Java：使用ArrayList時索引超出范圍？

Question

我正在嘗試通讀整數數據列表，找到最流行，最不流行的平均值，並報告以下內容...

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

我的代碼...

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);
    }


    }   

}

我遇到了錯誤...

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)

而且我似乎無法弄清楚原因。 我不認為我在使用ArrayList時不必擔心outboundsexceptions嗎？ 哦，這是我第一次使用ArrayList，所以如果我的代碼非常難看，我深表歉意。 任何幫助都將不勝感激！

Answer 1

在最后一次迭代中，您嘗試對數組外的索引使用get 。

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

假設total = 10 ，這意味着數組在0-9到0-9比在i = 9的動作.get(i+1)產生的.get(10) ==異常！

修復：適當的修復將停止數組在其之前的一個索引。
更改：

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

有了這個：

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

Answer 2

在每個函數中，您都有：

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

在while語句中，您將i加i++ ，因此當list.get(i) == list.get(i+1) ，它將導致異常。您必須在while語句中檢查i值：

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

如果i = max （i =總數），則i+1 （i =總數+ 1）將導致異常。

Answer 3

您需要將for循環更改為

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

否則嘗試訪問[i + 1]將導致異常。

我不認為我在使用ArrayList時不必擔心outboundsexceptions

是的，您在訪問特定索引時會這樣做。 與使用普通Array相比，僅當您向ArrayList添加內容時，您才不必擔心大小。