Kotlin Java Interop
Call Java from Kotlin and understand how Kotlin compares to Java. Platform types, nullability, SAM conversions, and more.
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Comparison to Java
Kotlin fixes a series of issues that Java suffers from....
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Calling Java from Kotlin
Kotlin is designed with Java interoperability in mind. Existing Java code can be called from Kotlin ...
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Calling Kotlin from Java
Kotlin code can be easily called from Java. Instances of a Kotlin class can be created and used in J...
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Using Java records in Kotlin
Records in Java are classes for storing immutable data with a fixed set of components. They have con...
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Strings in Java and Kotlin
This guide shows how to do typical string tasks in Java and Kotlin, to help you migrate and write id...
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Collections in Java and Kotlin
Collections are groups of a variable number of items that are significant to the problem and commonl...
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Nullability in Java and Kotlin
Nullability is the ability of a variable to hold null. Dereferencing null causes NullPointerExceptio...
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Standard input
Java Scanner is slow; use it only when you need its specific features (e.g. hasNext(), useDelimiter(...
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The @Jvm annotations toolbox: when to use each
When you publish Kotlin code that Java teammates will consume, the @Jvm annotations control how your...
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Platform types & nullability annotations
Every reference in Java can be null, but most Java code carries no nullability information. When Kot...
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SAM conversions & fun interface
A SAM (Single Abstract Method) interface has exactly one abstract method — Runnable, Callable, Compa...
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Calling Kotlin from Java in depth
Java cannot see Kotlin the way Kotlin sees itself: there are no top-level functions, no companion ob...
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@Throws & checked exceptions
Kotlin has no checked exceptions: every exception is unchecked, and the compiler never forces you to...
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Generics interop: variance & wildcards
Java expresses variance at the use site with wildcards (? extends T, ? super T). Kotlin prefers decl...
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Collections & arrays interop
Kotlin splits Java's single List into two views: a read-only List<T> and a MutableList<T>. They are ...
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Property interop: getters, setters & accessor targets
Kotlin properties and Java accessor methods are two views of the same idea. A Java getX()/setX() pai...
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vararg interop & the spread operator
A Kotlin vararg parameter and a Java varargs parameter (T...) compile to the same thing on the JVM: ...
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Calling Java builders & fluent APIs from Kotlin
Many Java libraries use the builder pattern: a chain of methods that each return this (or a new buil...
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Coroutines & suspend functions from Java
A Kotlin suspend function cannot be called directly from Java: under the hood the compiler adds a hi...
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Scope functions for cleaner Java interop
Kotlin's scope functions — let, run, with, apply, also — shine when you work with Java objects that ...
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Bridging Java functional interfaces & method references
Java 8 added java.util.function interfaces — Function, BiFunction, Supplier, Consumer, Predicate, an...
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Annotations, const & typealias across the boundary
Kotlin can declare and consume Java annotations. To create an annotation usable from both languages ...
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Companion objects & @JvmStatic from Java
Kotlin has no `static` keyword. The role of Java statics is played by companion objects, top-level f...
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Sealed classes, exhaustive when & Java switch
A `sealed` class or interface restricts its subtypes to a known, closed set declared in the same mod...
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Inline value classes & @JvmInline
A value class wraps a single value to give it a distinct type — a `UserId` that is not interchangeab...
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Default arguments & @JvmOverloads
Kotlin functions can declare default values for parameters, so one declaration covers many call shap...
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Extension functions seen from Java
An extension function lets you call `"abc".shout()` as if `shout` were a method on `String`, without...
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Java Optional ↔ Kotlin nullable
Java 8 introduced `Optional<T>` to express 'a value that may be absent'. Kotlin solves the same prob...
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