Object expressions (anonymous objects) in Kotlin - Time & Space Complexity

Time Complexity: Object expressions (anonymous objects)

O(n)

Understanding Time Complexity

Let's see how the time cost changes when we use object expressions, also called anonymous objects, in Kotlin.

We want to know how the program's work grows as we create more of these objects.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

fun createObjects(n: Int) {
    repeat(n) {
        val obj = object {
            val x = it
            fun greet() = "Hello $x"
        }
        println(obj.greet())
    }
}

This code creates n anonymous objects, each with a property and a function, then prints a greeting.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Creating an anonymous object and calling its function inside a loop.
• How many times: Exactly n times, once per loop iteration.

How Execution Grows With Input

Each time we increase n, we create one more object and call its function once.

Input Size (n)	Approx. Operations
10	10 object creations and 10 function calls
100	100 object creations and 100 function calls
1000	1000 object creations and 1000 function calls

Pattern observation: The work grows evenly with n. Double n, double the work.

Final Time Complexity

Time Complexity: O(n)

This means the time needed grows in a straight line with the number of objects created.

Common Mistake

[X] Wrong: "Creating anonymous objects inside a loop is free or constant time because they are anonymous."

[OK] Correct: Each object creation takes time, and doing it n times adds up linearly.

Interview Connect

Understanding how object creation inside loops affects time helps you write clear and efficient Kotlin code, a skill valued in many coding challenges.

Self-Check

What if we moved the anonymous object creation outside the loop and reused it? How would the time complexity change?

Practice

1. What is an object expression in Kotlin?

easy

A. A named class declaration

B. A way to create an anonymous object with properties and functions

C. A function that returns a string

D. A variable declaration with a fixed value

Solution

1. Step 1: Understand object expressions

   Object expressions create unnamed objects on the spot with properties and functions.

2. Step 2: Differentiate from other options

   Named classes, functions, and variables are different concepts and do not create anonymous objects.

3. Final Answer:

   A way to create an anonymous object with properties and functions -> Option B

4. Quick Check:

   Object expression = anonymous object [OK]

Hint: Anonymous objects = quick unnamed objects [OK]

Common Mistakes:

• Confusing object expressions with named classes
• Thinking object expressions are functions
• Assuming object expressions are variables

2. Which of the following is the correct syntax to create an anonymous object in Kotlin?

easy

A. val obj = object() { val x = 10 }

B. val obj = class { val x = 10 }

C. val obj = object { val x = 10 }

D. val obj = fun() { val x = 10 }

Solution

1. Step 1: Recall Kotlin object expression syntax

   The correct syntax uses object { ... } without parentheses.

2. Step 2: Check each option

   val obj = object { val x = 10 } matches the correct syntax. val obj = class { val x = 10 } uses class which is invalid here. val obj = fun() { val x = 10 } uses fun() which is a function, not an object. val obj = object() { val x = 10 } incorrectly adds parentheses after object.

3. Final Answer:

   val obj = object { val x = 10 } -> Option C

4. Quick Check:

   Correct syntax = object { ... } [OK]

Hint: Use 'object { }' without parentheses for anonymous objects [OK]

Common Mistakes:

• Adding parentheses after 'object'
• Using 'class' instead of 'object'
• Confusing functions with object expressions

3. What will be the output of this Kotlin code?

val obj = object {
    val name = "Alice"
    fun greet() = "Hello, $name!"
}
println(obj.greet())

medium

A. Runtime error

B. Hello, name!

C. Compilation error

D. Hello, Alice!

Solution

1. Step 1: Analyze the anonymous object

   The object has a property name with value "Alice" and a function greet() that returns "Hello, $name!".

2. Step 2: Evaluate the print statement

   Calling obj.greet() returns "Hello, Alice!" which is printed.

3. Final Answer:

   Hello, Alice! -> Option D

4. Quick Check:

   obj.greet() = "Hello, Alice!" [OK]

Hint: Object properties accessed inside functions print correctly [OK]

Common Mistakes:

• Expecting variable name printed instead of value
• Thinking anonymous objects cause errors
• Confusing string interpolation syntax

4. Identify the error in this Kotlin code using an object expression:

val obj = object {
    val x = 5
    fun printX() {
        println(y)
    }
}

medium

A. Property 'y' is not defined in the object

B. Missing parentheses after 'object'

C. Function printX() should return a value

D. Cannot use println inside object expressions

Solution

1. Step 1: Check properties and functions inside the object

   The object defines property x but tries to print y which is not defined.

2. Step 2: Understand the error cause

   Accessing undefined property y causes a compilation error.

3. Final Answer:

   Property 'y' is not defined in the object -> Option A

4. Quick Check:

   Undefined property access = error [OK]

Hint: Check all properties used inside object functions exist [OK]

Common Mistakes:

• Assuming missing parentheses after 'object' cause error
• Thinking functions must return a value
• Believing println is disallowed inside objects

5. How can you use an object expression to create a one-time comparator for sorting a list of strings by length in Kotlin?

hard

A. list.sortedWith(object : Comparator<String> { override fun compare(a: String, b: String) = a.length - b.length })

B. list.sortedWith(object Comparator<String> { fun compare(a: String, b: String) = a.length - b.length })

C. list.sortedWith(object : Comparator<String> { fun compare(a: String, b: String) = a.length - b.length })

D. list.sortedWith(object Comparator<String> { override fun compare(a: String, b: String) = a.length - b.length })

Solution

1. Step 1: Understand object expression with interface implementation

   To create a comparator, use object : Comparator<String> and override compare method.

2. Step 2: Check syntax correctness

   list.sortedWith(object : Comparator<String> { override fun compare(a: String, b: String) = a.length - b.length }) correctly uses override fun compare. Options B and D miss the colon before interface or override keyword. list.sortedWith(object : Comparator<String> { fun compare(a: String, b: String) = a.length - b.length }) misses override keyword.

3. Final Answer:

   list.sortedWith(object : Comparator<String> { override fun compare(a: String, b: String) = a.length - b.length }) -> Option A

4. Quick Check:

   Correct syntax = object : Interface { override fun method() } [OK]

Hint: Use 'object : Interface { override fun method() }' for anonymous implementations [OK]

Common Mistakes:

• Omitting colon before interface name
• Missing override keyword on method
• Using 'object Interface' instead of 'object : Interface'