Hashmap in Java | Internal Working of Hashmap in Java | Hashmap Implementation | DSA-One Course #30

Hi guys, In this video, we're going to learn how HashMap works internally.

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[amitanand]

Few other questions I have come across which everyone should prepare beforehand:
1. What is bucketing in hashmap ?
2. What is loadfactor ?
3. Why hashmap is not recommended in a multi threaded environment ?
4. Implement a hashmap using custom class as key
5. is it mandatary to override equals and hashcode method ? What will happen if not overridden ?
6. What is the difference between hashmap and concurrent hashmap also explain the working of concurrent hashmap .

[DensonGeorge]

I am not a beginner. I still find the DSA 1 series very helpful to revise the concepts and learn new things for interviews.

[tech_wizard]

Please add questions for every video of DSA course which would be enough to crack tech giant's for beginners

[nitishprasadkushwaha]

Bucketing in HashMap is the process of storing multiple values in a single location (bucket) of the internal array of the HashMap, which is indexed using a hash function. The hash function calculates the hash code of the key and uses it as an index in the array. If multiple keys have the same hash code, they are stored in the same bucket as linked nodes.

Load factor is a measure of how full a HashMap is allowed to get before its capacity is automatically increased. It is a float value that ranges from 0.0 to 1.0. When the number of entries in the HashMap exceeds the load factor multiplied by the current capacity, the capacity is increased, and the entries are rehashed.

HashMap is not recommended in a multi-threaded environment because it is not thread-safe. If multiple threads access a HashMap concurrently, it may result in an inconsistent state of the HashMap, leading to data loss or unexpected results. ConcurrentHashMap is recommended for use in a multi-threaded environment.
Here's an example implementation of a HashMap using a custom class as a key:

class Person {
private String name;
private int age;

public Person(String name, int age) {
this.name = name;
this.age = age;
}

// getters and setters

@Override
public int hashCode() {
return Objects.hash(name, age);
}

@Override
public boolean equals(Object o) {
if (this == o) return true;
if (!(o instanceof Person)) return false;
Person person = (Person) o;
return age == person.age &&
Objects.equals(name, person.name);
}
}

class CustomHashMap<K, V> {
private List<Entry<K, V>>[] buckets;
private int capacity;
private int size;

private static class Entry<K, V> {
K key;
V value;

public Entry(K key, V value) {
this.key = key;
this.value = value;
}

// getters and setters
}

public CustomHashMap(int capacity) {
this.buckets = new List[capacity];
this.capacity = capacity;
this.size = 0;
}

public void put(K key, V value) {
int index = key.hashCode() % capacity;
if (buckets[index] == null) {
buckets[index] = new LinkedList<>();
}
for (Entry<K, V> entry : buckets[index]) {
if (entry.key.equals(key)) {
entry.value = value;
return;
}
}
buckets[index].add(new Entry<>(key, value));
size++;
}

public V get(K key) {
int index = key.hashCode() % capacity;
if (buckets[index] == null) {
return null;
}
for (Entry<K, V> entry : buckets[index]) {
if (entry.key.equals(key)) {
return entry.value;
}
}
return null;
}

public int size() {
return size;
}

// other methods
}
It is highly recommended to override the equals() and hashCode() methods when using custom objects as keys in a HashMap. If these methods are not overridden, the default implementations in the Object class are used, which compares object references rather than object contents. This can lead to unexpected behavior, where two objects that are considered equal by their contents are not considered equal by the HashMap. As a result, the HashMap may not be able to retrieve the values correctly.

[mousumi]

Thank you bhaiya for this series🌸.i'm really grateful.

[GuruPrasadShukla]

best video on hashmaps in whole coder community!

Great, very well and easily explained

[anujpotdar]

I was asked this yesterday, could not answer. I started searching for solutions, most of them were too long and difficult to understand. This one taught the concept very well, keep such awesome videos, and more power to you Anuj!

[parthparmar]

thanks for the extremely good content and hardwork.

[surajdhotre]

U really gr8 bhaiya.Your video hleping me a lots....god bless u 😇

[tanmoydutta]

I was also asked the internal implementation of this in JPMC interview (technical round).

[itz_me_imraan]

The videos are too gud... Plz try to maintain consistency in DSA course🙏🙏...

[awwush]

are Abhi aapka hi videos dekhra tha ARRAYS superb!!

[unboxingsillystuffs]

Thank you Anuj..you explained very well.

[chaitanyaasati]

Yeah, It helped to understand the concept.

[CodeCult]

Excellent explanation bhai.
Bhai, on which basis an initial length of hash table get decided??
Is it same like an internal working of arraylist for initial length of hashtable or something else??

And bhai, please increase the frequency of videos on DSA-ONE. I'm eagerly waiting for upcoming videos.

[vishaldas]

Yes it was helpful btw plzz make a video on how to convert that linked list into bst in hashed map as u said is done in Java 8..🙃❣️

[ShankarKumar-kolt]

This video is very Helpful for me Thank you Anuj Bhaiya.

[sathishrajasekar]

Thank you Anuj, The Video was helpful.

[rameezkhan]

The way of explanation is excellent👌👌👌