JAVA Hashmap , Hashtable, Hashset, LinkedHashSet, LinkedHashMap, ConcurrentHashMap

1. Hashmap vs concurrent hashmap 
2. hashset vs hashmap 
3. internal working of hashmap 
4. how hashset avoid duplicate values in collection?
5. diff b/w hashmap vs hashtable 
6. does hashmap allow null value ?
7. diff b/w hashmap vs hashtable vs hashset
8. hashcode and hashing technique
9. diff b/w hashset vs linked hashset 
10. diff b/w hashmap vs linked hashmap 

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🧠 The HASHING MASTER TEMPLATE

Almost all HashMap / HashSet interview questions follow this flow:

Key → hashCode() → hash → bucket → equals()

Meaning:

1. Key diya
2. hashCode generate hua
3. bucket choose hua
4. equals check hua

This single template answers half of Java collection questions.

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1️⃣ Internal Working of HashMap

Template answer:

Key → hashCode → bucket index → store entry → collision handling

Steps:

1. key ka hashCode generate hota hai
2. hash function se bucket index milta hai
3. bucket me key-value pair store hota hai
4. collision ho to equals() check hota hai

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2️⃣ How HashSet avoids duplicates

Template:

Element → hashCode → bucket → equals check

Logic:

• HashSet internally HashMap use karta hai
• agar same hashCode + equals true → duplicate reject

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3️⃣ HashMap vs ConcurrentHashMap

Pattern:

Thread safety → Locking → Performance

HashMap	ConcurrentHashMap
Not thread safe	Thread safe
No synchronization	Segment locking
Faster single thread	Better multi thread

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4️⃣ HashSet vs HashMap

Pattern:

Storage → Data → Structure

HashSet	HashMap
Stores only values	Stores key-value
Internally uses HashMap	Base structure
Unique elements	Unique keys

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5️⃣ HashMap vs Hashtable

Pattern:

Thread safety → Null support → Performance

HashMap	Hashtable
Not synchronized	Synchronized
Allows null	No null
Faster	Slower

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6️⃣ Does HashMap allow null?

Answer pattern:

Key rule → Value rule

1 null key allowed
Multiple null values allowed

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7️⃣ HashMap vs Hashtable vs HashSet

Pattern:

Type → Storage → Thread safety

Collection	Stores	Thread safe
HashMap	key-value	No
Hashtable	key-value	Yes
HashSet	values	No

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8️⃣ hashCode and hashing technique

Template:

Object → hashCode() → hash → bucket

Meaning:

• hashCode unique integer generate karta hai
• usse bucket index decide hota hai

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9️⃣ HashSet vs LinkedHashSet

Pattern:

Ordering → Performance

HashSet	LinkedHashSet
No order	Maintains insertion order
Faster	Slightly slower

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🔟 HashMap vs LinkedHashMap

Pattern:

Ordering → Data structure

HashMap	LinkedHashMap
No order	Maintains insertion order
Hash table	Hash table + linked list

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🧠 One MASTER Pattern for All

Hashing = hashCode + equals + bucket

And comparison questions follow:

Storage
Thread safety
Ordering
Performance

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⭐ Ultra Short Memory Trick

HashMap → key value
HashSet → unique values
Hashtable → thread safe map
LinkedHashMap → ordered map
LinkedHashSet → ordered set
ConcurrentHashMap → thread safe map

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✅ If you want, I can also show you the biggest hidden pattern in Java collections interviews:

There are only 5 mental models behind almost every collection question.
Once you learn them, you can answer even unseen questions easily.

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🧠 The 4-Attribute Template (Collections)

Whenever interviewer asks anything about HashMap / HashSet / ConcurrentHashMap / LinkedHashMap / Hashtable, analyze with these 4 attributes:

1 Storage
2 Ordering
3 Thread Safety
4 Internal Structure / Working

You can answer almost any collection question using this.

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Example 1

HashMap vs ConcurrentHashMap

Using template:

Attribute	HashMap	ConcurrentHashMap
Storage	Key-value	Key-value
Ordering	No order	No order
Thread safety	Not thread safe	Thread safe
Structure	Hash table	Segmented hash table

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Example 2

HashSet vs HashMap

Attribute	HashSet	HashMap
Storage	Values only	Key-value
Ordering	No order	No order
Thread safety	Not thread safe	Not thread safe
Structure	Uses HashMap internally	Hash table

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Example 3

HashMap vs Hashtable

Attribute	HashMap	Hashtable
Storage	Key-value	Key-value
Ordering	No order	No order
Thread safety	Not synchronized	Synchronized
Structure	Hash table	Hash table

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Example 4

HashSet vs LinkedHashSet

Attribute	HashSet	LinkedHashSet
Storage	Values	Values
Ordering	No order	Insertion order
Thread safety	Not thread safe	Not thread safe
Structure	Hash table	Hash table + linked list

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Example 5

HashMap vs LinkedHashMap

Attribute	HashMap	LinkedHashMap
Storage	Key-value	Key-value
Ordering	No order	Insertion order
Thread safety	Not thread safe	Not thread safe
Structure	Hash table	Hash table + linked list

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Questions that use Internal Structure

Some questions from your list fall under internal working instead of comparison.

Example template:

Key → hashCode() → bucket → equals()

This answers:

• Internal working of HashMap
• Hashing technique
• How HashSet avoids duplicates

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🔑 Final Template (2 Parts)

Comparison questions

Storage
Ordering
Thread Safety
Structure

Working questions

Key → hashCode → bucket → equals

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⭐ One-Line Memory Trick

Collections = Storage + Order + Thread + Structure
Hashing = hashCode + bucket + equals

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If you want, I can also show you something very powerful:

There is a single mental map that explains ALL Java Collections (List, Set, Map) and makes 90% collection questions trivial.

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Good idea 👍 Bhupendra. If we apply the 4-attribute template to all your collections, we get one master table you can revise quickly before an interview.

Java Hash Collections – Master Table

Collection	Storage	Ordering	Thread Safety	Internal Structure / Working
HashMap	Key–Value pairs	No order	Not thread-safe	Hash table, bucket array, collision via linked list / tree
ConcurrentHashMap	Key–Value pairs	No order	Thread-safe (concurrent access)	Segmented / bucket-level locking
Hashtable	Key–Value pairs	No order	Thread-safe (synchronized)	Hash table
LinkedHashMap	Key–Value pairs	Maintains insertion order	Not thread-safe	Hash table + doubly linked list
HashSet	Values only (unique)	No order	Not thread-safe	Uses HashMap internally
LinkedHashSet	Values only (unique)	Maintains insertion order	Not thread-safe	Hash table + linked list

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Extra Quick Rules (Interview)

Rule	Answer
Does HashMap allow null?	1 null key, multiple null values
How HashSet avoids duplicates?	Uses hashCode() + equals()
Hashing flow	Key → hashCode() → bucket → equals()
Collision handling	Linked list / Tree (Java 8+)

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Ultra-Short Memory Trick

Map → key-value
Set → unique values
Linked → maintains order
Concurrent / Hashtable → thread-safe

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If you want, I can also show you the real internal working diagram of HashMap (bucket → node → tree) that interviewers love to ask. It will make 3–4 of your questions automatically clear.