Object expressions (anonymous objects) in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create an anonymous object with a property name.

Kotlin

val obj = object {
    val [1] = "Kotlin"
}
println(obj.name)

Drag options to blanks, or click blank then click option'

Atype

Bvalue

Cname

Ddata

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different property name like value or type will cause an error when accessing obj.name.

The anonymous object has a property called name, so you must declare it as val name.

2fill in blank

medium

Complete the code to override the toString() method in an anonymous object.

Kotlin

val obj = object {
    override fun [1](): String {
        return "Anonymous Object"
    }
}
println(obj.toString())

Drag options to blanks, or click blank then click option'

AtoString

Bequals

ChashCode

Dcopy

Attempts:

3 left

💡 Hint

Common Mistakes

Overriding equals or hashCode instead of toString will not change the printed output.

To customize the string representation, override the toString() method.

3fill in blank

hard

Fix the error in the code by completing the anonymous object declaration that implements the interface Runnable.

Kotlin

val runnable = object : [1] {
    override fun run() {
        println("Running")
    }
}
runnable.run()

Drag options to blanks, or click blank then click option'

ACallable

BRunnable

CThread

DExecutor

Attempts:

3 left

💡 Hint

Common Mistakes

Using Callable or Thread instead of Runnable causes type errors.

To create an anonymous object that implements Runnable, use object : Runnable.

4fill in blank

hard

Fill both blanks to create an anonymous object with a property and a method.

Kotlin

val obj = object {
    val [1] = 10
    fun [2]() = [1] * 2
}
println(obj.doubleValue())

Drag options to blanks, or click blank then click option'

Avalue

BdoubleValue

Cnumber

Dcalculate

Attempts:

3 left

💡 Hint

Common Mistakes

Using different names for the property or method than those called causes errors.

The property is named value and the method is doubleValue which returns twice the property.

5fill in blank

hard

Fill all three blanks to create an anonymous object that implements an interface and overrides a method.

Kotlin

interface Greeter {
    fun greet(): String
}

val greeter = object : [1] {
    override fun [2]() = "Hello, [3]!"
}
println(greeter.greet())

Drag options to blanks, or click blank then click option'

AGreeter

Bgreet

CWorld

DGreeting

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong interface or method names causes compilation errors.

The anonymous object implements Greeter, overrides greet(), and returns "Hello, World!".

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'