97 Things Every Java Programmer Should Know

Chapter 16. Declarative Expression Is the Path to Parallelism

Russel Winder

In the beginning, Java was an imperative, object-based programming language. Indeed, it still is. Over the years, though, Java has evolved, at each stage becoming more and more a language of declarative expression. Imperative is all about the code explicitly telling the computer what to do. Declarative is about the code expressing a goal abstracting over the way in which the goal is achieved. Abstraction is at the heart of programming, and so the move from imperative code to declarative code is a natural one.

At the core of declarative expression is the use of higher-order functions, functions that take functions as parameters and/or return functions. This was not an integral part of Java originally, but with Java 8 it moved front and center: Java 8 was a turning point in the evolution of Java, allowing replacement of imperative expression with declarative expression.

An example—trivial but nonetheless indicative of the main issue—is to write a function that returns a List containing the squares of the argument List to the function. Imperatively, we might write:

List<Integer> squareImperative(final List<Integer> datum) {
  var result = new ArrayList<Integer>();
  for (var i = 0; i < datum.size(); i++) {
    result.add(i, datum.get(i) * datum.get(i));
  }
  return result;
}

The function creates an abstraction over ...