Apply function behavior and use cases in Kotlin - Step-by-Step Execution

Concept Flow - Apply function behavior and use cases

Start with an object

↓

Call apply on object

↓

Inside apply: execute block with 'this' as object

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Modify object properties

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Return the original object

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Use modified object

The apply function runs a block on an object, lets you modify it using 'this', and returns the same object for chaining.

Execution Sample

Kotlin

val person = Person().apply {
    name = "Alice"
    age = 30
}
println(person)

Creates a Person, sets name and age inside apply, then prints the modified person.

Execution Table

Step	Action	Object State (name, age)	Return Value	Output
1	Create Person object	name=null, age=0	Person instance
2	Enter apply block	name=null, age=0	Person instance
3	Set name = "Alice"	name=Alice, age=0	Person instance
4	Set age = 30	name=Alice, age=30	Person instance
5	Exit apply block	name=Alice, age=30	Person instance
6	Print person	name=Alice, age=30	Person instance	Person(name=Alice, age=30)

💡 apply returns the original object after executing the block, allowing property changes.

Variable Tracker

Variable	Start	After Step 3	After Step 4	Final
person.name	null	Alice	Alice	Alice
person.age	0	0	30	30

Key Moments - 2 Insights

Why does apply return the original object and not the block's last expression?

Because apply is designed to return the object it was called on, enabling chaining. See execution_table step 5 where return value is the Person instance, not the last assignment.

How does 'this' work inside the apply block?

Inside apply, 'this' refers to the object, so you can set properties directly without naming the object. See execution_table steps 3 and 4 where name and age are set without prefix.

Visual Quiz - 3 Questions

Look at the execution_table at step 4, what is the value of person.age?

A 0

B 30

C null

D Unchanged

💡 Hint

Check the Object State column at step 4 in execution_table.

At which step does apply return the original object?

A Step 3

B Step 6

C Step 5

D Step 2

💡 Hint

Look at the Return Value column in execution_table.

If you replaced apply with let, what would change in the return value?

A Return value would be the last expression in the block

B Return value would be the original object

C Return value would be null

D Return value would be Unit

💡 Hint

Recall that apply returns the object, but let returns the block result.

Concept Snapshot

apply function syntax:
object.apply { this.property = value; ... }
- Executes block with 'this' as object
- Returns the original object
- Useful for initializing or modifying objects inline
- Enables chaining after modifications

Full Transcript

The apply function in Kotlin lets you run a block of code on an object, using 'this' to refer to it. Inside the block, you can set or change properties easily. After the block finishes, apply returns the same object, not the block's last expression. This helps you write clean code to create and modify objects in one step. For example, creating a Person and setting name and age inside apply, then printing the person shows the updated values. Remember, apply returns the original object, so you can chain calls. This is different from let, which returns the block's result.