Kotlin Reference

Quick reference tables for Kotlin keywords, operators, and standard library.

Keywords

Name	Description
`val`	Read-only variable
`var`	Mutable variable
`fun`	Function declaration
`class`	Class declaration
`object`	Singleton object
`interface`	Interface declaration
`when`	Pattern matching expression
`is`	Type check
`as`	Type cast
`in`	Range/collection membership, contravariance
`out`	Covariance
`suspend`	Mark suspendable function
`data`	Auto-generate data class methods
`sealed`	Restricted class hierarchy
`open`	Allow class/method inheritance
`override`	Override parent member
`lateinit`	Delayed non-null initialization
`by`	Delegation

Null Safety Operators

Name	Description
`?.`	Safe call: returns null instead of throwing
`?:`	Elvis: returns fallback if left is null
`!!`	Not-null assertion: throws if null
`as?`	Safe cast: returns null if cast fails

Scope Functions

Name	Description
`let`	Execute block on nullable value, return block result
`run`	Execute block on object, return block result
`with`	Execute block on object (not extension), return result
`apply`	Configure object, return the object itself
`also`	Side effect on object, return the object itself

Collection Functions

Name	Description
`filter`	Keep elements matching condition
`map`	Transform each element
`flatMap`	Transform and flatten
`reduce`	Accumulate to single value
`fold`	Like reduce but with initial value
`find`	First matching element or null
`any`	True if any element matches
`all`	True if all elements match
`none`	True if no elements match
`groupBy`	Group elements by key
`sortedBy`	Sort by selector
`zip`	Pair elements from two collections