SAM conversions for Java interfaces in Kotlin

Introduction

SAM conversions let you write simple Kotlin code when using Java interfaces with one method. It makes your code shorter and easier to read.

When you want to pass a simple action to a Java method that expects an interface with one method.

When you use Java libraries that have interfaces like Runnable or Comparator.

When you want to write cleaner Kotlin code calling Java code without writing full classes.

When you want to quickly create a small behavior without extra boilerplate.

Syntax

Kotlin

val runnable = Runnable { println("Hello from SAM") }
Thread(runnable).start()

You write a lambda instead of a full class implementing the interface.

This works only for Java interfaces with exactly one abstract method (SAM = Single Abstract Method).

Examples

Create a Runnable using a lambda and start a thread.

Kotlin

val runnable = Runnable { println("Run in thread") }
Thread(runnable).start()

Create a Comparator with a lambda to sort strings by length.

Kotlin

val comparator = Comparator<String> { a, b -> a.length - b.length }
val list = listOf("cat", "elephant", "dog")
println(list.sortedWith(comparator))

Use SAM conversion to handle a button click event in Java Swing.

Kotlin

val action = java.awt.event.ActionListener { e -> println("Button clicked") }
button.addActionListener(action)

Sample Program

This program shows how to use SAM conversions for Runnable and Comparator Java interfaces in Kotlin. It runs a thread and sorts a list.

Kotlin

fun main() {
    val runnable = Runnable { println("Hello from SAM conversion") }
    Thread(runnable).start()

    val comparator = Comparator<String> { a, b -> a.length - b.length }
    val words = listOf("apple", "banana", "kiwi")
    val sorted = words.sortedWith(comparator)
    println(sorted)
}

Important Notes

SAM conversions only work with Java interfaces, not Kotlin interfaces.

Behind the scenes, Kotlin creates an object implementing the interface for you.

This feature helps keep Kotlin code concise when working with Java libraries.

Summary

SAM conversions let you use lambdas for Java interfaces with one method.

This makes Kotlin code shorter and easier when calling Java APIs.

Use it for Runnable, Comparator, listeners, and other single-method interfaces.

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