SAM conversions for Java interfaces in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a Runnable using SAM conversion.

Kotlin

val runnable = Runnable { println([1]) }
runnable.run()

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

Aprintln("Hello from SAM")

B"Hello from SAM"

CHello from SAM

Drun()

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to put quotes around the string.

Trying to call println inside the lambda incorrectly.

The Runnable interface's single abstract method run() takes no arguments and returns Unit. Using SAM conversion, we provide a lambda that prints the string. The string must be quoted.

2fill in blank

medium

Complete the code to create a Comparator using SAM conversion that compares integers.

Kotlin

val comparator = Comparator<Int> { a, b -> a [1] b }
println(comparator.compare(5, 3))

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

A/

B+

C*

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Using addition instead of subtraction.

Using multiplication or division which do not give correct comparison.

Comparator's compare method returns a negative, zero, or positive number depending on order. Subtracting b from a gives the correct comparison result.

3fill in blank

hard

Fix the error in the SAM conversion for ActionListener.

Kotlin

val listener = ActionListener { e -> println(e.[1]) }

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

AactionCommand

BgetActionCommand()

CgetAction()

Dcommand

Attempts:

3 left

💡 Hint

Common Mistakes

Using the Java getter method syntax instead of Kotlin property.

Using incorrect method names.

In Kotlin, Java getter methods can be accessed as properties. So getActionCommand() becomes actionCommand.

4fill in blank

hard

Fill both blanks to create a Thread using SAM conversion and start it.

Kotlin

val thread = Thread([1])
thread.[2]()

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

A{ println("Running in thread") }

Bstart

Crun

DThread()

Attempts:

3 left

💡 Hint

Common Mistakes

Calling run() instead of start() on the thread.

Not providing a lambda as Runnable.

Thread constructor accepts a Runnable, which can be provided as a lambda. To run the thread, call start(), not run().

5fill in blank

hard

Fill all three blanks to create a Timer with delay 1000, an ActionListener using SAM conversion, and start the timer.

Kotlin

val timer = Timer([1], [2])
timer.[3]()

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

A1000

B{ e -> println("Timer fired") }

Cstart

Dstop

Attempts:

3 left

💡 Hint

Common Mistakes

Using stop() instead of start() to run the timer.

Not providing the correct delay value.

Timer constructor takes delay in milliseconds and an ActionListener. The listener can be a lambda. To run the timer, call start().

Practice

1.

What does SAM conversion allow you to do in Kotlin when working with Java interfaces?

easy

A. Automatically convert Kotlin functions to Java classes

B. Convert Java interfaces to Kotlin data classes

C. Use multiple methods in a Java interface as lambdas

D. Use a lambda expression instead of an anonymous class for interfaces with one method

Solution

1. Step 1: Understand SAM conversion purpose

   SAM conversion lets Kotlin treat Java interfaces with a single abstract method as functional interfaces.

2. Step 2: Identify how it simplifies code

   This allows using lambda expressions instead of verbose anonymous classes for such interfaces.

3. Final Answer:

   Use a lambda expression instead of an anonymous class for interfaces with one method -> Option D

4. Quick Check:

   SAM conversion = Lambda for single-method Java interfaces [OK]

Hint: Remember: SAM = Single Abstract Method = one lambda [OK]

Common Mistakes:

• Thinking SAM works for interfaces with multiple methods
• Confusing SAM conversion with Kotlin function to Java class conversion
• Assuming SAM converts Java interfaces to Kotlin classes

2.

Which of the following is the correct Kotlin syntax to use SAM conversion with the Java Runnable interface?

val r = ???

easy

A. Runnable { println("Running") }

B. Runnable.run { println("Running") }

C. Runnable.run() -> println("Running")

D. Runnable.run { -> println("Running") }

Solution

1. Step 1: Recall SAM conversion syntax for Runnable

   Runnable is a Java interface with one method run(), so Kotlin allows lambda syntax: Runnable { ... }

2. Step 2: Check each option's syntax

   Runnable { println("Running") } correctly uses Runnable { println("Running") }. Others misuse method calls or lambda syntax.

3. Final Answer:

   Runnable { println("Running") } -> Option A

4. Quick Check:

   Runnable lambda = Runnable { ... } [OK]

Hint: Use Runnable { } directly for SAM conversion [OK]

Common Mistakes:

• Trying to call run() method explicitly in lambda
• Using arrow syntax incorrectly inside Runnable
• Confusing method reference with lambda syntax

3.

What will be the output of this Kotlin code using SAM conversion with Comparator?

val list = listOf("apple", "banana", "cherry")
val sorted = list.sortedWith(Comparator { a, b -> b.length - a.length })
println(sorted)

medium

A. ["cherry", "banana", "apple"]

B. ["apple", "banana", "cherry"]

C. ["banana", "apple", "cherry"]

D. ["banana", "cherry", "apple"]

Solution

1. Step 1: Understand the Comparator lambda

   The lambda compares strings by length descending: b.length - a.length.

2. Step 2: Sort the list by string length descending

   "banana" (6), "apple" (5), "cherry" (6). Since "banana" and "cherry" have same length, their order depends on original order.

3. Step 3: Determine sorted order

   Between "banana" and "cherry", "banana" comes first in original list, so it stays first. Then "cherry", then "apple" last.

4. Final Answer:

   ["banana", "apple", "cherry"] -> Option C

5. Quick Check:

   Sorted by length descending = [banana, apple, cherry] [OK]

Hint: Comparator lambda: b.length - a.length sorts descending [OK]

Common Mistakes:

• Assuming ascending order instead of descending
• Mixing up a and b in comparator lambda
• Ignoring stable sort behavior for equal lengths

4.

Identify the error in this Kotlin code using SAM conversion with a Java interface ActionListener:

val listener = ActionListener { event ->
    println("Clicked")
}
button.addActionListener(listener)

medium

A. SAM conversion works only if the interface is a Java interface with one method

B. The lambda parameter name must be e, not event

C. ActionListener is a Kotlin interface, so SAM conversion does not apply

D. You must use an anonymous object instead of a lambda

Solution

1. Step 1: Check if ActionListener is a Java interface with one method

   ActionListener is a Java interface with a single method actionPerformed(ActionEvent).

2. Step 2: Verify SAM conversion applicability

   SAM conversion applies only to Java interfaces with exactly one abstract method, so it should work here.

3. Step 3: Identify the actual error

   If the code does not compile, it might be because the Kotlin compiler does not recognize ActionListener as a SAM interface due to missing Java interop or import.

4. Final Answer:

   SAM conversion works only if the interface is a Java interface with one method -> Option A

5. Quick Check:

   SAM conversion requires Java single-method interface [OK]

Hint: SAM only works with Java interfaces having one abstract method [OK]

Common Mistakes:

• Assuming Kotlin interfaces support SAM conversion
• Thinking parameter name must be fixed
• Believing anonymous objects are always required

5.

You want to use SAM conversion to simplify this Java interface usage in Kotlin:

public interface Task {
    void execute(int times);
}

Which Kotlin lambda expression correctly uses SAM conversion for Task?

hard

A. Task { println("Executed times") }

B. Task { times -> println("Executed $times times") }

C. Task.execute { times -> println("Executed $times times") }

D. Task.run { times -> println("Executed $times times") }

Solution

1. Step 1: Confirm Task is a Java interface with one method

   Task has one method execute(int times), so SAM conversion applies.

2. Step 2: Write correct lambda syntax for SAM conversion

   The lambda must accept one parameter (times) matching the method signature: Task { times -> ... }

3. Step 3: Check each option

   Task { times -> println("Executed $times times") } correctly uses Task { times -> println(...) }. Others misuse method calls or omit parameter.

4. Final Answer:

   Task { times -> println("Executed $times times") } -> Option B

5. Quick Check:

   SAM lambda matches method parameters = Task { times -> println("Executed $times times") } [OK]

Hint: Match lambda parameters to single method parameters exactly [OK]

Common Mistakes:

• Calling method name explicitly in lambda
• Omitting lambda parameter when method requires one
• Using wrong method name in lambda expression