下面是我总结的一些Java中函数式编程的要点，欢迎提问：

1. What is Functional Programing?

   • In mathematics, function is an expression that relates an input set to an output set.
   • Functional programming is a style of writing computer programs that treat computations as evaluating mathematical functions.

2. Lambda Calculus

   • Lambda calculus is a universal model, a formal system to express computations based on functional abstraction.
   • Tremendous impact : functional programing languages implements lambda calculus.

3. Concepts of functional programing
   3.1. First-Class and High-Order functions
   3.2. Pure functions
   3.3. Immutability
   3.4. Referential transparency

4. Functional programing techniques
   4.1. Composition : compose(), andThen()
   4.2. Monads : a monad allows us to wrap a value, apply a set of transformations, and get the value back with all transformations applied。
   such as : optional、stream、future etc, those have flatMap() method we call monad.
   4.3. Currying : a mathematical technique of converting a function that takes multiple arguments into a sequence of functions that take a single argument.
   Depends on : lambda expression and closures.
   Closures : Java has a limitation that variables the enclosing scope have to be final or effectively final.
   4.4. Recursion