Custom delegated properties in Kotlin - Time & Space Complexity

Time Complexity: Custom delegated properties

O(n)

Understanding Time Complexity

When using custom delegated properties in Kotlin, it's important to understand how the time to get or set a property changes as your program runs.

We want to know how the cost of accessing or updating these properties grows with usage.

Scenario Under Consideration

Analyze the time complexity of the following Kotlin code using a custom delegate.

class ExampleDelegate {
    private var storedValue: String = ""

    operator fun getValue(thisRef: Any?, property: kotlin.reflect.KProperty<*>): String {
        return storedValue
    }

    operator fun setValue(thisRef: Any?, property: kotlin.reflect.KProperty<*>, value: String) {
        storedValue = value
    }
}

class Example {
    var name: String by ExampleDelegate()
}
    

This code defines a custom delegate that stores and retrieves a string value for a property.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Each time the property name is accessed or updated, the delegate's getValue or setValue function runs.
• How many times: Once per property access or update; no loops or recursion inside these functions.

How Execution Grows With Input

Each property access or update runs a simple function that does a single assignment or return.

Number of Accesses (n)	Approx. Operations
10	10 simple get or set calls
100	100 simple get or set calls
1000	1000 simple get or set calls

Pattern observation: The time grows directly with the number of property accesses or updates, each costing about the same.

Final Time Complexity

Time Complexity: O(n)

This means the total time grows linearly with how many times you get or set the property.

Common Mistake

[X] Wrong: "Using a custom delegate makes property access slower in a way that depends on the size of stored data inside the delegate."

[OK] Correct: The delegate functions here do simple operations that take constant time regardless of stored data size, so each access or update costs about the same.

Interview Connect

Understanding how custom delegates work and their time cost helps you explain your code clearly and reason about performance in real projects.

Self-Check

What if the delegate stored data in a list and searched it on each get? How would the time complexity change?

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