java - 如何让数组列表仅保存在文本列表中多次出现的数字的第一次出现？

Question

我是java初学者，所以请放轻松。我正在尝试读取一个文件，并仅将文件中出现的第一次出现的重复数字保存到 ArrayList 中。

这是我到目前为止所拥有的:

import java.io.*;
import java.util.Scanner;

public class ListReader
{
  File numbers = new File("numbers.txt");
  //numbers.txt has numbers {26, 26, 44, 9, 64, 7, 19, 11, 80, 9}
  Scanner in = new Scanner(numbers);
  ArrayList<Integer> nums = new ArrayList<Integer>();
  while(in.hasNextInt())
  {
     nums.add(in.nextInt());
  } 
  int j = nums.size() - 2;
  for(int i = nums.size() - 1; i >= 0; i--)
  {
     if(nums.get(i) == nums.get(j))
     {
       nums.remove(nums.get(i)));
       j--;
     }
     else
     {
       j--;
     }
  }
  System.out.println(nums);
}

每次运行代码时，都会出现索引越界错误，我不知道如何避免。如果越界错误被纠正，是否能成功摆脱多余的26和9？

Answer 1

使用 set 是更简单的方法，但是，如果需要使用列表，请尝试以下操作:

public class ListReader {
//numbers.txt has numbers {26, 26, 44, 9, 64, 7, 19, 11, 80, 9}
ArrayList<Integer> nums = new ArrayList<>();
//I will manually put items into list, just to evade creating a file
//You keep doing your thing :)
void addItems() {
    nums.add(26);
    nums.add(26);
    nums.add(44);
    nums.add(9);
    nums.add(64);
    nums.add(7);
    nums.add(19);
    nums.add(11);
    nums.add(80);
    nums.add(9);
}
void removeExtraItems() {
    //Creating a list with extraItems that will be removed further
    ArrayList<Integer> extraItems = new ArrayList<>();
    //Whenever a copy is found from num arrayList - this list gets that copy
    ArrayList<Integer> numsCopy = new ArrayList<>();
    for (int i = 0; i < nums.size(); i++) {
        if(numsCopy.contains(nums.get(i))) {
            extraItems.add(nums.get(i));
        }
        else{numsCopy.add(nums.get(i));
        }
    }
    //for each extraItem, you check the num list backwards - if it finds it , it removes it and break;
    //if you go nums.remove(extraItems.get(i)
    //it will remove the first occurence
    //i guess that you dont want that
    for (int i = 0; i < extraItems.size(); i++) {
        for (int j = nums.size()-1; j >= 0; j--) {
            if(nums.get(j).equals(extraItems.get(i))){
                nums.remove(extraItems.get(i));
                break;
            }
        }
    }
}
//prints list
void printList() {
    for (int i = 0; i < nums.size(); i++) {
        System.out.println(nums.get(i));
    }
}
public static void main(String[] args) {
    ListReader lr = new ListReader();
    lr.addItems();
    lr.removeExtraItems();
    lr.printList();
}

}