3. The this Keyword in Kotlin
In Kotlin, this is used to refer to the current instance of a class. It plays a similar role to this in other programming languages, such as Java. The keyword is used within class methods to access instance variables and methods:
data class Person(val name: String) {
// An instance variable
var age: Int = 0
fun haveBirthday() {
this.age++
}
}
The this keyword explicitly references the current instance of Person in haveBirthday() to increment their age.
Finally, we’ll test the haveBirthday() function:
@Test
fun testHaveBirthday() {
val person = Person("Bob")
person.haveBirthday()
assertEquals(1, person.age)
}
4. Use Cases of this Keyword
In this section, we’ll explore several additional ways to utilize Kotlin’s this keyword.
4.1. Extension Function
When defining extension functions, we can use this to refer to the object reference the extension function is called on:
fun String.customLength(): Int {
return this.length
}
val length = "Hello, Kotlin".customLength()
Let’s verify our customLength() extension function:
@Test
fun testCustomLength() {
val input = "Hello, Kotlin"
val expectedLength = 13
val actualLength = input.customLength()
assertEquals(expectedLength, actualLength)
}
4.2. Constructor Chaining
When we have secondary constructors in a class, we must use this to call the primary constructor within the same class:
data class PersonData(val name: String, val age: Int) {
constructor(name: String) : this(name, 0)
}
Here, we have a PersonData data class with two constructors. We can instantiate this class with name and age using the primary constructor, or with just name using the secondary constructor. By using the secondary constructor, age defaults to zero from the internal usage of the primary constructor. The this keyword references the primary constructor from the secondary constructor.
Let’s see how we can test our secondary constructor:
@Test
fun testSecondaryConstructor() {
val person = PersonData("Alice")
assertEquals("Alice", person.name)
assertEquals(0, person.age)
}
5. Key Differences Between the it and this Keywords
Finally, let’s consider some key differences between these keywords that we should be aware of.
The it keyword is used as an implicit name for a single parameter in a lambda expression or an anonymous function. It allows us to refer to the single parameter of a lambda function without needing to explicitly name it.
Conversely, the this keyword refers to the current instance of an object reference, within a class or extension function. It accesses instance members, properties, and functions within a class or extension function to differentiate between instance members and local variables or parameters with the same name.
6. Conclusion
The it keyword simplifies lambda expressions with a single parameter. On the other hand, the this keyword is a reference to the current instance within a class and is used to access instance variables and methods.
As always, the full implementation of these examples is available over on GitHub.
