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Kotlin language specification

Version 1.9-rfc+0.1

Marat Akhin

Mikhail Belyaev


An exception type declaration is any type declaration that meets the following criteria:

Any object of an exception type may be thrown or caught.

Catching exceptions

A try-expression becomes active once the execution of the program enters it and stops being active once the execution of the program leaves it. If there are several active try-expressions, the one that became active last is currently active.

If an exception is thrown while a try-expression is currently active and this try-expression has any catch-blocks, those catch-blocks are checked for applicability for this exception. A catch-block is applicable for an exception object if the runtime type of this expression object is a subtype of the bound exception parameter of the catch-block.

Note: the applicability check is subject to Kotlin runtime type information limitations and may be dependent on the platform implementation of runtime type information, as well as the implementation of exception classes.

If a catch-block is applicable for the exception thrown, the code inside the block is evaluated and the value of the block is returned as the value of a try-expression. If the try-expression contains a finally-block, the body of this block is evaluated after the body of the selected catch block. If these evaluations results in throwing other exceptions (including the one caught by the catch-block), they are propagated as if none of the catch-blocks were applicable.

Important: the try-expression itself is not considered active inside its own catch and finally blocks.

If none of the catch-blocks of the currently active try-expression are applicable for the exception, the finally block (if any) is still evaluated, and the exception is propagated, meaning the next active try-expression becomes currently active and is checked for applicability.

If there are no active try-blocks, the execution of the program finishes, signaling that the exception has reached top level.

Throwing exceptions

Throwing an exception object is done using throw-expression. A valid throw expression throw e requires that:

Throwing an exception results in checking active try-blocks.

Note: Kotlin does not specify whether throwing exceptions involves construction of a program stack trace and how the actual exception handling is implemented. This is a platform-dependent mechanism.