Another angle of view: imperative/procedural vs functional/declarative

quote
https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/concepts/linq/functional-programming-vs-imperative-programming
 

Transitioning for OOP Developers
In traditional object-oriented programming (OOP), most developers are accustomed to programming in the imperative/procedural style. To switch to developing in a pure functional style, they have to make a transition in their thinking and their approach to development.
To solve problems, OOP developers design class hierarchies, focus on proper encapsulation, and think in terms of class contracts. The behavior and state of object types are paramount, and language features, such as classes, interfaces, inheritance, and polymorphism, are provided to address these concerns.
In contrast, functional programming approaches computational problems as an exercise in the evaluation of pure functional transformations of data collections. Functional programming avoids state and mutable data, and instead emphasizes the application of functions.

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collections

Q 15: What is the main difference between an ArrayList and a Vector? What is the main difference between HashMap
and Hashtable? What is the difference between a stack and a queue? LF DC PI CI FAQ
A 15:
Vector / Hashtable ArrayList / HashMap
Original classes before the introduction of Collections
API. Vector & Hashtable are synchronized. Any
method that touches their contents is thread-safe.
So if you don’t need a thread safe collection, use the ArrayList or
HashMap. Why pay the price of synchronization unnecessarily at
the expense of performance degradation.
Q. So which is better? As a general rule, prefer ArrayList/HashMap to Vector/Hashtable. If your application is a
multithreaded application and at least one of the threads either adds or deletes an entry into the collection
then use new Java collections API‘s external synchronization facility as shown below to temporarily synchronize
your collections as needed: CO
Map myMap = Collections.synchronizedMap (myMap); // single lock for the entire map
List myList = Collections.synchronizedList (myList); // single lock for the entire list