Fluent APIs and Types

This article was originally published on Linkedin

A frame from the earliest cartoon featuring Tweety and an hungry cat

I’d like to borrow an example from Mario Fusco to illustrate how to take advantage of types to make programming safer and more fun. Consider this Java class:

public class Cat {
    private Bird caughtBird;
    private boolean isFull = false;

    public Cat catchBird(Bird bird) {
        if (caughtBird != null) {
            throw new RuntimeException("Caught one bird already");
        caughtBird = bird;
        return this;

    public Cat eat() {
        if (caughtBird == null) {
            throw new RuntimeException("Must catch a bird first");
        this.caughtBird = null;
        this.isFull = true;
        return this;

    public static void main(String[] args) {
        Cat cat = new Cat();
        Bird bird = new Bird();
        cat.eat().catchBird(bird); // calls out of order; 
                                   // throws exception

The class uses the return this trick to enable method call chaining. However, there is a temporal dependency between the methods in this class, and if I call them out of order, I will get an exception at runtime. At runtime, possibly throwing an exception in the face of a user!

The nice thing is that we can use types to prevent this error at programming time. That is:

class HungryCat {
    public CatWithPrey catchBird(Bird bird) {
        return new CatWithPrey(bird);

class CatWithPrey {
    private Bird caughtBird;

    public CatWithPrey(Bird bird) {
        this.caughtBird = bird;

    public FullCat eat() {
        return new FullCat();

class FullCat {
    public HungryCat whackOnTheHead() {
        System.out.println("Whack! (Coughs up the bird)");
        return new HungryCat();

Now if you start with a HungryCat, the only option you have is to call method catchBird(...), and the IDE itself will guide you towards invoking the correct method:

Screenshot of the IDE showing how the autocomplete guides you to invoke the only method available
Autocomplete to the rescue

And this is how you do a fluent API: at every step, the IDE guides us to choose among the available methods, and the compiler prevents us to invoke methods out of order.

IDE screenshot showing example usage of the fluent code
The IDE shows the types returned at every step

The original example from Mario Fusco is just 2 slides out of this slideshare presentation, that contains a lot of other good stuff.

That’s all folks!