Classes in Kotlin are as simple as possible so that you can express the maximum amount of logic with the less code possible.

I’ll show quickly how you can start writing Kotlin classes, and the differences with Java classes.

1. Declare the class

It’s as easy as using the reserved word class and the name of the clase. You don’t need to use braces if the class doesn’t contain any code.

Spoiler: Have you seen it’s not using public visibility modifier? That’s because everything is public in Kotlin by default.

2. Add some properties

Fields don’t exist in Kotlin (or at least you can’t declare them). A class has properties.

To put it simple, a property substitutes a Java field + getter + setter. So let’s declare a couple of properties for our class:

If you want a custom setter, of course you can declare it:

Spoiler: as you can see, you can use variables directly into Strings (😱), without using the typical String.format

Have you realized the amount of code you’ll save with this? All the setters and getters and immediately  blown out.

3. Add a constructor

By default, you only need a constructor per class. You can add several, but you’ll see this use in a future article.

As there’s only one constructor, the way to write it can be simplified a lot too:

What happened here?

  1. The properties disappeared! In fact, they’re still there. But if the constructor arguments are annotated with var or val, the properties are generated inline.
  2. The constructor is written just after the definition of the class. You don’t need to create an extra method and assign the values of the properties.

4. You can now create functions inside the class

And use the properties inside them as you wish:

As the example above shows, functions can be written in a contracted way when a value is directly assigned. But there still exists the old “Java” way:

In this case, you need to define the return type.

5. Everything in Kotlin is closed by default

So it can’t be extended, and children (in case a class can be extended) can’t override its functions, unless it’s indicated with the reserved word open:

See how the parent constructor is called. Simple and clean!

Conclusion

In this article, you’ve seen some of the differences between Java and Kotlin classes. If you want to know more about them, I recommend you to subscribe to receive the rest of the articles I’ve prepared for you.

Author: Antonio Leiva

I’m in love with Kotlin. I’ve been learning about it for a couple of years, applying it to Android and digesting all this knowledge so that you can learn it with no effort.