java - ArrayList Java-大小为零

我的程序（包含在下面）不断出现此错误，列表大小为零，这弄乱了其他所有内容。我猜这意味着我没有正确添加元素，但是我无法弄清楚它出了什么问题。我在技术上面临挑战，因此对不起格式错误。谢谢你的帮助！

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();
}
}

最佳答案

问题是由

double mean = sum / list.size();

在getMean（）方法中。


程序创建一个对象r，该对象为空，表示

r.size（）= 0;
然后在第9行，调用r.toString（）方法，此方法调用getMean（）。由于r.size为0，因此程序会引发异常


线程“主”中的异常java.lang.ArithmeticException：/零