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Development lead Andrey Breslav has said that Kotlin is designed to be an industrial-strength object-oriented language, and a "better language" than Java, but still be fully interoperable with Java code, allowing companies to make a gradual migration from Java to Kotlin.[22]
Semicolons are optional as a statement terminator; in most cases a newline is sufficient for the compiler to deduce that the statement has ended.[23]
Kotlin variable declarations and parameter lists have the data type come after the variable name (and with a colon separator), similar to Pascal and TypeScript.
Variables in Kotlin can be read-only, declared with the val keyword, or mutable, declared with the var keyword.[24]
Class members are public by default, and classes themselves are final by default, meaning that creating a derived class is disabled unless the base class is declared with the open keyword.
In addition to the classes and methods (called member functions in Kotlin) of object-oriented programming, Kotlin also supports procedural programming with the use of functions.[25] Kotlin functions (and constructors) support default arguments, variable-length argument lists, named arguments and overloading by unique signature. Class member functions are virtual, i.e. dispatched based on the runtime type of the object they are called on.
Kotlin 1.3 adds (experimental) support for contracts[26] (inspired by Eiffel's design by contract[27] programming paradigm)