// functional

https://docs.oracle.com/javase/8/docs/api/java/lang/FunctionalInterface.html

https://www.geeksforgeeks.org/functional-programming-paradigm/

• Pure functions - immutability, deterministic
• Recursion
• Referential transparency
• Functions are First-Class and can be Higher-Order
• Variables are Immutable

Predicate methods

// lambda

Supplier [java.util.function.Supplier]

Function.identity()

https://stackoverflow.com/questions/28032827/java-8-lambdas-function-identity-or-t-t