Java HashMap Internal Working - Lecture Notes

Introduction

• Primary Focus: Understanding how HashMap works internally in Java.
• Common Interview Question: Often asked in Java developer interviews.

HashMap Structure

• Initial Capacity: Default is 16 buckets.
• Bucket: Each bucket is implemented as a linked list.
  • Node Structure: Contains key, value, hash, and next reference.

How put Method Works

1. Create Map Object: Initializes with 16 buckets (indexed from 0 to 15).
2. Adding Elements:
   • Example: Adding employee objects into the map.
   • Hash Calculation: Uses a hash function on the key to determine the bucket.
   • Index Calculation: Uses modulus operator to find the index.
3. Collision Handling:
   • Hashing Collision: Occurs when multiple nodes share the same index.
   • Equal Check: Check if the nodes are identical using equals method.
   • Node Placement: If different, add as a next node in the linked list.
   • Replacement: If same, replace the existing node.
4. Null Keys:
   • Always placed in bucket index 0.

Java 8 Improvements

• Linked List to Balanced Tree:
  • Converts to a balanced tree after reaching a certain threshold to optimize performance.
  • More efficient handling of collisions when more than 5 nodes exist in a bucket.

Summary of Operations

• Adding a Key-Value Pair:
  • Calculate hash from the key.
  • Identify the bucket index using hash.
  • Place node in the appropriate bucket.
• Collision Handling:
  • Use equals method to check node uniqueness.
  • Replace or add node based on equality check.

Diagrammatic Understanding

• Flow Diagram:
  • Illustrates the steps from adding a key-value pair to handling collisions.
  • Provides a visual representation of node placement and linked list structure.

Conclusion

• Key Interview Takeaway: Understanding node handling, especially collision handling, is crucial.
• Advanced Topic: Java 8 optimization with balanced trees is worth additional exploration.

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End of Lecture

• The session concludes with an invitation to explore further topics in Java programming.