HashMap Custom implementation in java
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 Custom HashMap >
 Entry<K,V>
 Putting 5 keyvalue pairs in HashMap (stepbystep)>
 Methods used in custom HashMap >
 What will happen if map already contains mapping for key?
 Complexity calculation of put and get methods in HashMap >
 put method  worst Case complexity >
 put method  best Case complexity >
 get method  worst Case complexity >
 get method  best Case complexity >
 Summary of complexity of methods in HashMap >
Custom HashMap >
This is very important and trending topic. In this post i will be explaining HashMap custom implementation in lots of detail with diagrams which will help you in visualizing the HashMap implementation.
I will be explaining how we will put and get keyvalue pair in HashMap by overriding
>equals method  helps in checking equality of entry objects.
>hashCode method  helps in finding bucket’s index on which data will be stored.
We will maintain bucket (ArrayList) which will store Entry (LinkedList).
Entry<K,V>
We store keyvalue pair by usingEntry<K,V>
Entry contains
 K key,
 V value and
 Entry<K,V>next (i.e. next entry on that location of bucket).
static class Entry<K, V> { K key; V value; Entry<K,V> next; public Entry(K key, V value, Entry<K,V> next){ this.key = key; this.value = value; this.next = next; } }
Putting 5 keyvalue pairs in custom HashMap (stepbystep)>
I will explain you the whole concept of HashMap by putting 5 keyvalue pairs in HashMap.
Initially, we have bucket of capacity=4. (all indexes of bucket i.e. 0,1,2,3 are pointing to null)
Let’s put first keyvalue pair in HashMap
Key=21, value=12
newEntry Object will be formed like this >
We will calculate hash by using our hash(K key) method  in this case it returns
key/capacity= 21%4= 1.
So, 1 will be the index of bucket on which newEntry object will be stored.
We will go to 1stindex as it is pointing to null we will put our newEntry object there.
At completion of this step, our HashMap will look like this
Let’s put second keyvalue pair in HashMap
Key=25, value=121
newEntry Object will be formed like this >
We will calculate hash by using our hash(K key) method  in this case it returns
key/capacity= 25%4= 1.
So, 1 will be the index of bucket on which newEntry object will be stored.
We will go to 1st index, it contains entry with key=21, we will compare two keys(i.e. compare 21 with 25 by using equals method), as two keys are different we check whether entry with key=21’s next is null or not, if next is null we will put our newEntry objecton next.
At completion of this step our HashMap will look like this
Let’s put third keyvalue pair in HashMap
Key=30, value=151
newEntry Object will be formed like this >
We will calculate hash by using our hash(K key) method  in this case it returns
key/capacity= 30%4= 2.
So, 2 will be the index of bucket on which newEntry object will be stored.
We will go to 2nd index as it is pointing to null we will put our newEntry object there.
At completion of this step, our HashMap will look like this
Let’s put fourth keyvalue pair in HashMap
Key=33, value=15
Entry Object will be formed like this >
We will calculate hash by using our hash(K key) method  in this case it returns
key/capacity= 33%4= 1,
So, 1 will be the index of bucket on whichnewEntry object will be stored.
We will go to 1st index 
>it contains entry with key=21, we will compare two keys (i.e. compare 21 with 33 by using equals method, as two keys are different, proceed to next of entry with key=21 (proceed only if next is not null).
>now, next contains entry with key=25, we will compare two keys (i.e. compare 25 with 33 by using equals method, as two keys are different, now next of entry with key=25 is pointing to null so we won’t proceed further, we will put our newEntry object on next.
At completion of this step our HashMap will look like this
Let’s put fifth keyvalue pair in HashMap
Key=35, value=89
Repeat above mentioned steps.
At completion of this step our HashMap will look like this
Must read: LinkedHashMap Custom implementation
Methods used in custom HashMap >
public void put(K newKey, V data)

Method allows you put keyvalue pair in HashMap
If the map already contains a mapping for the key, the old value is replaced.
provide complete functionality how to override equals method.
provide complete functionality how to override hashCode method.

public V get(K key)

Method returns value corresponding to key.

public boolean remove(K deleteKey)

Method removes keyvalue pair from HashMapCustom.

public void display()

Method displays all keyvalue pairs present in HashMapCustom.,
insertion order is not guaranteed, for maintaining insertion order refer LinkedHashMapCustom.

private int hash(K key)

Method implements hashing functionality, which helps in finding the appropriate bucket location to store our data.
This is very important method, as performance of HashMapCustom is very much dependent on this method's implementation.

What will happen if map already contains mapping for key?
If the map already contains a mapping for the key, the old value is replaced.
Complexity calculation of put and get methods in HashMap >
put method  worst Case complexity >
O(n).
But how complexity is O(n)?
Initially, let's say map is like this 
And we have to insert newEntry Object with Key=25, value=121
We will calculate hash by using our hash(K key) method  in this case it returns
key/capacity= 25%4= 1.
So, 1 will be the index of bucket on which newEntry object will be stored.
We will go to 1st index, it contains entry with key=21, we will compare two keys(i.e. compare 21 with 25 by using equals method), as two keys are different we check whether entry with key=21’s next is null or not, if next is null we will put our newEntry objecton next.
At completion of this step our HashMap will look like this
Now let’s do complexity calculation 
Earlier there was 1 element in HashMap and for putting newEntry Object we iterated on it. Hence complexity was O(n).
Note: We may calculate complexity by adding more elements in HashMap as well, but to keep explanation simple i kept less elements in HashMap.
put method  best Case complexity >
O(1).
But how complexity is O(n)?
Let's say map is like this 
And we have to insert newEntry Object with Key=30, value=151
We will calculate hash by using our hash(K key) method  in this case it returns
key/capacity= 30%4= 2.
So, 2 will be the index of bucket on which newEntry object will be stored.
We will go to 2nd index as it is pointing to null we will put our newEntry object there.
At completion of this step our HashMap will look like this
Now let’s do complexity calculation 
Earlier there 2 elements in HashMap but we were able to put newEntry Object in first go. Hence complexity was O(1).
get method  worst Case complexity >
O(n).
But how complexity is O(n)?
Initially, let's say map is like this 
And we have to get Entry Object with Key=25, value=121
We will calculate hash by using our hash(K key) method  in this case it returns
key/capacity= 25%4= 1.
So, 1 will be the index of bucket on which Entry object is stored.
We will go to 1st index, it contains entry with key=21, we will compare two keys(i.e. compare 21 with 25 by using equals method), as two keys are different we check whether entry with key=21’s next is null or not, next is not null so we will repeat same process and ultimately will be able to get Entry object.
Now let’s do complexity calculation 
There were 2 elements in HashMap and for getting Entry Object we iterated on both of them. Hence complexity was O(n).
Note: We may calculate complexity by using HashMap of larger size, but to keep explanation simple i kept less elements in HashMap.
get method  best Case complexity >
O(1).
But how complexity is O(n)?
Initially, let's say map is like this 
And we have to get Entry Object with Key=30, value=151
We will calculate hash by using our hash(K key) method  in this case it returns
key/capacity= 30%4= 2.
So, 2 will be the index of bucket on which Entry object is stored.
We will go to 2nd index and get Entry object.
Now let’s do complexity calculation 
There were 3 elements in HashMap but we were able to get Entry Object in first go.
Hence complexity was O(1).
Summary of complexity of methods in HashMap >
Operation/ method

Worst case

Best case

put(K key, V value)

O(n)

O(1)

get(Object key)

O(n)

O(1)

Data structure
Object (computer science)
Implementation
Java (programming language)
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