Collections interop behavior in Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What happens when you pass a Kotlin List to a Java method expecting a java.util.List?

Kotlin List is compiled to a java.util.List interface, so it can be passed directly to Java methods expecting java.util.List. This allows seamless interoperability without copying the collection.

intermediate

How does Kotlin handle mutability when interoperating with Java collections?

Kotlin distinguishes between mutable and read-only collections. When Kotlin code receives a Java collection, it treats it as mutable because Java collections are mutable by default. Conversely, Kotlin's read-only collections are seen as interfaces without mutability guarantees in Java.

intermediate

What is the difference between Kotlin's MutableList and Java's List in interop?

Kotlin's MutableList extends List and allows modification. Java's List interface is mutable by design. When Kotlin code calls Java methods returning List, it treats them as mutable, but Kotlin's read-only List interface does not guarantee mutability.

beginner

How does Kotlin handle Java arrays when interoperating?

Java arrays are seen as Kotlin Array types. Kotlin provides special support to convert between Java arrays and Kotlin arrays without copying, enabling efficient interop.

advanced

Why should you be careful when modifying collections passed between Kotlin and Java?

Because Kotlin distinguishes between mutable and read-only collections but Java collections are mutable by default, modifying collections passed between them can cause unexpected side effects. Always be aware of mutability contracts to avoid bugs.

When Kotlin passes a List to Java, what type does Java see?

A java.util.List

B Kotlin List

C ArrayList only

D MutableList only

Kotlin's List compiles to java.util.List, so Java sees it as java.util.List.

How does Kotlin treat Java collections it receives?

A As mutable collections

B As read-only collections

C As immutable collections

D As arrays

Java collections are mutable by default, so Kotlin treats them as mutable.

What happens if you modify a Kotlin read-only list passed to Java?

A Java can modify it freely

B It causes a compile error in Java

C It depends on the underlying implementation

D It is always immutable

Read-only in Kotlin is a compile-time concept; Java can modify if the underlying collection supports it.

How are Java arrays seen in Kotlin?

A As Kotlin Lists

B As Kotlin Arrays

C As MutableLists

D As read-only collections

Java arrays map directly to Kotlin Arrays for efficient interop.

Why is mutability important in Kotlin-Java collection interop?

A Because Kotlin does not support Java collections

B Because Java collections are always read-only

C Because Kotlin collections are always immutable

D Because mutability differences can cause unexpected bugs

Mutability differences can cause bugs if not handled carefully.

Explain how Kotlin collections are represented when passed to Java and how mutability is handled.

Think about how Kotlin's read-only and mutable collections map to Java's collections.

Describe potential issues when modifying collections shared between Kotlin and Java.

Consider what happens if Java modifies a Kotlin read-only list.