Next we’ll examine Kotlin generics. This is an advanced topic, but one that you are bound to encounter as you continue to use Kotlin. Generics also allow the compiler to better check our code—and we always want that. So let’s do this!
We’ve already seen generics at use when using Kotlin’s containers like
which require a type parameter when Kotlin cannot infer it:
Remember: runtime errors cause things to fail right in the user’s face. This is not good! Compiler errors, in contrast, must be caught in development. So transforming runtime errors to compiler errors helps us produce more correct programs. This is good! Because Kotlin knows what is stored in the list, it knows what we can and can’t do with the list contents:
OK—so now we know how to use classes that accept type parameters. But how about using the in our own classes? This turns out to not be too hard! Let’s explore together.
To start, let’s design a class that does not support type parameters,
but can still accept any object by using
Next, let’s look at how to add a generic type parameter to our example above.
This will allow the compiler to ensure that all of the values added to
LastN are the same type!
There are a few important things to understand about how Kotlin compiles generic classes. First, the type parameter is not a variable. You can use them in most places that you would normally use a type, but you can’t assign to them:
One useful way to think about what happens when your program is compiled is that the compiler replaces the type parameters with types used in your code. So, given this generic class:
If I create a
Example<String>, it’s almost as if I had written this code:
You can use any name to name your type parameters. But Kotlin inherits Java’s established conventions. Specifically:
E for element (like a list),
V for key and value (like a map),
N for number
Just like Kotlin classes, interfaces can also accept type parameters. That includes one interface that we are fairly familiar with by now!
You may have noticed above that when we implemented
LastN we used a
List rather than an array.
That wasn’t an accident!
Let’s see why and what problems arise with generic arrays.
(What follows is bonus bonus material, but very cool!)
Let’s consider another example where we’d like to use generics:
Let’s explore how we can complete the example above using another feature of Kotlin’s generics system: bounded type parameters.
This lesson has barely scratched the surface of what is possible with Kotlin generics. If you want to learn more, this is a good place to start.
You regularly see generic type parameters in Kotlin documentation. Now that we’ve discussed a bit about how to use type parameters in our own code, we’re more prepared to understand documentation for classes that use them. Let’s look at one familiar example together and discuss how to identify and interpret these type parameters.
Let's determine if a binary tree is height balanced. A tree is height balanced if the height of any node's two subtrees (right and left) never differ by more than 1.
Provide a method named
isBalanced accepts a
BinaryTree<*> and returns
true if the tree is balanced, and
A few hints on this problem:
Need more practice? Head over to the practice page.