This is one of the most essential Java API every Java developer should learn. This API provides implementations of standard data structure in Java-like linked list, set, stack, queue, hash table, priority queue, and others.

At least you should know about all everyday objects like ArrayList, HashMap, HashSet, LinkedHashSet, TreeSet, etc. Each of them has its different property like ArrayList is a dynamic array that can grow, HashMap is a standard implementation of the hash table and can be used to store key-value pairs.

Similarly, HashSet is a set implementation that doesn’t allow duplicate elements. I strongly suggest you check Java Fundamentals: Collections course by Richard Warburton on Pluralsight to learn Java Collection Framework in depth.