Some Guava features were really useful for Java 7 application because Guava was bringing APIs missing in the JDK. The introduction of Kotlin and Java 8+ fixed some of these limitations. When migrating an application to Kotlin and/or using a Java 8+ target, it is recommended to prefer the provided native APIs over Guava to ease maintenance: developers don’t need to learn how to use two APIs and can stick to the default one.

This rule raises an issue when the Guava APIs listed below are used.

Guava API Kotlin API

com.google.common.base.Joiner.on

kotlin.collections.Iterable.joinToString or kotlin.Array.joinToString

com.google.common.base.Predicate

Use function with type (T) → Boolean as a replacement for Predicate<T>

com.google.common.base.Function

Use function with type (T) → R as a replacement for Function<T, R>

com.google.common.base.Supplier

Use function with type () → T as a replacement for Supplier<T>

com.google.common.io.Files.createTempDir

kotlin.io.path.createTempDirectory

com.google.common.collect.ImmutableSet.of

kotlin.collections.setOf

com.google.common.collect.ImmutableList.of

kotlin.collections.listOf

com.google.common.collect.ImmutableMap.of

kotlin.collections.mapOf
Guava API Java 8 API

com.google.common.io.BaseEncoding.base64

java.util.Base64

com.google.common.io.BaseEncoding.base64Url

java.util.Base64

com.google.common.base.Optional

java.util.Optional*

com.google.common.base.Optional.of

java.util.Optional.of*

com.google.common.base.Optional.absent

java.util.Optional.empty*

com.google.common.base.Optional.fromNullable

java.util.Optional.ofNullable*

*: Note that this rule will also raise issues for the use of Guava Optional and recommend using the Java 8+ equivalent instead. In most Kotlin-only cases, you will probably be better off using Kotlin’s null-safe type system directly, without wrapping Optional constructs. In some cases, such as those involving Java interoperability, it may be necessary to use Optional in Kotlin, however.