r1.add(4); 
        System.out.println("The item at index 13 for r1 is: " + r1.get(13));
        System.out.println("r1 and r2 are equal: " + r1.equals(r2));
        System.out.println("r1 and r2 overlap this number of times: " + r1.countOverlap(r1, r2));
    }

private ArrayList<Integer> list;

public ListStat() {
list = new ArrayList<Integer>();
}

public ListStat(int N) {
list = new ArrayList<Integer>(N);
for (int i=0; i<N; i++) {
    list.add(i, i+1);
}
}

public void add(int num) {
list.add(num);
}

public int get(int index) {
return list.get(index);
}

public int getListLength(){
return list.size();
}

public double getMean() {
int sum = 0;
for (int i=0; i<list.size(); i++) {
    sum = sum + list.get(i);
}
double mean = sum/list.size();
return mean;
}

public int getSum() {
int sum = 0;
for (int i=0; i<list.size(); i++) {
    sum = sum + list.get(i);
}
return sum;
}

public boolean equals(ListStat r) {
if (r.equals(list))
return true;
else
return false;
}

public static int countOverlap(ListStat r1, ListStat r2) {
int count = 0;
for (int i=0; i<r1.getListLength(); i++) {
    for (int j=0; j<r2.getListLength(); j++) {
        if (r1.get(i)==r2.get(j)) {
            count++;
        }
    }
}
return count;
}

public double getStdDev() {
int sum = 0;
double standardDev = 0.0;
for (int i=0; i<list.size(); i++) {
    sum = sum + list.get(i);
}
double mean = sum/list.size();
double standsum=0;
for (int i=0; i<list.size(); i++) {
    standardDev = (list.get(i) - mean) * (list.get(i) - mean);
    standsum=standsum+standardDev;
}
standsum = standsum/list.size();
standsum = Math.sqrt(standsum);
return standsum;
}

public String toString() {
return "The list length is " + getListLength() + ", the mean is " + getMean() + ", the sum is " + getSum() + ", and the standard deviation is " + getStdDev();  
}
}

Answer 1

The problem is caused by

double mean = sum / list.size();

in getMean() method.

1. The program create an object r, which is empty, means

   r.size() = 0;

2. Then at line 9, call r.toString() method, this method invoke getMean(). Since r.size is 0, so the program throw an exception

Exception in thread “main” java.lang.ArithmeticException: / by zero