HashMap

Overview

• HashMap stores data as key-value pairs
• Keys are unique
• Values can be duplicated
• Not synchronized (not thread-safe)
• Allows:
  • One null key
  • Multiple null values

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Key Characteristics

Feature	Description
Structure	Array of buckets
Duplicates	Keys not allowed
Order	Not guaranteed
Thread Safety	No
Performance	O(1) average

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Constructors

Constructor	Description
HashMap()	Default capacity (16) and load factor (0.75)
HashMap(int initialCapacity)	Creates map with specified initial capacity
HashMap(int initialCapacity, float loadFactor)	Custom capacity and load factor
HashMap(Map<? extends K, ? extends V> m)	Creates map with entries from another map

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How HashMap Works Internally

put method internal

get method internal

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Time Complextity

Operation	Average	Worst Case
Put	O(1)	O(log n)
Get	O(1)	O(log n)
Remove	O(1)	O(log n)

Important Concepts

1. hashCode() + equals()

• hashCode() → bucket selection
• equals() → key comparison

1. Capacity is always rounded to power of 2

Actual threshold:

• threshold = capacity * loadFactor