Package net.sf.saxon.expr

Class PseudoExpression

java.lang.Object
net.sf.saxon.expr.Expression
net.sf.saxon.expr.PseudoExpression
All Implemented Interfaces:
ExportAgent, Locatable, IdentityComparable, Traceable
Direct Known Subclasses:
DefaultedArgumentExpression, Pattern, SortKeyDefinition, SortKeyDefinitionList
public abstract class PseudoExpression extends Expression
A pseudo-expression is an object that can appear as a node on the expression tree, but which cannot actually be evaluated in its own right. An example is a sort key definition. This has to be a node on the tree, because it contains subexpressions, so recursive traversals of the tree need to process it. But evaluating a sort key definition throws an exception.

A constraint for pseudo-expressions is that rewrite methods (simplify, typecheck, promote etc) always return an object of the same class (nearly always the same object)

  • Constructor Details Link icon

    • PseudoExpression Link icon

      public PseudoExpression()

  • Method Details Link icon

    • getImplementationMethod Link icon

      public int getImplementationMethod()
      Description copied from class: Expression
      An implementation of Expression must provide at least one of the methods evaluateItem(), iterate(), or process(). This method indicates which of these methods is provided directly. The other methods will always be available indirectly, using an implementation that relies on one of the other methods.
      Specified by:
      getImplementationMethod in class Expression
      Returns:
      the implementation method, for example Expression.ITERATE_METHOD or Expression.EVALUATE_METHOD or Expression.PROCESS_METHOD

    • computeCardinality Link icon

      protected final int computeCardinality()
      Description copied from class: Expression
      Compute the static cardinality of this expression
      Specified by:
      computeCardinality in class Expression
      Returns:
      the computed cardinality, as one of the values StaticProperty.ALLOWS_ZERO_OR_ONE, StaticProperty.EXACTLY_ONE, StaticProperty.ALLOWS_ONE_OR_MORE, StaticProperty.ALLOWS_ZERO_OR_MORE. May also return StaticProperty.ALLOWS_ZERO if the result is known to be an empty sequence, or StaticProperty.ALLOWS_MANY if if is known to return a sequence of length two or more.

    • getItemType Link icon

      public ItemType getItemType()
      Description copied from class: Expression
      Determine the static item type of the expression, as precisely possible. All expression return sequences, in general; this method determines the type of the items within the sequence, assuming that (a) this is known in advance, and (b) it is the same for all items in the sequence.

      This method should always return a result, though it may be the best approximation that is available at the time.

      Specified by:
      getItemType in class Expression
      Returns:
      a value such as Type.STRING, Type.BOOLEAN, Type.NUMBER, Type.NODE, or Type.ITEM (meaning not known at compile time)

    • evaluateItem Link icon

      public final Item evaluateItem(XPathContext context) throws XPathException
      Description copied from class: Expression
      Evaluate an expression as a single item. This always returns either a single Item or null (denoting the empty sequence). No conversion is done. This method should not be used unless the static type of the expression is a subtype of "item" or "item?": that is, it should not be called if the expression may return a sequence. There is no guarantee that this condition will be detected.
      Overrides:
      evaluateItem in class Expression
      Parameters:
      context - The context in which the expression is to be evaluated
      Returns:
      the node or atomic value that results from evaluating the expression; or null to indicate that the result is an empty sequence
      Throws:
      XPathException - if any dynamic error occurs evaluating the expression

    • effectiveBooleanValue Link icon

      public boolean effectiveBooleanValue(XPathContext context) throws XPathException
      Description copied from class: Expression
      Get the effective boolean value of the expression. This returns false if the value is the empty sequence, a zero-length string, a number equal to zero, or the boolean false. Otherwise it returns true.
      Overrides:
      effectiveBooleanValue in class Expression
      Parameters:
      context - The context in which the expression is to be evaluated
      Returns:
      the effective boolean value
      Throws:
      XPathException - if any dynamic error occurs evaluating the expression

    • evaluateAsString Link icon

      public final UnicodeString evaluateAsString(XPathContext context) throws XPathException
      Description copied from class: Expression
      Evaluate an expression as a String. This function must only be called in contexts where it is known that the expression will return a single string (or where an empty sequence is to be treated as a zero-length string). Implementations should not attempt to convert the result to a string, other than converting () to "". This method is used mainly to evaluate expressions produced by compiling an attribute value template.
      Overrides:
      evaluateAsString in class Expression
      Parameters:
      context - The context in which the expression is to be evaluated
      Returns:
      the value of the expression, evaluated in the current context. The expression must return a string or (); if the value of the expression is (), this method returns "".
      Throws:
      XPathException - if any dynamic error occurs evaluating the expression

    • iterate Link icon

      public final SequenceIterator iterate(XPathContext context) throws XPathException
      Description copied from class: Expression
      Return an Iterator to iterate over the values of a sequence. The value of every expression can be regarded as a sequence, so this method is supported for all expressions. This default implementation handles iteration for expressions that return singleton values: for non-singleton expressions, the subclass must provide its own implementation.
      Overrides:
      iterate in class Expression
      Parameters:
      context - supplies the context for evaluation
      Returns:
      a SequenceIterator that can be used to iterate over the result of the expression
      Throws:
      XPathException - if any dynamic error occurs evaluating the expression

    • process Link icon

      public void process(Outputter output, XPathContext context) throws XPathException
      Description copied from class: Expression
      Process the instruction
      Overrides:
      process in class Expression
      Parameters:
      output - the destination for the result
      context - The dynamic context, giving access to the current node, the current variables, etc.
      Throws:
      XPathException - if a dynamic error occurs

    • getElaborator Link icon

      public Elaborator getElaborator()
      Make an elaborator for this expression
      Overrides:
      getElaborator in class Expression
      Returns:
      an appropriate Elaborator