java - 使用 hashCode 和 Arrays.equals 时潜在的哈希问题

Question

正如我的代码中的注释所解释的，任务是从给定的输入文件中查找彼此排列的字符串对的数量。例如，“ABCD”和“BCDA”是彼此的排列，意味着已找到一对。

我的程序的主要部分如下:

/**
 * Finds the number of pairs of strings that are permutations of each other.
 * 
 * A hash map is created with a hash code generated from the array formed using the getFrequency
 * method as key and a pair containing a string array and the number of times a permutation of that 
 * particular string array has been found as value.
 * 
 * If a permutation is already in the hash table previously, increment the counter.
 */
public static int findPairs(String fileName) {
    try {
        //Sets up the necessary file readers
        FileReader dataFile = new FileReader(fileName);
        BufferedReader bufferedDataFile = new BufferedReader(dataFile);

        String line = bufferedDataFile.readLine();

        //Finds the number of entries in the file
        int num = Integer.parseInt(line);

        int counter = 0;
        int accumulator = 0;

        HashMap<Integer, Pair> store = new HashMap<>();

        for (int i = 0; i < num; i++) {
            String current = bufferedDataFile.readLine();
            int[] currentArr = getFrequency(current);
            int currHashCode = Arrays.hashCode(currentArr);

            if (store.containsKey(currHashCode)) {
                Pair pairToCheck = store.get(currHashCode);
                int[] arrToCheck = pairToCheck.getArr();

                //Double checking, in case there is a collision and unequal arrays 
                //have the same hashCode
                if (Arrays.equals(currentArr, arrToCheck)) {
                    counter = pairToCheck.getCount();
                    pairToCheck.updateCount();
                } else {
                    //if the current bucket is not empty, and not a permutation of the input string,
                    //continue to conduct a linear  probe
                    while (pairToCheck != null && !Arrays.equals(currentArr, arrToCheck)) {
                        currHashCode++;
                        pairToCheck = store.get(currHashCode);
                        arrToCheck = pairToCheck.getArr();
                    }

                    //if the current bucket is empty, add the new pair into the position
                    if (pairToCheck == null) {
                        counter = 0;
                    //otherwise, a permutation has been found later in the linear probe!
                    } else {
                        counter = pairToCheck.getCount();
                        pairToCheck.updateCount();
                    }
                }
            //no such permutation in the hash table yet!    
            } else {
                counter = 0;
            }

            //Updates the accumulator using the counter. If there were already other strings
            //which are permutations of the current string, the current string will be able to
            //form a pair with each of these strings.
            accumulator += counter;

            //Updates the hash map only if the permutation has not been stored previously
            if (counter == 0) {
                Pair newPair = new Pair(currentArr, 1);
                store.put(currHashCode, newPair);
            }
        }

        //Close the file reader
        bufferedDataFile.close();

        return accumulator;
    } catch (Exception e) {
        System.out.println(e);
    }

    //In the event of an error, return -1
    return -1;
}

对 Java 的 hashCode 和 Arrays 实现进行此类操作可能会导致哪些潜在问题？这尤其是因为我已经获得了一些需要通过的私有(private)测试用例，虽然我可以通过其中的一些测试用例，但有一个我反复失败。我怀疑这与我处理碰撞的方式有关......但是尽管我已经检查了多次，但我仍然不确定错误可能出在哪里。非常感谢任何帮助!

编辑:根据请求，这是我的 getFrequency 方法:

public static int[] getFrequency(String s) {
    //There are 128 legal ascii characters
    int[] charArr = new int[128];

    //Iterate through the given string, and increment the count for a character using its 
    //ascii value to locate its position in the array
    for (int i = 0; i < s.length(); i++) {

        char c = s.charAt(i);
        int ascii = (int) c;
        charArr[ascii] += 1;    
    }

    return charArr;
}

编辑2:和配对:

public class Pair {

   private int[] m_arr;
   private int m_count;

   public Pair(int[] arr, int count) {
       this.m_arr = arr;
       this.m_count = count;
   }

   public int[] getArr() {
       return this.m_arr;
   }

   public int getCount() {
       return this.m_count;
   }

   public void updateCount() {
       this.m_count++;
   }

}

Answer 1

查找字谜词是一个已知问题。通常的解决方案是对字符串进行排序并比较排序后的字符串。排序时，“ABCD”和“BCDA”都会变成“ABCD”。

将排序后的字符串存储在一个集合中可以让您轻松找到匹配项。创建一个类，将字符串分别保存在排序版本和未排序版本中，以便于检索字符串的未排序版本。

您的哈希函数不好，因为“BB”将哈希为与“AC”相同的值。对字符串的排序版本使用更好的哈希函数。