SAM conversions for Java interfaces in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of SAM conversion with Runnable

What is the output of this Kotlin code using SAM conversion for a Java Runnable interface?

Kotlin

val thread = Thread(Runnable { println("Hello from thread") })
thread.start()
Thread.sleep(100)

A Compilation error

B Hello from thread

C No output

D Runtime exception

💡 Hint

SAM conversion lets you pass a lambda where a single-method Java interface is expected.

The lambda is converted to a Runnable instance. When thread.start() runs, it prints the message.

❓ Predict Output

intermediate

SAM conversion with Comparator interface

What does this Kotlin code print when using SAM conversion for Java's Comparator interface?

Kotlin

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

A [cherry, banana, apple]

B [apple, banana, cherry]

C Compilation error

D [banana, cherry, apple]

💡 Hint

The comparator sorts strings by descending length.

The comparator compares string lengths in reverse order, so 'banana' (6), 'cherry' (6), then 'apple' (5). Since 'banana' and 'cherry' have same length, their order is stable.

🔧 Debug

advanced

Why does this SAM conversion fail?

This Kotlin code tries to use SAM conversion with a Java interface but fails. Why? interface MyInterface { fun doWork() fun extra() } fun test() { val obj = MyInterface { println("Work done") } }

A MyInterface has more than one abstract method, so SAM conversion is not applicable.

B Kotlin does not support SAM conversions for interfaces declared in Kotlin.

C The lambda syntax is incorrect for SAM conversion.

D Missing override keyword for doWork method.

💡 Hint

SAM conversion works only for interfaces with exactly one abstract method.

SAM conversion applies only to Java interfaces with a single abstract method. Here, MyInterface has two abstract methods, so Kotlin cannot convert the lambda.

📝 Syntax

advanced

Correct SAM conversion syntax for Java Callable

Which option correctly uses SAM conversion in Kotlin for the Java Callable interface to return the string "Done"?

A val c = Callable { "Done" }

B val c = Callable<String> { return "Done" }

C val c = Callable<String> { "Done"; println("Done") }

D val c = Callable<String> { println("Done") }

💡 Hint

The lambda should return the value directly without explicit return keyword.

Option A correctly uses SAM conversion with a lambda returning "Done". Option A has an invalid return statement inside lambda. Option A returns Unit, not String. Option A returns last expression which is println result (Unit).

🚀 Application

expert

Using SAM conversion with Java's ActionListener in Kotlin

You want to create a Java Swing JButton in Kotlin and add an ActionListener using SAM conversion. Which code snippet correctly prints "Button clicked" when the button is pressed?

A

val button = JButton("Click")
button.addActionListener(ActionListener { e -> println("Button clicked") })

B

val button = JButton("Click")
button.addActionListener(ActionListener { println("Button clicked") })

C

val button = JButton("Click")
button.addActionListener { e -> println("Button clicked") }

D

val button = JButton("Click")
button.addActionListener { println("Button clicked") }

💡 Hint

SAM conversion lets you pass a lambda with the event parameter for ActionListener.

Option C uses SAM conversion with a lambda that takes the event parameter 'e' and prints the message. Option C misses the event parameter, which is required. Option C and C use explicit ActionListener constructor which is valid but verbose; Option C is invalid syntax because ActionListener is a functional interface, not a constructor taking a lambda directly.