What is Hashmap? How it works? Difference between Hashmap and Concurrent Hashmap. Find Middle node of LinkedList. Difference between JVM, JRE and JDK,etc.

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Hashmap is a data structure that stores key-value pairs and provides constant time complexity for basic operations.

Hashmap uses hashing to store and retrieve values based on their keys.
Concurrent Hashmap is a thread-safe version of Hashmap.
Middle node of LinkedList can be found using two pointers, one moving at twice the speed of the other.
JVM is a virtual machine that executes Java bytecode.
JRE is a runtime environment that provides libraries and tools for executing Java programs.
JDK is a development kit that includes JRE and tools for developing Java programs.