Kotlin: What is the difference between Apply and Also

Question

What is the difference between apply and also. From what I know the following code does the same thing:

apply

val person = Person().apply {     name = "Tony Stark"     age = 52     // More such stuff } 

also

val person = Person().also {   it.name = "Tony Stark"   it.age = 52   // More such stuff } 

Is there any difference and should I use one over the other? Also, are there some cases where one would work and the other won't?

Answer 1

TL;DR Difference

The also function takes a lambda in which you refer to the object you called the function on (receiver T) with either it (implicit name) or a custom name.

val person = Person().also {     it.name = "Tony Stark" } 

With apply, on the other hand, a function literal with receiver is used so inside the passed in lambda you can access the receiver’s members directly, as you see in the following. The receiver can be referenced by this.

val person = Person().apply {     name = "Tony Stark" } 

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also

Declaration:

inline fun <T> T.also(block: (T) -> Unit): T (source) 

Calls the specified function block with this (the receiver) value as its argument and returns this (the receiver) value.

apply

Declaration:

inline fun <T> T.apply(block: T.() -> Unit): T (source) 

Calls the specified function block with this value as its receiver and returns this (the receiver) value.

when to use what

Usage examples are explained in this thread.