Converting Iteration with transformation

Transforming while Iterating

In the previous articles in this tutorial series we looked at converting loops with if or conditional statements in the imperative style to the functional style. In this article we'll see how to convert an imperative style iteration that transforms data to the functional style. In addition, we'll also refactor code that mixes transforming data with code that picks select elements before the transformation.

Anytime we are transforming data in an imperative style loop, we can use the map() function in the functional style. Let's see how.

From Imperative to Functional Style

Here's an example of an iteration, using the foreach, that transforms to uppercase a collection of names:

List<String> names = List.of("Jack", "Paula", "Kate", "Peter");
  
for(String name: names) {
  System.out.println(name.toUpperCase());
}

Each step through the iteration, the name variable is bound to a new value. As the iteration advances from one element to the next in the given collection, each name is transformed to uppercase using the toUpperCase() function and the resulting value is printed. We have already seen, in the previous article, how to convert the imperative style foreach to the functional style—using the stream() internal iteration. If we merely apply what we have seen before, the resulting functional style code would be rather unwieldy, with the lambda passed to forEach performing both transformation and printing, like so:

List<String> names = List.of("Jack", "Paula", "Kate", "Peter");
  
names.forEach(name -> System.out.println(name.toUpperCase())); //Don't do this

Even though the above functional style code will produce the same results as the imperative style code, the lambda passed to the forEach() function is not cohesive, hard to read, and hard to change.

Before refactoring the imperative style code above to the function style, we should first refactor the imperative style to yet another imperative style implementation to make each line more cohesive, like so:

List<String> names = List.of("Jack", "Paula", "Kate", "Peter");
  
for(String name: names) {
  String nameInUpperCase = name.toUpperCase();
  System.out.println(nameInUpperCase);
}

From the previous articles in this series, we know that the for can turn into a stream() and the printing of the value can be done from within the forEach(). That leaves us with the transformation, the call to the toUpperCase() function. Such transformations can be done using the map() operation on the stream().

List<String> names = List.of("Jack", "Paula", "Kate", "Peter");
  
names.stream()
  .map(name -> name.toUpperCase())
  .forEach(nameInUpperCase -> System.out.println(nameInUpperCase));

The map() operation transforms the data to a different value based on the function invoked from within the lambda expression that's passed to map(). In this example, each name is transformed to its uppercase value. The transformed value is then printed out from within the lambda expression passed to forEach().

We can make the code a bit more concise by using method references instead of lambda expressions, like so:

List<String> names = List.of("Jack", "Paula", "Kate", "Peter");
  
names.stream()
  .map(String::toUpperCase)
  .forEach(System.out::println);

Use the map() function to transform data while iterating over a collection of data.

Picking Elements to Transform

Suppose, in the middle of the iteration, we want to pick some values from the collection based on some condition and apply a transformation only on those elements. For example, what if we want to transform and print only names of length 4? In the imperative style we could do the following:

List<String> names = List.of("Jack", "Paula", "Kate", "Peter");
  
for(String name: names) {
  if(name.length() == 4) {
    System.out.println(name.toUpperCase());
  }
}

We already know that the imperative style if can be refactored to the filter() function in the functional style. We can perform the transformation, using map(), after the filter() operation, like so:

List<String> names = List.of("Jack", "Paula", "Kate", "Peter");
  
names.stream()
  .filter(name -> name.length() == 4)
  .map(String::toUpperCase)
  .forEach(System.out::println);

The filter() function discards data that's not desired and passes on only the values we like to use. The map() function transforms the values it sees after the filter.

Mappings

Anywhere you see transformation of data within a for loop, use the map() function to perform the transformation in the functional style. In addition, if the body of the loop has a if statement to selectively some value for transformation, then use the stream() API with the call to the filter() method before using the map() method.