functional programming - define

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Although Java is not a functional language, Java 8 Lamda Expressions and Method References allow we to program in a functional style.

We can think OO abstracts data, FP abstracts behavior.

In computer science, functional programming is a programming paradigm. It set once and never changed.

for example:

// functional/Strategize.java
// (c)2017 MindView LLC: see Copyright.txt
// We make no guarantees that this code is fit for any purpose.
// Visit http://OnJava8.com for more book information.

interface Strategy {
  String approach(String msg);
}

class Soft implements Strategy {
  public String approach(String msg) {
    return msg.toLowerCase() + "?";
  }
}

class Unrelated {
  static String twice(String msg) {
    return msg + " " + msg;
  }
}

public class Strategize {
  Strategy strategy;
  String msg;

  Strategize(String msg) {
    strategy = new Soft(); // [1]
    this.msg = msg;
  }

  void communicate() {
    System.out.println(strategy.approach(msg));
  }

  void changeStrategy(Strategy strategy) {
    this.strategy = strategy;
  }

  public static void main(String[] args) {
    Strategy[] strategies = {
      new Strategy() { // [2]
        public String approach(String msg) {
          return msg.toUpperCase() + "!";
        }
      },
      msg -> msg.substring(0, 5), // [3] // lambda expression
      Unrelated::twice // [4] // Java 8 method reference
    };
    Strategize s = new Strategize("Hello there");
    s.communicate();
    for (Strategy newStrategy : strategies) {
      s.changeStrategy(newStrategy); // [5]
      s.communicate(); // [6]
    }
  }
}
/* Output:
hello there?
HELLO THERE!
Hello
Hello there Hello there
*/

references:

1. On Java 8 - Bruce Eckel

2. https://en.wikipedia.org/wiki/Functional_programming#Concepts

3. https://github.com/wangbingfeng/OnJava8-Examples/blob/master/functional/Strategize.java