Collection types can be numerous. They are created for various purposes, implementing standard interfaces (ICollection, ICompare, IList, IEnumerable, IEnumerator) which allow cycles (i.e. foreach) to work with them. However, in books, they are usually first divided into generic and non-generic.
The feature of non-generic collections (ArrayList, Hashtable, SortedList, Stack) is that they contain elements only of class Object. This makes them universal (since you can store elements of different types in a single collection), but to add a value to such a collection, it must first be converted to an object. This process is called Boxing and occurs automatically. The reverse process, Unboxing, must be specified in the code when accessing a collection element, for example, int a = (int)list[0].
If you add not values, but objects to a non-generic collection - an Upcasting operation (casting to the base type Object) occurs instead of boxing. The reverse operation (downcasting) must be specified, for example, Dog dog = (Dog)list[0].
Non-generic collections are type-unsafe, because if you try to get a collection element, for example, int a = (int)list[0], which is actually of type string, an error will occur.
Generic collections (List, LinkedList, Dictionary, HashSet) do not have these problems. Because they are created strictly for a specific type. It's easier for the programmer, faster (since no boxing or upcasting is required) and type-safe. The only caveat (as the books say) is that it makes the program code larger, as the compiler creates a separate collection for each type.
Here are some PDF books about collections:
C# Collections: A Detailed Presentation
by Rick Miller
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Java Collections
2013 by John Zukowski
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Hands-On Data Structures and Algorithms with Python: Store, manipulate and access data effectively and boost the performance of your applications
2022 by Dr. Basant Agarwal
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