Keywords

Keywords are reserved in every source context and cannot be used as ordinary identifiers.

Declarations

package, import, as, class, interface, enum, extends, implements, public, private

Control flow

if, else, for, switch, case, break, continue, return, try, catch, finally, throw

Values and type operations

true, false, null, this, super, is, as

Basic type names such as Integer, Boolean, and Void are predefined by the language and cannot be redefined either.

Compatibility

A new keyword can break old source, so stable releases should prefer contextual keywords or enable them through a new language version. The current specification has no backtick-escape syntax for using keywords as identifiers.

Keywords are case-sensitive: class is a keyword, while Class can be a type name. Keywords use ASCII characters only so visually similar characters cannot hinder review.

value is a contextual keyword only in a top-level value TypeName declaration header; fields, parameters, locals, and functions may still be named value.

annotation is a contextual keyword only in a top-level annotation declaration header. Targets and retention policies are ordinary interface names in std.annotation.

extension is a contextual keyword only in a top-level extension-function declaration header.

Neither Module nor module is a keyword. The former is a bootstrap interface; the module entry point and bootstrap factory named module are ordinary functions. Parameter labels name, version, exports, and dependencies are ordinary identifiers.