如何在Java中实现一个散列表

import java.util.HashMap;
import java.util.Map;
public class Test {
    public static void main(String[] args) {
        Map map = new HashMap<>();
        String doc = "yue ban fei yu";
        String[] words = doc.split(" ");
        for (String s : words) {
            if (!map.containsKey(s)) {
                map.put(s, 1);
            } else {
                map.put(s, map.get(s) + 1);
            }
        }
        System.out.println(map);
    }
}

import java.util.HashMap;
import java.util.Map;
public class Main {
    public static void main(String[] args) {
        int[] cnt = new int[20000];
        String doc = "a b c d";
        String[] words = doc.split(" ");
        int a = 0;
        int b = 0;
        int c = 0;
        int d = 0;
        for (String s : words) {
           if (s == "a") a++;
           if (s == "b") b++;
           if (s == "c") c++;
           if (s == "d") d++;   
        }
    }
}

import java.util.HashMap;
import java.util.Map;
public class Main {
    public static void main(String[] args) {
        int[] cnt = new int[20000];
        String doc = "a b c d";
        String[] words = doc.split(" ");
        for (String s : words) {
            int tmp = 0;
            for (char c: s.toCharArray()) {
                tmp *= 26;
                tmp += (c - 'a');
            }
            cnt[tmp]++;
        }
        String target = "a";
        int hash = 0;
        for (char c: target.toCharArray()) {
            hash *= 26;
            hash += c - 'a';
        }
        System.out.println(cnt[hash]);
    }
}

static final int hash(Object key) {
    int h;
    return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}

public int hashCode() {
    // The hash or hashIsZero fields are subject to a benign data race,
    // making it crucial to ensure that any observable result of the
    // calculation in this method stays correct under any possible read of
    // these fields. Necessary restrictions to allow this to be correct
    // without explicit memory fences or similar concurrency primitives is
    // that we can ever only write to one of these two fields for a given
    // String instance, and that the computation is idempotent and derived
    // from immutable state
    int h = hash;
    if (h == 0 && !hashIsZero) {
        h = isLatin1() ? StringLatin1.hashCode(value)
                       : StringUTF16.hashCode(value);
        if (h == 0) {
            hashIsZero = true;
        } else {
            hash = h;
        }
    }
    return h;
}

public static int hashCode(byte[] value) {
    int h = 0;
    int length = value.length >> 1;
    for (int i = 0; i < length; i++) {
        h = 31 * h + getChar(value, i);
    }
    return h;
}

s[0]*31^(n-1) + s[1]*31^(n-2) + ... + s[n-1]