Functional programming techniques - Java 8 in Action: Lambdas, Streams, and Functional-Style Programming - page 371

Chapter 14. Functional programming techniques
This chapter covers
 First-class citizens, higher-order functions, currying, and partial application
 Persistent data structures
 Lazy evaluation and lazy lists as generalizing Java streams
 Pattern matching and how to simulate it in Java
 Referential transparency and caching
In chapter 13 you saw how to think functionally; thinking in terms of side-effect-free methods
can help you write more maintainable code. In this chapter, we introduce more advanced
functional programming techniques. You can view this chapter as a mix of practical techniques
to apply in your codebase as well as academic information that will make you a more
knowledgeable programmer. We discuss higher-order functions, currying, persistent data
structures, lazy lists, pattern matching, caching with referential transparency, and combinators.
14.1. Functions everywhere
In chapter 13 we used the phrase functional-style programming to mean that the behavior of
functions and methods should be like that of mathematical-style functions—no side effects.
Functional-language programmers often use the phrase with more generality to mean that
functions may be used like other values: passed as arguments, returned as results, and stored in
data structures. Such functions that may be used like other values are referred to as first-class
functions . This is exactly what Java 8 adds over previous versions of Java: you may use any
method as a function value, using the :: operator to create a method reference , and lambda
expressions (for example, (int x) -> x + 1) to directly express function values. In Java 8 it's
perfectly valid to store the method Integer.parseInt in a variable by using a method reference as
follows:
Function<String, Integer> strToInt = Integer::parseInt;
14.1.1. Higher-order functions
So far we've mainly used the fact that function values are first class only in order to pass them to
Java 8 stream-processing operations (as in chapters 4 - 7 ) and to achieve the very similar effect
of behavior parameterization when we passed Apple::isGreen-Apple as a function value to