Package net.sf.saxon.expr

Class AxisGetExpression

java.lang.Object
net.sf.saxon.expr.Expression
net.sf.saxon.expr.UnaryExpression
net.sf.saxon.expr.AxisGetExpression
All Implemented Interfaces:
ExportAgent, GeneralizedAxisExpression, Locatable, IdentityComparable, Traceable
public final class AxisGetExpression extends UnaryExpression implements GeneralizedAxisExpression
An AxisGetExpression represents a step such as child::get(XYZ) or descendant::get(XYZ), where XYZ is an expression that evaluates to a selector

  • Constructor Details

    • AxisGetExpression

      public AxisGetExpression(int axis, Expression selector)
      Constructor for an AxisExpression whose origin is the context item
      Parameters:
      axis - The axis to be used in this AxisExpression: relevant constants are defined in class AxisInfo.
      selector - The expression within the "get()"
      See Also:

  • Method Details

    • getOperandRole

      protected OperandRole getOperandRole()
      Get the usage (in terms of streamability analysis) of the single operand
      Specified by:
      getOperandRole in class UnaryExpression
      Returns:
      the operand usage

    • getExpressionName

      public String getExpressionName()
      Get a name identifying the kind of expression, in terms meaningful to a user.
      Overrides:
      getExpressionName in class Expression
      Returns:
      a name identifying the kind of expression, in terms meaningful to a user. The name will always be in the form of a lexical XML QName, and should match the name used in explain() output displaying the expression.

    • typeCheck

      public Expression typeCheck(ExpressionVisitor visitor, ContextItemStaticInfo contextInfo) throws XPathException
      Type-check the expression
      Overrides:
      typeCheck in class UnaryExpression
      Parameters:
      visitor - an expression visitor
      contextInfo - Information available statically about the context item: whether it is (possibly) absent; its static type; its streaming posture.
      Returns:
      the original expression, rewritten to perform necessary run-time type checks, and to perform other type-related optimizations
      Throws:
      XPathException - if an error is discovered during this phase (typically a type error)

    • optimize

      public Expression optimize(ExpressionVisitor visitor, ContextItemStaticInfo contextInfo) throws XPathException
      Description copied from class: UnaryExpression
      Perform optimisation of an expression and its subexpressions.

      This method is called after all references to functions and variables have been resolved to the declaration of the function or variable, and after all type checking has been done.

      Overrides:
      optimize in class UnaryExpression
      Parameters:
      visitor - an expression visitor
      contextInfo - the static type of "." at the point where this expression is invoked. The parameter is set to null if it is known statically that the context item will be undefined. If the type of the context item is not known statically, the argument is set to Type.ITEM_TYPE
      Returns:
      the original expression, rewritten if appropriate to optimize execution
      Throws:
      XPathException - if an error is discovered during this phase (typically a type error)

    • getCost

      public double getCost()
      Return the estimated cost of evaluating an expression. This is a very crude measure based on the syntactic form of the expression (we have no knowledge of data values). We take the cost of evaluating a simple scalar comparison or arithmetic expression as 1 (one), and we assume that a sequence has length 5. The resulting estimates may be used, for example, to reorder the predicates in a filter expression so cheaper predicates are evaluated first.
      Overrides:
      getCost in class Expression
      Returns:
      the estimated cost

    • equals

      public boolean equals(Object other)
      Is this expression the same as another expression?
      Overrides:
      equals in class UnaryExpression
      Parameters:
      other - the other operand; the result is false if this is not an Expression
      Returns:
      true if the other operand is an expression and if it can be determined that the two expressions are equivalent, in the sense that they will always return the same result.

    • computeHashCode

      protected int computeHashCode()
      get HashCode for comparing two expressions
      Overrides:
      computeHashCode in class UnaryExpression
      Returns:
      a computed hash code

    • copy

      public Expression copy(RebindingMap rebindings)
      Copy an expression. This makes a deep copy.
      Specified by:
      copy in class Expression
      Parameters:
      rebindings - a mutable list of (old binding, new binding) pairs that is used to update the bindings held in any local variable references that are copied.
      Returns:
      the copy of the original expression

    • computeSpecialProperties

      protected int computeSpecialProperties()
      Get the static properties of this expression (other than its type). The result is bit-signficant. These properties are used for optimizations. In general, if property bit is set, it is true, but if it is unset, the value is unknown.
      Overrides:
      computeSpecialProperties in class UnaryExpression
      Returns:
      the special properties, as a bit-significant integer

    • getItemType

      public ItemType getItemType()
      Determine the data type of the items returned by this expression
      Overrides:
      getItemType in class UnaryExpression
      Returns:
      Type.NODE or a subtype, based on the NodeTest in the axis step, plus information about the content type if this is known from schema analysis

    • getStaticUType

      public UType getStaticUType(UType contextItemType)
      Get the static type of the expression as a UType, following precisely the type inference rules defined in the XSLT 3.0 specification.
      Overrides:
      getStaticUType in class Expression
      Parameters:
      contextItemType - the static type of the context item for the expression evaluation
      Returns:
      the static item type of the expression according to the XSLT 3.0 defined rules

    • getIntrinsicDependencies

      public int getIntrinsicDependencies()
      Determine the intrinsic dependencies of an expression, that is, those which are not derived from the dependencies of its subexpressions. For example, position() has an intrinsic dependency on the context position, while (position()+1) does not. The default implementation of the method returns 0, indicating "no dependencies".
      Overrides:
      getIntrinsicDependencies in class Expression
      Returns:
      a set of bit-significant flags identifying the "intrinsic" dependencies. The flags are documented in class net.sf.saxon.value.StaticProperty

    • computeCardinality

      protected int computeCardinality()
      Determine the cardinality of the result of this expression
      Overrides:
      computeCardinality in class UnaryExpression
      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.

    • isSubtreeExpression

      public boolean isSubtreeExpression()
      Determine whether the expression can be evaluated without reference to the part of the context document outside the subtree rooted at the context node.
      Overrides:
      isSubtreeExpression in class Expression
      Returns:
      true if the expression has no dependencies on the context node, or if the only dependencies on the context node are downward selections using the self, child, descendant, attribute, and namespace axes.

    • getAxis

      public int getAxis()
      Get the axis
      Specified by:
      getAxis in interface GeneralizedAxisExpression
      Returns:
      the axis number, for example AxisInfo.CHILD

    • getImplementationMethod

      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

    • iterate

      public SequenceIterator iterate(XPathContext context) throws XPathException
      Evaluate the path-expression in a given context to return a NodeSet
      Overrides:
      iterate in class Expression
      Parameters:
      context - the evaluation context
      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

    • iterate

      public SequenceIterator iterate(GNode origin)
      Iterate the axis from a given starting node, without regard to context
      Parameters:
      origin - the starting node
      Returns:
      the iterator over the axis

    • export

      public void export(ExpressionPresenter destination) throws XPathException
      Diagnostic print of expression structure. The abstract expression tree is written to the supplied output destination.
      Specified by:
      export in interface ExportAgent
      Overrides:
      export in class UnaryExpression
      Parameters:
      destination - the expression presenter used to display the structure
      Throws:
      XPathException - if the export fails, for example if an expression is found that won't work in the target environment.

    • toString

      public String toString()
      Represent the expression as a string. The resulting string will be a valid XPath 3.0 expression with no dependencies on namespace bindings other than the binding of the prefix "xs" to the XML Schema namespace.
      Overrides:
      toString in class UnaryExpression
      Returns:
      the expression as a string in XPath 3.0 syntax

    • toShortString

      public String toShortString()
      Description copied from class: Expression
      Produce a short string identifying the expression for use in error messages
      Overrides:
      toShortString in class UnaryExpression
      Returns:
      a short string, sufficient to identify the expression

    • getStreamerName

      public String getStreamerName()
      Description copied from class: Expression
      Get the (partial) name of a class that supports streaming of this kind of expression
      Overrides:
      getStreamerName in class Expression
      Returns:
      the partial name of a class that can be instantiated to provide streaming support in Saxon-EE, or null if there is no such class

    • getPreconditions

      public Set<Expression> getPreconditions()
      Find any necessary preconditions for the satisfaction of this expression as a set of boolean expressions to be evaluated on the context node
      Returns:
      A set of conditions, or null if none have been computed

    • getElaborator

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