A visitor of program elements, in the style of the visitor design
pattern. Classes implementing this interface are used to operate
on an element when the kind of element is unknown at compile time.
When a visitor is passed to an element's
accept method, the
visitXyz method most applicable
to that element is invoked.
Classes implementing this interface may or may not throw a
NullPointerException if the additional parameter
null; see documentation of the implementing class for
WARNING: It is possible that methods will be added to this interface to accommodate new, currently unknown, language structures added to future versions of the Java™ programming language. Therefore, visitor classes directly implementing this interface may be source incompatible with future versions of the platform. To avoid this source incompatibility, visitor implementations are encouraged to instead extend the appropriate abstract visitor class that implements this interface. However, an API should generally use this visitor interface as the type for parameters, return type, etc. rather than one of the abstract classes.
Note that methods to accommodate new language constructs could
be added in a source compatible way if they were added as
default methods. However, default methods are only
available on Java SE 8 and higher releases and the
javax.lang.model.* packages bundled in Java SE 8 were required to
also be runnable on Java SE 7. Therefore, default methods
were not used when extending
to cover Java SE 8 language features. However, default methods
are used in subsequent revisions of the
packages that are only required to run on Java SE 8 and higher
| the return type of this visitor's methods. Use |
| the type of the additional parameter to this visitor's
methods. Use |