将ArrayList中的元素添加和删除为hashmap中的值

Question

所以我有一个看起来像这样的文件：

    1st 2nd­ nth
    e1­, ­­v1, 1
    e1, v3, 2
    e1, v4, 4
    e1, v5, 7
    e2, v1, 1
    ., ­., .
    ., ­., .
    ., ­., .

其中我希望第一列是hashmap的键(e1,或e2,或e3),值是一个名为“ratings”的ArrayList，我希望第二列的值（int）位于ArrayList的第n个索引中。

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

public class Setup
{
    public static void Setup(String[] args)
    {
        String user;
        int value, location;
        //Create a Hashmap that holds a string key and an ArrayList value
        HashMap<String, ArrayList<Integer>> userRatings = new HashMap<String, ArrayList<Integer>>();
        try
        {
            BufferedReader bufferReader = new BufferedReader(new FileReader("Student list.txt")); //read from file
            String line, sentence; //declare two string variables
            String[] sData; //declare a string array (store the contents here)
            line = bufferReader.readLine(); //Read the line
            while (line != null) //While there is a line, do this:
            {
                line = bufferReader.readLine();
                sData  = line.split(", "); //Into the string array, enter individual values in the line split by the ", " characters
                int iData[] = new int[sData.length]; //Create an int array the size of the string array
                user = sData[0];
                for (int i = 0; i <sData.length; i++) //fill the int array with the int-version of the string array
                {
                    iData[i] = Integer.parseInt(sData[i]); //pass the strings as integers into the integer array
                }
                value = iData[1];
                location = iData[2];
                if(!userRatings.containsKey(user)) //The user does not have ratings.
                {
                    ArrayList<Integer> ratings = new ArrayList<Integer>();
                    //                     ratings = userRatings.get(user);
                    for (int j = 0; j < 50; j++)
                    {
                        ratings.add(j);
                    }
                    System.out.println(user + " " + userRatings.get(user));

                }
                else //The user has ratings
                {
                    userRatings.get(user).add(location,value);
                    System.out.println(user + " " + userRatings.get(user));
                }
            }
            bufferReader.close();
        }    catch (FileNotFoundException e)
        {
            System.out.println("File does not exist or could not be found.");
        }
        catch (IOException e)
        {
            System.out.println("Can't read from file");
        }
        catch (NullPointerException e)
        {
        }
    }
}

总而言之：文件第一列中的每个字符串在hashmap（userList）中都有自己的键，程序将检查它是否有这个键，如果没有键，它将创建一个新的arraylist作为这个键的值。arrayList将填充50个索引，其中它们将包含“0”。在此之后，arraylist将从文件中添加新的值，其中第二列中的整数将添加到第N列的相应值处。

我如何填充arraylist，以及我如何编辑它，以便如果我想在用户e6的第n个索引处添加一个新的整数，我可以？

Answer 1

当给定的键不存在于映射中时，您必须添加一个新的键值对

if(!userRatings.containsKey(user)) //The user does not have ratings.
{
   ArrayList<Integer> ratings = new ArrayList<Integer>();                   
   for (int j = 0; j < 50; j++) {
       ratings.add(j);
  }
  userRatings.put(user, ratings); // place new mapping 
  System.out.println(user + " " + userRatings.get(user));
} else //The user has ratings
{
    ArrayList<Integer> ratings  = userRatings.get(user);
    ratings.add(location,value); // Update your list with location and value
    System.out.println(user + " " + userRatings.get(user)); 
}

idata[i]=integer.parseint(Sdata[i])；鉴于您的文件内容，这将不起作用，它将抛出NumberFormatException，因为v1无法解析为int。

相反,您可以执行以下操作：

value = Integer.parseInt(sData[1].trim().substring(1));
location = Integer.parseInt(sData[2].trim());

ArrayList<Integer> first = userRatings.get(e1);
ArrayList<Integer> second = userRatings.get(e2);

//Taking the smallest size will ensure that we don't get IndexOutOfBoundsException.

int length = first.size() < second.size() ? first.size() : second.size();

for(int iDx = 0; iDx < legth; iDx++){
   //compare content
}

方法2

ArrayList<Integer> first = userRatings.get(e1);
ArrayList<Integer> second = userRatings.get(e2);

Arrays.equals(first.toArray(), second.toArray());