Converting foreach with if

Iterating with foreach

In the previous articles in this tutorial series we looked at converting loops written in the imperative style to the functional style. In this article we'll see how to convert an imperative style iteration using foreach to the functional style. In addition, we'll also see how to pick select elements using if transforms to the functional style.

Java 5 introduced the very popular foreach syntax. For example, to iterate over a collection of Strings representing names, we'd write something like for(String name: names). Under the hood, the foreach is converted, at the bytecode level, to use an Iterator—while the iterator tells us there is another element, fetch the next element for processing. In other words, the foreach is a nice concise syntax sugar for iteration with a while loop over the elements provided by an Iterator. We can convert a foreach into the functional style quite easily. Let's see how.

From Imperative to Functional Style

Here's an example of iteration, using the foreach, over a collection of names:

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

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. Converting the imperative style foreach to the functional style is a straight up use of the forEach internal iterator method. It is called an internal iterator because the advancing to the next element is handled internally and automatically instead of externally or explicitly.

Let's refactor the loop to use functional style.

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

That was pretty straightforward, the for loop turned into a call to the forEach() method on the collection. Each step through the iteration, the lambda provided to the forEach() as an argument is invoked with the next element in the collection.

A slightly variation of this iteration, using stream(), is shown next.

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

The forEach() method is available both on a Collection<T> and on a Stream<T>. Functions like filter(), which we'll use soon, are available only on a Stream<T> and not on the Collection. This is by design in order to provide efficiency when multiple intermediate operations may precede the terminal operation like forEach(), findFirst(), etc.

Picking select elements with if

Suppose, in the middle of the iteration, we want to pick some values from the collection based on some condition. For example, what if we want to 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);
  }
}

For the functional style, the filter method of Stream becomes a direct replacement of the imperative style if. The filter method will allow an element in the collection to pass through to the next stage in the functional pipeline if the predicate, passed in as a lambda, to the filter() method evaluates to true; otherwise, the value is discarded from further processing.

Let's convert the previous code to functional style:

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

The filter() method acts like a gate, it opens to let some elements pass through and closes to reject or discard some elements, as the iteration moves forward.

We saw in the previous articles the functional style equivalent for the traditional for loops. In this article we saw how the imperative style foreach of Java 5 transforms into an elegant syntax in the functional style. Furthermore, the if condition within a loop of the imperative style translates to a call to the filter() method of the Stream API.

Mappings

Anywhere you see a foreach loop, use the forEach() method directly on the collection. If the body of the foreach has a if statement to selectively pick some value, then use the stream() API with the call to the filter() method.