Object expressions (anonymous objects) in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of an anonymous object with overridden method

What is the output of this Kotlin code using an anonymous object?

Kotlin

val obj = object {
    fun greet() = "Hello from anonymous object"
}
println(obj.greet())

A Hello from anonymous object

B Compilation error: anonymous object cannot have functions

C null

D Runtime error: Unresolved reference greet

💡 Hint

Anonymous objects can have functions and properties just like regular classes.

The anonymous object defines a function greet() which returns the string. Calling greet() prints that string.

🧠 Conceptual

intermediate

Type of anonymous object when assigned to a variable

Consider this Kotlin code snippet: val obj = object { val x = 10 } What is the static type of 'obj'?

A Any

B Int

C Unit

D The exact anonymous object type with property 'x'

💡 Hint

When assigned to a val, the compiler infers the full anonymous object type.

When an anonymous object is assigned to a variable, its full type including properties is preserved.

❓ Predict Output

advanced

Output when anonymous object is returned from a function

What is the output of this Kotlin code?

Kotlin

fun create() = object {
    val name = "Kotlin"
    fun greet() = "Hello, $name!"
}

val obj = create()
println(obj.greet())

A Compilation error: Cannot access greet() on anonymous object returned from function

B Hello, $name!

C Hello, Kotlin!

D Runtime error: Unresolved reference greet

💡 Hint

Functions returning anonymous objects have their type erased to Any unless explicitly typed.

When a function returns an anonymous object without an explicit type, the caller sees it as Any, so greet() is not accessible.

🔧 Debug

advanced

Why does this anonymous object code cause a compilation error?

Identify the cause of the compilation error in this Kotlin code: val obj: Any = object { fun sayHi() = "Hi" } println(obj.sayHi())

Kotlin

val obj: Any = object {
    fun sayHi() = "Hi"
}
println(obj.sayHi())

A Anonymous objects cannot have functions

B The variable obj is typed as Any, which does not have sayHi() method

C Missing parentheses when calling sayHi

D Cannot assign anonymous object to Any type

💡 Hint

Check the declared type of obj and what methods it exposes.

The variable obj is declared as type Any, which does not have sayHi(), so calling obj.sayHi() causes a compilation error.

🚀 Application

expert

Using anonymous objects to implement an interface

What is the output of this Kotlin code?

Kotlin

interface Speaker {
    fun speak(): String
}

val speaker = object : Speaker {
    override fun speak() = "I am anonymous"
}

println(speaker.speak())

A Runtime error: speak() not implemented

B Compilation error: anonymous object must implement all interface methods

C I am anonymous

D I am Speaker

💡 Hint

Anonymous objects can implement interfaces and override their methods.

The anonymous object implements Speaker and overrides speak(), so calling speak() prints the returned string.

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'