Functional programming teaches some very important development sessions. In top:

• lazy evaluation - deffer computation till the moment is not needed anymore (see iterators, filtering collections, lazy val, functions to compute things, Provider/Supplier/Memoization like objects)
• immutable data
  • vavr pattern for changing collections (by using AtomicReference)
    • https://stackoverflow.com/questions/56223825/vavr-set-field-should-be-volatile-atomic-or-declared-other-way
    • https://stackoverflow.com/questions/44147719/lock-free-atomic-update-to-immutable-map
  • no need to make protection copies to pass them around
    • iterators on old data that work
• control structures replacements
  • try...catch replaced with Try data structure
  • loops for/while/repeat/until with Monads
  • synchronous calls replaced with Future and async/await semantics
• removal of non objectual primitives
  • Option, Rich pattern

Huge Coding Improvements (the Big Lessons)

• Basic -> C/C++/Pascal (code & parameters reuse, procedural control structures)
• C++ -> STL (standard library usage, reuse through libraries)
• C++ -> Java (automatic memory management & no pointer arithmetic & OS independence)
• Java -> Spring Framework (adding Dependency Injection)
• Java -> Scala (get rid of loops and exceptions)
  • functional immutable collections (www.vavr.io, old javaslang.io)/ Streams from java8 standard libraries
  • lambda expressions (Java8)
  • types for: exceptions, options(java's Optional)