Thinking functionally - Java 8 in Action: Lambdas, Streams, and Functional-Style Programming - page 370

 A function is referentially transparent if it always returns the same result value when called with the
same argument value. Iterative constructs such as while loops can be replaced by recursion.
 Tail recursion may be a better practice than classic recursion in Java because it opens the way to
eventual compiler optimization.