Custom delegated properties in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a delegated property using the 'by' keyword.

Kotlin

val myValue: String [1] MyDelegate()

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

Aby

Bwith

Cusing

Dfrom

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'with' or 'from' instead of 'by' causes syntax errors.

Forgetting the keyword entirely.

In Kotlin, delegated properties use the 'by' keyword to delegate the property implementation to another object.

2fill in blank

medium

Complete the operator function to get the value of the delegated property.

Kotlin

operator fun getValue(thisRef: Any?, property: KProperty<*>): String [1] "Hello from delegate"

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

Ayield

Bprint

Creturn

Dthrow

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'print' instead of 'return' does not provide the value.

Using 'throw' causes an exception.

The 'getValue' operator function must return the value of the property, so it uses 'return'.

3fill in blank

hard

Fix the error in the setValue operator function to correctly assign the new value.

Kotlin

operator fun setValue(thisRef: Any?, property: KProperty<*>, value: String) {
    [1] = value
}

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

Aproperty

BstoredValue

Cfield

DthisRef

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to assign to 'property' or 'thisRef' causes errors.

Using 'field' outside of property setter is invalid.

The delegate must assign the new value to its own storage variable, here named 'storedValue'.

4fill in blank

hard

Fill both blanks to create a delegate that stores and returns an integer value.

Kotlin

class IntDelegate {
    private var [1] = 0
    operator fun getValue(thisRef: Any?, property: KProperty<*>): Int = [2]
}

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

AstoredValue

Bvalue

Dproperty

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'value' in getValue is incorrect because it's not defined there.

Using 'property' as a value causes errors.

The delegate uses 'storedValue' to keep the integer and returns it in getValue.

5fill in blank

hard

Fill all three blanks to complete a delegate that logs and stores a string property.

Kotlin

class LoggingDelegate {
    private var [1] = ""
    operator fun getValue(thisRef: Any?, property: KProperty<*>): String {
        println("Getting value of ${property.name}")
        return [2]
    }
    operator fun setValue(thisRef: Any?, property: KProperty<*>, value: String) {
        println("Setting value of ${property.name} to $value")
        [3] = value
    }
}

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

AstoredValue

Dvalue

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'value' as a variable name for storage causes confusion.

Forgetting to return the stored value in getValue.

The delegate uses 'storedValue' to keep the string, returns it in getValue, and assigns it in setValue.

Practice

1. What is the main purpose of using custom delegated properties in Kotlin?

easy

A. To define extension functions for a class

B. To create multiple constructors for a class

C. To control how a property's value is stored and retrieved

D. To override the default toString() method

Solution

1. Step 1: Understand property delegation concept

   Delegated properties let you customize how a property's value is handled behind the scenes.

2. Step 2: Identify the purpose of custom delegation

   Custom delegated properties specifically allow control over getting and setting property values.

3. Final Answer:

   To control how a property's value is stored and retrieved -> Option C

4. Quick Check:

   Custom delegation = control property access [OK]

Hint: Delegation controls property get/set behavior [OK]

Common Mistakes:

• Confusing delegation with constructors
• Thinking delegation defines functions
• Mixing delegation with method overriding

2. Which of the following is the correct syntax to declare a custom delegated property in Kotlin?

easy

A. val prop = Delegate()

B. val prop by Delegate()

C. val prop: Delegate by this

D. val prop delegate Delegate()

Solution

1. Step 1: Recall Kotlin delegation syntax

   Delegated properties use the by keyword followed by a delegate instance.

2. Step 2: Match correct syntax

   val prop by Delegate() correctly declares a delegated property.

3. Final Answer:

   val prop by Delegate() -> Option B

4. Quick Check:

   Delegation uses 'by' keyword [OK]

Hint: Look for 'by' keyword with delegate instance [OK]

Common Mistakes:

• Using '=' instead of 'by' for delegation
• Incorrect delegate type declaration
• Missing parentheses for delegate constructor

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

class Delegate {
    operator fun getValue(thisRef: Any?, property: kotlin.reflect.KProperty<*>): String {
        return "Hello from delegate"
    }
}

class Example {
    val message: String by Delegate()
}

fun main() {
    val e = Example()
    println(e.message)
}

medium

A. Hello from delegate

B. null

C. Compilation error

D. Hello from Example

Solution

1. Step 1: Understand getValue operator

   The getValue function returns the string "Hello from delegate" when the property is accessed.

2. Step 2: Trace property access in main()

   Accessing e.message calls getValue and prints the returned string.

3. Final Answer:

   Hello from delegate -> Option A

4. Quick Check:

   Delegate getValue returns string [OK]

Hint: getValue defines returned property value [OK]

Common Mistakes:

• Expecting default property value instead of delegate's
• Confusing getValue with setValue
• Assuming compilation error due to missing parameters

4. Identify the error in this Kotlin code using a custom delegated property:

class Delegate {
    operator fun getValue(thisRef: Any?, property: kotlin.reflect.KProperty<*>): Int {
        return 42
    }
}

class Example {
    var number: Int by Delegate()
}

fun main() {
    val e = Example()
    e.number = 10
    println(e.number)
}

medium

A. Delegate class must be an interface

B. Property number should be val, not var

C. getValue must return String, not Int

D. Delegate class lacks setValue operator function

Solution

1. Step 1: Check property mutability and delegation

   The property number is var, so delegate must support both get and set.

2. Step 2: Verify delegate methods

   The Delegate class only defines getValue, missing setValue needed for var.

3. Final Answer:

   Delegate class lacks setValue operator function -> Option D

4. Quick Check:

   var needs getValue and setValue [OK]

Hint: var properties need both getValue and setValue [OK]

Common Mistakes:

• Forgetting setValue for var properties
• Assuming getValue return type must be String
• Thinking delegate must be interface

5. You want to create a custom delegated property that logs every time the property is read or written. Which of the following delegate implementations correctly achieves this?

class LoggingDelegate<T>(var value: T) {
    operator fun getValue(thisRef: Any?, property: kotlin.reflect.KProperty<*>): T {
        println("Getting value of ${property.name}")
        return value
    }

    operator fun setValue(thisRef: Any?, property: kotlin.reflect.KProperty<*>, newValue: T) {
        println("Setting value of ${property.name} to $newValue")
        value = newValue
    }
}

class Example {
    var data: String by LoggingDelegate("initial")
}

fun main() {
    val e = Example()
    e.data = "new"
    println(e.data)
}

hard

A. Logs on get and set, prints 'Setting value of data to new' then 'Getting value of data' and 'new'

B. Only logs on get, no output on set

C. Throws error because delegate class is not generic

D. Does not compile due to missing constructor parameters

Solution

1. Step 1: Analyze delegate methods

   The delegate defines getValue and setValue with print statements logging property access.

2. Step 2: Trace main function output

   Setting e.data = "new" prints setting log, then printing e.data triggers get log and prints value.

3. Final Answer:

   Logs on get and set, prints 'Setting value of data to new' then 'Getting value of data' and 'new' -> Option A

4. Quick Check:

   Delegate logs on get/set correctly [OK]

Hint: Check both getValue and setValue print logs [OK]

Common Mistakes:

• Missing setValue causes no set logging
• Assuming delegate must be non-generic
• Forgetting to pass initial value to delegate