# Interface ModuleElement.DirectiveVisitor<R,P>

Type Parameters:
  `R` - the return type of this visitor's methods. Use `Void` for visitors that do not need to return results.
  `P` - the type of the additional parameter to this visitor's methods. Use `Void` for visitors that do not need an additional parameter.

Enclosing interface:
  `ModuleElement`

public static interface ModuleElement.DirectiveVisitor<R,P>
A visitor of module directives, in the style of the visitor design pattern. Classes implementing this interface are used to operate on a directive when the kind of directive is unknown at compile time. When a visitor is passed to a directive's `accept` method, the `visitXyz` method applicable to that directive is invoked.

Classes implementing this interface may or may not throw a `NullPointerException` if the additional parameter `p` is `null`; see documentation of the implementing class for details.

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. Methods to accommodate new language constructs will be added in a source compatible way using default methods.

Since:
  9

- ## Method Summary

  Modifier and Type
  Method
  Description
  `default R`
  `visit(ModuleElement.Directive d)`
  Visits any directive as if by passing itself to that directive's `accept` method and passing `null` for the additional parameter.
  `default R`
  `visit(ModuleElement.Directive d, P p)`
  Visits any directive as if by passing itself to that directive's `accept` method.
  `R`
  `visitExports(ModuleElement.ExportsDirective d, P p)`
  Visits an `exports` directive.
  `R`
  `visitOpens(ModuleElement.OpensDirective d, P p)`
  Visits an `opens` directive.
  `R`
  `visitProvides(ModuleElement.ProvidesDirective d, P p)`
  Visits a `provides` directive.
  `R`
  `visitRequires(ModuleElement.RequiresDirective d, P p)`
  Visits a `requires` directive.
  `default R`
  `visitUnknown(ModuleElement.Directive d, P p)`
  Visits an unknown directive.
  `R`
  `visitUses(ModuleElement.UsesDirective d, P p)`
  Visits a `uses` directive.

- ## Method Details

  - ### visit

    default R visit(ModuleElement.Directive d)
    Visits any directive as if by passing itself to that directive's `accept` method and passing `null` for the additional parameter.
    Implementation Requirements:
      The default implementation is `d.accept(v, null)`.
    Parameters:
      `d` - the directive to visit
    Returns:
      a visitor-specified result

  - ### visit

    default R visit(ModuleElement.Directive d, P p)
    Visits any directive as if by passing itself to that directive's `accept` method.
    Implementation Requirements:
      The default implementation is `d.accept(v, p)`.
    Parameters:
      `d` - the directive to visit
      `p` - a visitor-specified parameter
    Returns:
      a visitor-specified result

  - ### visitRequires

    R visitRequires(ModuleElement.RequiresDirective d, P p)
    Visits a `requires` directive.
    Parameters:
      `d` - the directive to visit
      `p` - a visitor-specified parameter
    Returns:
      a visitor-specified result

  - ### visitExports

    R visitExports(ModuleElement.ExportsDirective d, P p)
    Visits an `exports` directive.
    Parameters:
      `d` - the directive to visit
      `p` - a visitor-specified parameter
    Returns:
      a visitor-specified result

  - ### visitOpens

    R visitOpens(ModuleElement.OpensDirective d, P p)
    Visits an `opens` directive.
    Parameters:
      `d` - the directive to visit
      `p` - a visitor-specified parameter
    Returns:
      a visitor-specified result

  - ### visitUses

    R visitUses(ModuleElement.UsesDirective d, P p)
    Visits a `uses` directive.
    Parameters:
      `d` - the directive to visit
      `p` - a visitor-specified parameter
    Returns:
      a visitor-specified result

  - ### visitProvides

    R visitProvides(ModuleElement.ProvidesDirective d, P p)
    Visits a `provides` directive.
    Parameters:
      `d` - the directive to visit
      `p` - a visitor-specified parameter
    Returns:
      a visitor-specified result

  - ### visitUnknown

    default R visitUnknown(ModuleElement.Directive d, P p)
    Visits an unknown directive. This can occur if the language evolves and new kinds of directive are added.
    Implementation Requirements:
      The default implementation throws `new UnknownDirectiveException(d, p)`.
    Parameters:
      `d` - the directive to visit
      `p` - a visitor-specified parameter
    Returns:
      a visitor-specified result
    Throws:
      `UnknownDirectiveException` - a visitor implementation may optionally throw this exception