Thursday, April 30, 2020

How HashMap Works Internally in Java

In this Java tutorial you'll learn how HashMap works internally in Java, which is also a favorite Java Collections interview question. Note that code snippets used here are from JDK 10.

There are four things you should know about HashMap before going into internal working of HashMap in Java-

  • HashMap works on the principal of hashing.
  • Map.Entry interface- This interface gives a map entry (key-value pair). HashMap in Java stores both key and value object, in bucket, as an object of Node class which implements this nested interface Map.Entry. Read more about Map.Entry interface here.
  • hashCode()- HashMap provides put(key, value) method for storing and get(key) method for retrieving values from HashMap. When put() method is used to store (Key, Value) pair, HashMap implementation calls hashcode on Key object to calculate a hash that is used to find a bucket where Entry object will be stored.
    When get() method is used to retrieve value, again key object (passed with the get() method) is used to calculate a hash which is then used to find a bucket where that particular key is stored.
  • equals()- equals() method is used to compare objects for equality. In case of HashMap key object is used for comparison, also using equals() method Map knows how to handle hashing collision (hashing collision means more than one key having the same hash value, thus assigned to the same bucket). In that case objects are stored in a linked list, refer figure for more clarity.
    Where hashCode() method helps in finding the bucket where that key is stored, equals() method helps in finding the right key as there may be more than one key-value pair stored in a single bucket.

** Bucket term used here is actually an index of array, that array is called table in HashMap implementation. Thus table[0] is referred as bucket0, table[1] as bucket1 and so on.

How elements are stored internally in Java HashMap

HassMap class in Java internally uses an array called table of type Node to store the elements which is defined in the HashMap class as-

transient Node[] table;
Node is defined as a static class with in a Hashmap.
static class Node implements Map.Entry {
  final int hash;
  final K key;
  V value;
  Node next;

As you can see for each element four things are stored in the following fields-

  • hash- For storing Hashcode calculated using the key.
  • key- For holding key of the element.
  • value- For storing value of the element.
  • next- To store reference to the next node when a bucket has more than one element and a linkedlist is formed with in a bucket to store elements.

Following image shows how Node(key-value pair) objects are stored internally in table array of the HashMap class.

HashMap internal implementation in Java

Importance of hashCode() and equals() method in HashMap

How important it is to have a proper hash code and equals method can be seen through the help of the following program, explanation of this example will also help in understanding the working of HashMap's put()method (next section).

public class HashMapTest {
  public static void main(String[] args) {
    Map <Key, String> cityMap = new HashMap<Key, String>();
    cityMap.put(new Key(1, "NY"),"New York City" );
    cityMap.put(new Key(2, "ND"), "New Delhi");
    cityMap.put(new Key(3, "NW"), "Newark");
    cityMap.put(new Key(4, "NP"), "Newport");

    System.out.println("size before iteration " + cityMap.size());
    Iterator <Key> itr = cityMap.keySet().iterator();
    while (itr.hasNext()){
    System.out.println("size after iteration " + cityMap.size());    

// This class' object is used as key
// in the HashMap
class Key{
  int index;
  String Name;
  Key(int index, String Name){
    this.index = index;
    this.Name = Name;
  // A very bad implementation of hashcode
  // done here for illustrative purpose only 
  public int hashCode(){
    return 5;
  // A very bad implementation of equals
  // done here for illustrative purpose only 
  public boolean equals(Object obj){
    return true;


size before iteration 1
size after iteration 1

How put() method of Java HashMap works internally

Lets get through the above example to see what is happening, this will also help in understanding how put() method of HashMap works internally.

Notice that I am inserting 4 values in the HashMap, still in the output it says size is 1 and iterating the map gives me the last inserted entry. Why is that? Answer lies in, how hashCode() and equals() method are implemented for the key Class. Have a look at the hashCode() method of the class Key which always returns "5" and the equals() method which is always returning "true".

When a value is put into HashMap it calculates a hash using key object and for that it uses the hashCode() method of the key object class (or its parent class). Based on the calculated hash value HashMap implementation decides which bucket should store the particular Entry object.

In my code the hashCode() method of the key class always returns "5". This effectively means, calculated hash value, is same for all the entries inserted in the HashMap. Thus all the entries are stored in the same bucket.

HashMap implementation uses equals() method to see if the key is equal to any of the already inserted keys (Recall that there may be more than one entry in the same bucket). Note that, with in a bucket key-value pair entries (Entry objects) are stored in a linked-list (Refer figure for more clarity). In case hash is same, but equals() returns false (which essentially means more than one key having the same hash or hash collision) Entry objects are stored, with in the same bucket, in a linked-list.

In my code, I am always returning true for equals() method so the HashMap implementation "thinks" that the keys are equal and overwrites the value. So, in a way using hashCode() and equals() I have "tricked" HashMap implementation to think that all the keys (even though different) are same, thus overwriting the values.

In a nutshell there are three steps in the internal implementation of HashMap put() method-

  • Using hashCode() method, hash value will be calculated. In which bucket particular entry will be stored is ascertained using that hash.
  • equals() method is used to find if such a key already exists in that bucket, if not found then a new node is created with the map entry and stored within the same bucket. A linked-list is used to store those nodes.
  • If equals() method returns true, it means that the key already exists in the bucket. In that case, the new value will overwrite the old value for the matched key.

How Java HashMap get() method works internally

As we already know how Entry objects are stored in a bucket and what happens in the case of Hash Collision it is easy to understand what happens when key object is passed in the get() method of the HashMap to retrieve a value.

Using the key (passed in the get() method) hash value will be calculated to determine the bucket where that Entry object is stored, in case there are more than one Entry object with in the same bucket (stored as a linked-list) equals() method will be used to find out the correct key. As soon as the matching key is found get() method will return the value object stored in the Entry object.

When null Key is inserted in a HashMap

HashMap in Java also allows null as key, though there can only be one null key in HashMap. While storing the Entry object HashMap implementation checks if the key is null, in case key is null, it is always mapped to bucket 0, as hash is not calculated for null keys.

HashMap implementation changes in Java 8

Though HashMap implementation in Java provides constant time performance O(1) for get() and put() methods but that is in the ideal case when the Hash function distributes the objects evenly among the buckets.

But the performance may worsen in the case hashCode() used is not proper and there are lots of hash collisions. As we know now that in case of hash collision entry objects are stored as a node in a linked-list and equals() method is used to compare keys. That comparison to find the correct key with in a linked-list is a linear operation so in a worst case scenario the complexity becomes O(n).

To address this issue in Java 8 hash elements use balanced trees instead of linked lists after a certain threshold is reached. Which means HashMap starts with storing Entry objects in linked list but after the number of items in a hash becomes larger than a certain threshold, the hash changes from using a linked list to a balanced tree, this improves the worst case performance from O(n) to O(log n).

Points to note-

  • HashMap works on the principal of hashing.
  • HashMap in Java uses the hashCode() method to calculate a hash value. Hash value is calculated using the key object. This hash value is used to find the correct bucket where Entry object will be stored.
  • HashMap uses the equals() method to find the correct key whose value is to be retrieved in case of get() and to find if that key already exists or not in case of put().
  • With in the internal implementation of HashMap hashing collision means more than one key having the same hash value, in that case Entry objects are stored as a linked-list with in a same bucket.
  • With in a bucket values are stored as Entry objects which contain both key and value.
  • In Java 8 hash elements use balanced trees instead of linked lists after a certain threshold is reached while storing values. This improves the worst case performance from O(n) to O(log n).

That's all for this topic How HashMap Works Internally in Java. If you have any doubt or any suggestions to make please drop a comment. Thanks!

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