Functions revisited

Unlike object-oriented programs, those written in a functional way don't hold any mutable state. Instead, code is made up with functions that take arguments and return values. Because there is no internal state nor side-effects involved that can alter the execution, all functions are deterministic. This is a really good feature because it implies that different executions of the same function, with the same parameters, would produce the same outcome.

The following snippet illustrates a function that does not mutate any internal state:

public Integer add(Integer a, Integer b) {
  return a + b;
}

The following is the same function written using Java's functional API:

public final BinaryOperator<Integer> add =
  new BinaryOperator<Integer>() {

    @Override
    public Integer apply(Integer a, Integer b) {
      return a + b;
    }
  };

The first example should be completely familiar to any Java developer; it follows the common syntax of a function that takes two...