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An Introduction to Functional Programming in Java 8: Part 4 - Splitter

Hello everybody,

In the last parts, we’ve learned about the most basic ways to program functional in Java. We used functions as Objects, Streams and so much more.

Today, we want to create a useful example with them. You will train the paradigms of the last parts and learn how the optimize the run time of your program.

Example: Split a Stream by a specific filter

You might have stumbled over a problem like this before: You have a collection of objects and want to split them by a specific filter. After that, you want to perform some action with all elements who passed the test and another action with the ones that didn’t passed the test.

The Basic (and Slow) Approach

public <T> void splitAndPerform(Collection<T> items, Predicate<T> splitBy, Consumer<T> passed, Consumer<T> notPassed) {

This approach works, but it is pretty slow. The run time for splitting alone is O(2n), because we go through the whole collection 2 times: Ones to get all the items that passed the test and ones to get all the items that didn’t passed it.

But what if we would create a splitter on our own? It’d sort all items in the collection, depending on whether or not they pass the test, into one List or the other. This would lower the run time to split the collection to O(n), because you just have to call filter once, not twice.

So let’s do this.

The Better Approach

1. Split the Collection

The first step to building our splitter is to, you guessed it, split it.

In our splitBy() function, we want to take a Predeciate<T> as a parameter and return a new Splitter object, which is basically an object which consists of two Lists. One List consists of all the objects that passed the test, and the other one of all the objects that didn’t pass the test.

public class Splitter<T> {

    private List<T> passed;
    private List<T> notPassed;

    private Splitter(List<T> passed, List<T> notPassed) {
        this.passed = passed;
        this.notPassed = notPassed;

    public static <T> Splitter<T> splitBy(Collection<T> items,Predicate<T> test) {
        List<T> passed = new LinkedList<T>();
        List<T> notPassed = new LinkedList<T>();
                .forEach(item -> {

        return new Splitter<T>(passed, notPassed);


As you can see, we used the factory method pattern to make the creation of the Splitter nicer.

Now that we can create a Splitter object, we want to give it some functionality.

2. Work With the Split Lists

We want to work with the Lists in the same way that we can work with Streams. But we don’t want to recreate every function that a Stream has for our two lists. That’s where a nice design pattern comes in handy. I heard about it in this talk and it’s a very cool way to use lambdas. We basically create two new functions, called workOnPassedItems and workOnNotPassedItems. They take a Consumer<Stream<T>>. Therefore, we can create a lambda and work inside it with a normal looking stream. This method will then be applied onto the List of passed and not passed items.

public class Splitter<T> {


    public Splitter<T> workWithPassed(Consumer<Stream<T>> func) {
        return this;

    public Splitter<T> workWithNotPassed(Consumer<Stream<T>> func) {
        return this;


We used the cascade design pattern to make the use of multiple methods nicer. You will see it in the example down below.

And that’s basically our Splitter! Now, we can try out some examples on how to use it.

Example 1: Showing numbers, but squaring all odd numbers

In this very first example we want to operate over a list of numbers. For each even number, we just want to print this number out. But for every odd number, we want to square them before we print them out.

So first off, we want to split the list into even and odd numbers. After that, we can work on the lists as already told.

public void workOnNumbers() {
    List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);

    Splitter.splitBy(numbers, num -> num%2 == 0)
            .workWithPassed(passed ->
                    passed.forEach(even -> System.out.println("" + even + " -> " + even)))
            .workWithNotPassed(notPassed ->
           -> odd * odd)
                            .forEach(odd -> System.out.println("" + Math.sqrt(odd) + " -> " + odd)

Example 2: Sending all Winners a Confirmation and all Losers a Cancellation

Now, we have a List of Candidates. All of these people have a method called hasWon(), which gives back a boolean. We want to split the List into Winners and Losers. After that, we want to send all Winners a confirmation that they won, and all losers a cancellation that they lost. So let’s do this!

public void sendEmails(List<Candidates> candidates) {
    Splitter.splitBy(candidates, Candidates::hasWon)
            .workWithPassed(winners ->
                    winners.forEach(winner -> Email.send(winner.getEmail(), "You won!"))
    .workWithNotPassed(losers ->
            losers.forEach(loser -> Email.send(loser.getEmail(), "You lost, sorry!"))

Update: On Using partitioningBy

After the post’s out for a day now, a lot of you have given feedback. And I really appreciate it! You told me about ways to make the Splitter class better.

One of the discussed things is the use of Stream.collect(Collectors.partitioningBy(Predicate<T> test)). And I totally agree that it could be useful in our scenario.

It partitions the Stream depending on a test function. So for us, the map would look something like this: {true: passed, false: notPassed}. After that, we just take the two lists out of the map and go on.

So as Philboyd_Studge mentioned on Reddit, the new splitBy method could look something like this:

public static <T> Splitter<T> splitBy(Collection<T> items,Predicate<T> test) {

        Map<Boolean, List<T>> map =

        return new Splitter<T>(map.get(true), map.get(false));

And I have to admit that this takes out a lot of noise. It just looks nicer than the splitBy method above. So thank you for that!

So what’s the Splitter’s Right to Exist?

The Splitter’s purpose is to demonstrate how you can work with functions as objects. Its purpose is not to replace some methods in the JDK.

It’s a class for learning and playing around. If you want to tweak around, just do it. Please leave a comment on what you have learned or where you have optimized the class, so that others can learn from it too.

In addition, we learned about design patterns. Some of you even told me that they learned new design patterns in this blog post.

The patterns help to make the syntax nicer. When you split your collection in two parts, it’s nicer to use the cascade pattern in the Splitter than to handle a Map.


That’s it for today!

We have learned how to create our first useful class with the help of Streams. We also optimized the run time of our program with it. Therefore, we have trained our knowledge about design patterns like the factory method and the cascade pattern.

Lastly, we have tried out our splitter with some examples.

Next time, we will talk about RxJava. It’s a framework that uses the observer pattern for asynchronous tasks and builds upon functional programming.

Are there any things that your splitter should do too? Please let me know in the comments. I’d love to hear your feedback too!

Thanks for reading and have a nice day,


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