Collections and difference between the different collections, hashmap

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Explanation of collections and difference between them, with focus on hashmap.

Collections are data structures that store and manipulate groups of objects.
ArrayList, LinkedList, HashSet, and TreeMap are some examples of collections.
HashMap is a collection that stores key-value pairs and allows for fast retrieval of values based on their keys.
HashMap uses hashing to store and retrieve values, while TreeMap uses a red-black tree.
HashMap allows for null values and keys, while TreeMap does not.
HashMap is not synchronized, while Hashtable is synchronized.

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