SAM conversions for Java interfaces in Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does SAM stand for in the context of Java interfaces?

SAM stands for Single Abstract Method. It means an interface with exactly one abstract method.

beginner

How does Kotlin support SAM conversions for Java interfaces?

Kotlin allows you to pass a lambda expression where a Java SAM interface is expected, automatically converting the lambda to an instance of that interface.

beginner

Why are SAM conversions useful in Kotlin when working with Java interfaces?

They let you write concise and readable code by using lambdas instead of creating anonymous classes for Java interfaces with one abstract method.

intermediate

Can Kotlin perform SAM conversions for Kotlin interfaces?

No, Kotlin only supports SAM conversions automatically for Java interfaces, not for Kotlin interfaces.

beginner

Show a simple Kotlin example of SAM conversion with a Java interface Runnable.

Java interface Runnable has one method run(). In Kotlin, you can write: val r = Runnable { println("Running") } which creates a Runnable instance using a lambda.

What is required for an interface to be eligible for SAM conversion in Kotlin?

A It must have no methods.

B It must have exactly one abstract method.

C It must be a Kotlin interface.

D It must have multiple abstract methods.

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

Can Kotlin perform SAM conversions automatically for Kotlin interfaces?

A Only for interfaces with default methods.

B Yes, always.

C Only if the interface is marked with a special annotation.

D No, only for Java interfaces.

Kotlin supports SAM conversions automatically only for Java interfaces, not Kotlin interfaces.

What Kotlin syntax is used to create a SAM instance from a Java interface?

A A lambda expression.

B An anonymous class.

C A data class.

D A companion object.

You use a lambda expression to create an instance of a Java SAM interface in Kotlin.

Which Java interface is commonly used to demonstrate SAM conversion in Kotlin?

A Cloneable

B Serializable

C Runnable

D Comparator

Runnable has a single abstract method run(), making it a classic example for SAM conversion.

What benefit does SAM conversion provide when calling Java code from Kotlin?

A More concise and readable code using lambdas.

B Faster runtime performance.

C Automatic null safety.

D Better memory management.

SAM conversions let you write shorter and clearer code by using lambdas instead of anonymous classes.

Explain what SAM conversion is and how Kotlin uses it with Java interfaces.

Think about interfaces with one method and how lambdas replace anonymous classes.

Describe the limitations of SAM conversions in Kotlin, especially regarding Kotlin interfaces.

Consider which interfaces Kotlin can convert automatically and which it cannot.

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