Java programmers will be used to "casting": converting a reference to some generic type (e.g., Animal) to a more specific type (e.g., Axolotl). The Kotlin compiler frequently can automatically cast objects, through "smart casts".

In this snippet, the compiler knows that critter is a Frog in the is Frog branch of the when. So, it automatically casts critter to be a Frog, so we can safely call hop() on it. The same holds true for the smart cast of critter to Axolotl in the is Axolotl branch.

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Abstract Classes Adding Functions to Classes Adding Properties to Classes Chaining to Superclass Functions Checking Inheritance Via is Classes and Instances Constructor Init-Only Parameters Constructors Data Class enum and Functions enum and when() enum Class enum name and ordinal enum valueOf() enum values() Formal Constructor Declaration Hello, World! (In a Class) Inheritance Inheritance and Constructors Inheritance and Functions Inheritance and Overriding Functions Init Block Inner Classes Inner Classes and Outer this Inner Classes and this is and when Manual Casts Nested Classes Nested Classes and Private Access Non-Nested Sealed Classes override is Transitive Overriding Properties Passing this as a Parameter Printing the Class of Numbers Sealed Classes Sealed Classes and when Secondary Constructors Smart Casts The this Pseudo-Property
Chaining to Superclass Functions Checking Inheritance Via is Inheritance Inheritance and Constructors Inheritance and Functions Inheritance and Overriding Functions is and when Manual Casts override is Transitive Overriding Properties Smart Casts
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