Improve Kotlin Code Review — Part II
In the second blog, I will tell you to advance concepts of Kotlin so you can improve code and code review techniques.
In the second blog, I will tell you to advance concepts of Kotlin so you can improve code and code review techniques.
Use Generics
Generics in Kotlin are a way to create classes, interfaces, and methods that can work with different types of data. By using generics, you can write code that is more flexible, reusable, and type-safe.
For example, let’s say you want to create a Box class that can hold any type of object. You can define this class using generics as follows:
class Box<T>(val item: T) {
fun getItem(): T {
return item
}
}In this example, the Box class is defined with a type parameter T. The item variable is of type T, which means it can hold any type of object. The getItem method returns an object of type T.
You can then create instances of the Box class with different types of objects:
val box1 = Box("Hello")
val box2 = Box(42)In this example, box1 holds a String object, while box2 holds an Int object.
Generics can also be used in function and method declarations. For example:
fun <T> printList(list: List<T>) {
for (item in list) {
println(item)
}
}In this example, the printList function takes a list of any type T, and prints each item in the list.
Use Kotlin In and Out Keywords in generics
Kotlin’s in and out keywords are used to define variance of generic types, specifically in the context of subtyping.
in is used to declare a generic type as a contravariant type parameter, which means that the type can only be consumed or used as an input. This is useful when you have a generic type that is a consumer of other types, such as a function that takes a parameter of that generic type. When using the in keyword, you can safely pass in a subtype of the declared type parameter.
For example:
interface Consumer<in T> {
fun consume(item: T)
}
class AnimalConsumer : Consumer<Animal> {
override fun consume(item: Animal) {
// Consume the Animal object
}
}
class DogConsumer : Consumer<Dog> {
override fun consume(item: Dog) {
// Consume the Dog object
}
}In this example, the Consumer interface is defined with a type parameter T that is contravariant using the in keyword. The AnimalConsumer and DogConsumer classes both implement the Consumer interface with different type arguments. Since the Consumer interface is contravariant, it is safe to pass a Dog object to the AnimalConsumer instance.
out is used to declare a generic type as a covariant type parameter, which means that the type can only be produced or used as an output. This is useful when you have a generic type that is a producer of other types, such as a function that returns a value of that generic type. When using the out keyword, you can safely return a subtype of the declared type parameter.
For example:
interface Producer<out T> {
fun produce(): T
}
class AnimalProducer : Producer<Animal> {
override fun produce(): Animal {
return Animal()
}
}
class DogProducer : Producer<Dog> {
override fun produce(): Dog {
return Dog()
}
}In this example, the Producer interface is defined with a type parameter T that is covariant using the out keyword. The AnimalProducer and DogProducer classes both implement the Producer interface with different type arguments. Since the Producer interface is covariant, it is safe to return a Dog object from the AnimalProducer instance.
In summary, in and out keywords are used to define the variance of a generic type parameter in Kotlin. in is used to declare a contravariant type parameter, which can only be consumed or used as an input, while out is used to declare a covariant type parameter, which can only be produced or used as an output.
Use Delegates in Recycler Adapter
The by keyword in Kotlin is used for delegation. It allows you to delegate some of the implementation details of a class to another object. This can be useful for simplifying the code and making it more modular.
In the context of a RecyclerView adapter, you can use the by keyword to delegate some of the implementation details to another object, such as a delegate or helper class. Here's an example:
class MyAdapter(private val items: List<MyItem>)
: RecyclerView.Adapter<MyViewHolder>() {
// Define a separate class that implements the adapter logic
private val adapterDelegate by lazy { MyAdapterDelegate() }
override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): MyViewHolder {
return adapterDelegate.onCreateViewHolder(parent, viewType)
}
override fun onBindViewHolder(holder: MyViewHolder, position: Int) {
adapterDelegate.onBindViewHolder(holder, position, items[position])
}
override fun getItemCount(): Int {
return items.size
}
// Define the adapter delegate class
private class MyAdapterDelegate {
fun onCreateViewHolder(parent: ViewGroup, viewType: Int): MyViewHolder {
// Create the view holder for the item view
}
fun onBindViewHolder(holder: MyViewHolder, position: Int, item: MyItem) {
// Bind the data to the view holder
}
}
}In this example, we define a separate MyAdapterDelegate class that implements the adapter logic, and we use the by keyword to delegate to an instance of that class. We use the lazy keyword to ensure that the adapter delegate is only created when it's needed, which can help improve performance.
Delegation can also be used for notifying a RecyclerView adapter when the data set has changed. This can be useful for separating the data and adapter concerns, and making the code more modular.
Here’s an example of using delegation to notify a RecyclerView adapter:
class MyAdapter(private val items: MutableList<MyItem>)
: RecyclerView.Adapter<MyViewHolder>()
, MutableList<MyItem> by items {
override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): MyViewHolder {
// Create the view holder for the item view
}
override fun onBindViewHolder(holder: MyViewHolder, position: Int) {
// Bind the data to the view holder
}
override fun getItemCount(): Int {
return items.size
}
}In this example, we use the by keyword to delegate the implementation of the MutableList<MyItem> interface to the items list. This means that any methods called on the adapter that are not implemented by the adapter itself will be delegated to the items list.
For example, if we call add(item: MyItem) on the adapter, it will be delegated to the items list, and the adapter will be notified that the data set has changed.
By using delegation in this way, we can separate the data and adapter concerns, and make the code more modular. We can also take advantage of the built-in functionality of the MutableList interface, such as add, remove, and clear, without having to implement these methods ourselves in the adapter.
Use applyIf extension function
The applyIf extension function is a popular utility function in Kotlin that allows you to conditionally apply a lambda expression to an object. Here's an example of how you can define the applyIf function:
inline fun <T> T.applyIf(condition: Boolean, block: T.() -> Unit): T {
return if (condition) {
this.apply(block)
} else {
this
}
}In this example, the applyIf function is defined as an extension function on any type T. It takes a boolean condition parameter and a lambda expression block as parameters. The block parameter is a function that can be invoked on the object being passed in.
The inline keyword is used to optimize the generated code and reduce the overhead of the function call. It's a good practice to use the inline keyword when defining utility functions like this that are called frequently.
The applyIf function uses the apply function to apply the lambda expression to the object if the condition is true. If the condition is false, it returns the object without applying the lambda expression.
Here’s an example of how you can use the applyIf function:
val text: String? = "hello world"
val result = text.applyIf(!text.isNullOrEmpty()) {
println("The length of the text is ${this.length}")
}In this example, the applyIf function is used to conditionally apply a lambda expression to the text object if it's not null or empty. The lambda expression prints the length of the text to the console. The applyIf function returns the text object with or without applying the lambda expression depending on the condition.
Overall, the applyIf extension function is a convenient utility function in Kotlin that can help you write cleaner and more concise code. It allows you to conditionally apply a lambda expression to an object, which can be useful for cases where you need to perform an operation only if a certain condition is met.
Use withNotNull
The withNotNull extension function is a popular utility function in Kotlin that allows you to execute a lambda expression on a non-null object. Here's an example of how you can define the withNotNull function:
inline fun <T : Any, R> T?.withNotNull(block: T.() -> R): R? {
return if (this != null) {
this.block()
} else {
null
}
}In this example, the withNotNull function is defined as an extension function on any type T that is not null. It takes a lambda expression block as a parameter, which is executed on the object being passed in.
The inline keyword is used to optimize the generated code and reduce the overhead of the function call. It's a good practice to use the inline keyword when defining utility functions like this that are called frequently.
The withNotNull function checks if the object is not null before executing the lambda expression. If the object is not null, it applies the lambda expression to the object and returns the result. If the object is null, it returns null.
Here’s an example of how you can use the withNotNull function:
val text: String? = "hello world"
val result = text.withNotNull {
println("The length of the text is ${this.length}")
}In this example, the withNotNull function is used to execute a lambda expression on the text object only if it's not null. The lambda expression prints the length of the text to the console. The withNotNull function returns the result of the lambda expression if the text object is not null, or null if it is null.
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