Java：使用ArrayList时索引超出范围？

Question

I'm trying to read through a data list of integers, find the most popular, least popular, and average and report the following...

MOST POPULAR NUMBERS
The following numbers were picked 263 times: 41

LEAST POPULAR NUMBERS
The following numbers were picked 198 times: 20

AVERAGE
The Average was 228.545455 times.
The following numbers were picked 228 times:  5 22
The following numbers were picked 229 times:  2  7 12 40

My code...

import java.util.*;
import java.io.*;
import java.util.Arrays;
import java.util.Collections;
public class Hmwk {

    public static void main(String[] args) throws FileNotFoundException {
        Scanner input=new Scanner (new File ("input.txt"));
        int counter = 0;
        ArrayList<Integer> numberList = new ArrayList<Integer>(45);
        while(input.hasNextInt()){
            int in = input.nextInt();
            numberList.add(in);
            counter++;
        }
        mostPopular(numberList,counter);
        leastPopular(numberList,counter);
        average(numberList,counter);


    }
public static void mostPopular(ArrayList<Integer> list, int total){
    Collections.sort(list);
    int popular = 0;
    int counter = 0;
    int counterTwo = 0;
    for (int i=0; i<total-1; i++){
        while(list.get(i) == list.get(i+1)){
            counter++;
            i++;
        }
        if(counter > counterTwo){
            counterTwo = counter;
            popular = i;
        }
    }
    System.out.printf("MOST POPULAR NUMBERS");
    System.out.printf("The following number was picked",counterTwo,"times:", popular);

}   
public static void leastPopular(ArrayList<Integer> list, int total){
    Collections.sort(list);
    int unpopular=0;
    int counter = 0;
    int counterTwo = 0;
    for (int i=0; i<total-1; i++){
        while(list.get(i) == list.get(i+1)){
            counter++;
            i++;

        if(counter < counterTwo){
            counterTwo = counter;
            unpopular = i;
        }
        }

    }
    System.out.printf("LEAST POPULAR NUMBERS");
    System.out.printf("The following number was picked",counterTwo,"times:", unpopular);
}

public static void average(ArrayList<Integer> list, int total){
    int sum = 0;
    int counter = 0;
    ArrayList<Integer> average = new ArrayList<Integer>(45);
    for (int i=0; i<total-1; i++){
        while(list.get(i) == list.get(i+1)){
            counter++;
            i++;
        }
        average.add(counter);
    }


    for (int i = 0; i <average.size(); i++){
        sum+= average.get(i);
    }
    double average2 = sum/total;
    System.out.printf("AVERAGE");
    System.out.printf("The Average was",average,"times.");
    double ceiling = Math.ceil(average2) ;
    double floor = Math.floor(average2);
    int counter2 = 0;
    Collections.sort(list);
    for (int i=0; i<total-1; i++){
        while(list.get(i) == list.get(i+1)){
            counter2++;
            i++;
        }
        if(counter2 == ceiling){
            System.out.printf("The following number was picked", ceiling,"times:",i);
        }
        if (counter2 == floor){
            System.out.printf("The following number was picked", floor,"times:",i);
    }


    }   

}

I'm getting the error...

Exception in thread "main" java.lang.IndexOutOfBoundsException: Index: 2555, Size: 2555
    at java.util.ArrayList.RangeCheck(Unknown Source)
    at java.util.ArrayList.get(Unknown Source)
    at Hmwk.mostPopular(Hmwk.java:31)
    at Hmwk.main(Hmwk.java:19)

And I can't seem to figure out why. I didn't think I needed to worry about outofboundsexceptions when using ArrayList? Oh and this is my first time using ArrayList so if my code is extremely ugly, I apologize. Any and all help is much appreciated!

Answer 1

At your last iteration you are trying to use get on an index out of the array.

int counterTwo = 0;
for (int i=0; i<total; i++){
    while(list.get(i) == list.get(i+1)){

Let's say total = 10 that means the array is from 0-9 than when we are at the very last iteration you are using at i = 9 the action .get(i+1) resulting with .get(10) == Exception!

Fix: A proper fix will to stop the array one index before it.
Change:

for (int i=0; i<total; i++){

With this:

for (int i=0; i<total-1; i++){

Answer 2

In each function, you have:

for (int i=0; i<total; i++){
while(list.get(i) == list.get(i+1)){
        counter++;
        i++;
    }

and in while statement you increase i by i++ , so when list.get(i) == list.get(i+1) , it will cause an exception.You have to check i value inside while statement:

   while(list.get(i) == list.get(i+1)){
       counter++;
       i++;
       if(i == total-1) break;
   }

And if i = max (i = total) then i+1 (i = total + 1) will cause an exception.

Answer 3

Your need to change your for loop to

for (int i=0; i<total-1; i++){

otherwise trying to access [i+1] will result in the exception.

I didn't think I needed to worry about outofboundsexceptions when using ArrayList

yes, you do when accessing a specific index. Only when you are adding stuff to the ArrayList you don't need to worry about the size, in contrast to when you use normal Arrays.