Apply function behavior and use cases in Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the apply function do in Kotlin?

The apply function executes a block of code on an object and returns the object itself. It is useful for initializing or configuring an object.

intermediate

How is apply different from let in Kotlin?

apply returns the object it was called on, while let returns the result of the lambda expression. apply is used for object configuration, let for transformations.

beginner

Show a simple example of using apply to initialize an object.

val person = Person().apply { this.name = "Alice" this.age = 30 } // person is now initialized with name and age

intermediate

Why is apply useful for builder-style code?

apply lets you write clean code by configuring an object inside a block without repeating the object name, improving readability and reducing boilerplate.

intermediate

Can apply be used with nullable objects?

Yes, but you should use the safe call operator ?.apply to avoid null pointer exceptions. The block runs only if the object is not null.

What does the Kotlin apply function return?

A The object it was called on

B The result of the lambda block

C A new object

D Null

apply returns the original object after running the block, allowing chaining.

Which Kotlin function is best for configuring an object without repeating its name?

A let

B run

C apply

D also

apply lets you configure an object inside a block where this refers to the object, so you don't repeat the name.

How do you safely use apply on a nullable object?

A Use <code>!!.apply</code>

B Use <code>?.apply</code>

C Use <code>apply</code> directly

D Use <code>let</code> instead

Using ?.apply runs the block only if the object is not null, preventing errors.

Which of these is a typical use case for apply?

A Initializing an object

B Transforming a value

C Filtering a list

D Mapping a collection

apply is commonly used to initialize or configure objects.

What is the receiver object inside the apply block?

A Null

B A new object created inside the block

C The lambda parameter

D The object <code>apply</code> was called on

Inside apply, this refers to the original object the function was called on.

Explain how the apply function works in Kotlin and give an example of when you would use it.

Think about configuring an object without repeating its name.

Describe the difference between apply and let in Kotlin and when to use each.

Focus on what each function returns and how you use the object inside.

Practice

1. What does the apply function do in Kotlin?

easy

A. Runs code on an object and returns the same object

B. Creates a new object and returns it

C. Runs code and returns the last expression result

D. Runs code without returning anything

Solution

1. Step 1: Understand the purpose of apply

   The apply function executes a block of code on the object it is called on.

2. Step 2: Identify the return value of apply

   It always returns the original object after applying the block, not a new value.

3. Final Answer:

   Runs code on an object and returns the same object -> Option A

4. Quick Check:

   apply returns the object itself [OK]

Hint: Remember: apply returns the original object after changes [OK]

Common Mistakes:

• Thinking apply returns the last expression result
• Confusing apply with let or run
• Assuming apply creates a new object

2. Which of the following is the correct syntax to use apply on an object person to set its name property to "Alice"?

easy

A. person.apply { name = "Alice" }

B. apply(person) { name = "Alice" }

C. person.apply(name = "Alice")

D. person.apply { setName("Alice") }

Solution

1. Step 1: Identify the correct lambda syntax for apply

   apply takes a lambda with receiver where this is the object, so properties can be set directly.

2. Step 2: Check each option's syntax

   person.apply { name = "Alice" } correctly uses person.apply { name = "Alice" }. apply(person) { name = "Alice" } is invalid syntax. person.apply(name = "Alice") wrongly passes a parameter. person.apply { setName("Alice") } assumes a setter method which may not exist.

3. Final Answer:

   person.apply { name = "Alice" } -> Option A

4. Quick Check:

   Use object.apply { property = value } syntax [OK]

Hint: Use curly braces with apply to set properties inside [OK]

Common Mistakes:

• Passing parameters directly to apply
• Using parentheses instead of curly braces
• Calling non-existent setter methods

3. What is the output of this Kotlin code?

val list = mutableListOf<Int>().apply {
    add(1)
    add(2)
    add(3)
}
println(list)

medium

A. Compilation error

B. []

C. null

D. [1, 2, 3]

Solution

1. Step 1: Understand what apply does on the list

   The apply block adds 1, 2, and 3 to the mutable list.

2. Step 2: Check the returned value of apply

   apply returns the original list after adding the elements, so list contains [1, 2, 3].

3. Final Answer:

   [1, 2, 3] -> Option D

4. Quick Check:

   apply adds elements and returns the list [OK]

Hint: apply returns the object after running code inside [OK]

Common Mistakes:

• Expecting apply to return last added element
• Thinking list is empty after apply
• Confusing apply with run or let

4. Identify the error in this Kotlin code using apply:

val map = mutableMapOf()
map.apply {
    put("a", 1)
    put("b", 2)
}
println(map.get("c"))

medium

A. Syntax error in apply block

B. Compilation error due to missing return

C. No error; output is null

D. Runtime error: key "c" not found

Solution

1. Step 1: Check the apply block usage

   The apply block correctly adds two entries to the map.

2. Step 2: Understand the map.get("c") call

   Since key "c" does not exist, get returns null, which is printed without error.

3. Final Answer:

   No error; output is null -> Option C

4. Quick Check:

   Accessing missing key returns null, no error [OK]

Hint: Missing keys in map.get return null, no crash [OK]

Common Mistakes:

• Expecting runtime error for missing key
• Thinking apply must return a value explicitly
• Confusing map access syntax

5. You want to create and configure a StringBuilder in Kotlin to build the string "Hello World!" using apply. Which code snippet correctly does this?

hard

A. val result = StringBuilder().apply { append("Hello"); append(" World!") }

B. val result = StringBuilder().apply { append("Hello"); append(" World!") }.toString()

C. val result = StringBuilder().apply { append("Hello World!") }

D. val result = StringBuilder().apply { append("Hello"); append(" World!") }.toString

Solution

1. Step 1: Understand apply returns the object itself

   The apply block configures the StringBuilder, but to get the string, toString() must be called after.

2. Step 2: Check each option's correctness

   val result = StringBuilder().apply { append("Hello"); append(" World!") }.toString() calls toString() after apply, returning the built string. val result = StringBuilder().apply { append("Hello World!") } returns the StringBuilder object, not the string. val result = StringBuilder().apply { append("Hello"); append(" World!") } misses toString(). val result = StringBuilder().apply { append("Hello"); append(" World!") }.toString misses parentheses on toString, causing error.

3. Final Answer:

   val result = StringBuilder().apply { append("Hello"); append(" World!") }.toString() -> Option B

4. Quick Check:

   Call toString() after apply to get string [OK]

Hint: Call toString() after apply to get final string [OK]

Common Mistakes:

• Forgetting to call toString() after apply
• Missing parentheses on toString
• Expecting apply to return string automatically