Query about internal implementation of HashMap

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I am going through HashMap implementation and referring to this link: How does Java implement hash tables? I am finding that "A HashMap contains an array of buckets in order to contain its entries". So, I have few questions-

  1. What is the type of the array of buckets.
  2. Since array has drawbacks (e.g. fixed size and allowed only homogeneous data).Then why we are using arrays despite of these drawbacks.

3.In case of same hashcode for a key or collision it uses linked list.How it gets(search) the reference of the second, third node etc.

Thanks in adv.


  1. What is the type of the array of buckets.

That depends on the map you make, if you make a HashMap<Integer, String> then the buckets will be of those types, able to contain those types of objects

  1. Since array has drawbacks (e.g. fixed size and allowed only homogeneous data).Then why we are using arrays despite of these drawbacks.

Because the drawbacks are worth it compared to the performance gain. Because arrays are a fixed size, a lot of checks can be skipped (i.e. does this index exist?). You can read more about that here; https://en.wikiversity.org/wiki/Java_Collections_Overview and Why not always use ArrayLists in Java, instead of plain ol' arrays?

  1. In case of same hashcode for a key or collision it uses linked list.How it gets(search) the reference of the second, third node etc.

That is explained here better than I can; What happens when a duplicate key is put into a HashMap?

How Map/HashMap Works Internally in Java, , get(Object key) calls hashCode() on the key object and uses the returned hashValue to find a bucket location where keys and values are stored as an Entry object. Entry object stores in the bucket as (hash, key, value, bucket index). Then, the value object is returned. HashMap is the data structure used in Java to store key-value pairs, where the average retrieval time for get() and put() operations is constant i.e. O(1). Hashmap uses the array of Nodes(named as…


  1. It's an internal Object which contains the key, value and a reference to the next node in the bucket (to realize a single linked list)
  2. A fixed size of a power of 2 is needed for the array. The index of the array for a given key is based on a logical AND (&) of the hashcode of the key and the size of the array which is the actual "magic" of a hash table.
  3. The linked list in a bucket is needed to deal with hashcode collisions. This is the reason for the worst case complexity of O(n) of HashMap.get() - happens if all keys have the same hashcode and the searched key is the last one in the bucket.

If the hashmaps grows there is a very expensive rehash function, because the array has to grow to the next power of 2 aswell. In this case every bucket has to recalulate its index. In this case a new array is constructed. This means there is no dynamic data structure needed.

You can avoid rehashes if you create a new hashmap with a suitable capacity argument.

How HashMap works in Java, , you must be aware of hashCode() and equals() method. How does HashMap work internally in java is one of the most asked core java interview questions. Most of the candidates do not give the satisfactory explanation. This question shows that candidate has good knowledge of Collection. So this question should be in your TO-DO list before appearing for the interview.


From the OpenJDK8 code source :

  1. The bins are either Lists or Trees, depending on the amount of elements they hold
  2. The homogeneity of arrays isn't a problem in this context, and access speed primes over the cost of resizing the array
  3. The HashMap always iterate over all the values with a same hash, testing whether they have the correct key :
final Node<K,V> getNode(int hash, Object key) {
    Node<K,V>[] tab; Node<K,V> first, e; int n; K k;
    if ((tab = table) != null && (n = tab.length) > 0 &&
        (first = tab[(n - 1) & hash]) != null) {
        if (first.hash == hash && // always check first node
            ((k = first.key) == key || (key != null && key.equals(k))))
            return first;
        if ((e = first.next) != null) {
            if (first instanceof TreeNode)
                return ((TreeNode<K,V>)first).getTreeNode(hash, key);
            do {
                if (e.hash == hash &&
                    ((k = e.key) == key || (key != null && key.equals(k))))
                    return e;
            } while ((e = e.next) != null);
        }
    }
    return null;

}

Working of HashMap in Java, This method is provided by Object class. You can override this in your class to provide your own implementation. HashMap uses equals() to compare the key  In this article, we will discuss the Internal Working of HashMap in Java and how hashmap’s get and put method works internally. As we know that HashMap is a key and value collection in java. The HashMap stores the data in key and value format. It provides the basic implementation of the Map interface of Java.


Does HashSet and HashMap both use Hashtable ? Or Hashtable is , Let's note down the internal working of put method in hashmap. First of all, the key object is checked for null . If the key is null , the value is stored in table[0] position. Then on next step, a hash value is calculated using the key's hash code by calling its hashCode() method. Now indexFor(hash, table. initialCapacity - the initial capacity. The implementation performs internal sizing to accommodate this many elements. concurrencyLevel - the estimated number of concurrently updating threads. The implementation performs internal sizing to try to accommodate this many threads. In the ConcurrentHashMap Api , you will find the following constants.


Internal Working of HashMap in Java, HashMap Internal Implementation Analysis in Java important in order to understand the internal working functionality of HashMap in OpenJDK. data into the cluster with data streamers, executing queries, or using other  I have been trying to understand the internal implementation of java.util.HashMap and java.util.HashSet.. Following are the doubts popping in my mind for a while: Whats is the importance of the @Override public int hashcode() in a HashMap/HashSet?


How HashMap works in Java, HashMap is perhaps the most popular implementation of Map interface in Java If you want to fully understand the internal workings of HashMap, the best We would query some SQL database somewhere and then need to  Basically when a bucket becomes too big (currently: TREEIFY_THRESHOLD = 8), HashMap dynamically replaces it with an ad-hoc implementation of tree map. This way rather than having pessimistic O(n) we get much better O(log n).