public static void hashMapInsight(HashMap<String, Integer> hashMap) {
    Class clsHashMap = hashMap.getClass();
    try {
        Field table = clsHashMap.getDeclaredField("table");
        table.setAccessible(true);
        Object[] buckets = (Object[]) table.get(hashMap);
        // System.out.println(Arrays.toString(bucket));
        Class Node = Class.forName("java.util.HashMap$Node");
        int maxLength = 0;
        StringBuilder longest = new StringBuilder();
        for (Object bucketNode : buckets) {
            if (bucketNode != null) {
                int i = 1;
                StringBuilder tmp = new StringBuilder();
                Field keyString = Node.getDeclaredField("key");
                keyString.setAccessible(true);
                tmp.append(keyString.get(bucketNode));
                tmp.append(" -> ");
                Field next = Node.getDeclaredField("next");
                next.setAccessible(true);
                while ((bucketNode = next.get(bucketNode)) != null) {
                    i++;
                    tmp.append(keyString.get(bucketNode));
                    tmp.append(" -> ");
                }
                if (i > maxLength) {
                    maxLength = i;
                    longest = tmp;
                }
            }
        }
        longest.delete(longest.length() - 4, longest.length());
        System.out.println("hashmap table size is: " + buckets.length);
        System.out.println("longest linkedlist is: " + longest.toString());
        System.out.println("Max length of hashmap bucket is: " + maxLength);
    } catch (NoSuchFieldException e) {
        e.printStackTrace();
    } catch (SecurityException e) {
        e.printStackTrace();
    } catch (IllegalAccessException e) {
        e.printStackTrace();
    } catch (ClassNotFoundException e) {
        e.printStackTrace();
    }
}

public static void main(String[] args) {
    int m;
 //   SeparateChainingHashST<String, Integer> map;
    HashMap<String, Integer> hashMap;
    for (int i = 4; i < 20; i++) {
        m = 1 << i;
        hashMap = new HashMap<>(m);
//        map = new SeparateChainingHashST<>(m);
        for (int j = 0; j < (int) (m * 0.75); j++) {
            String t = Integer.toString((int) (Math.random() * (m * 0.75)));
            // t = Integer.toString(j);
//            map.put(t, 0);
            hashMap.put(t, 0);
        }
//        System.out.println("-------------my hashMap-------------");
//        map.bucketStat();
        System.out.println("------------java hashMap------------");
        hashMapInsight(hashMap);
        System.out.println();
    }
}