net.sf.saxon.expr
Class AxisExpression

java.lang.Object
  extended by net.sf.saxon.expr.Expression
      extended by net.sf.saxon.expr.AxisExpression
All Implemented Interfaces:
Serializable, SourceLocator, LocationProvider, SaxonLocator, InstructionInfo, Locator

public final class AxisExpression
extends Expression

An AxisExpression is always obtained by simplifying a PathExpression. It represents a PathExpression that starts either at the context node or at some other singleton node, and uses a simple node-test with no filters. For example "*", "title", "./item", "@*", or "ancestor::chapter*".

An AxisExpression delivers nodes in axis order (not in document order). To get nodes in document order, in the case of a reverse axis, the expression should be wrapped in a call on reverse().

Since 9.4, an axis expression does not necessarily select starting from the context item. This allows an expression such as $x/abcd (very common in XQuery FLWOR expressions) to be computed without creating a new context.

See Also:
Serialized Form

Field Summary
 
Fields inherited from class net.sf.saxon.expr.Expression
EFFECTIVE_BOOLEAN_VALUE, EVALUATE_METHOD, EVENT_FEED_METHOD, ITEM_FEED_METHOD, ITERATE_METHOD, locationId, MAX_SEQUENCE_LENGTH, MAX_STRING_LENGTH, PROCESS_METHOD, PUSH_SELECTION, staticProperties, UNBOUNDED_LOWER, UNBOUNDED_UPPER, WATCH_METHOD
 
Constructor Summary
AxisExpression(byte axis, NodeTest nodeTest)
          Constructor for an AxisExpression whose origin is the context item
 
Method Summary
 PathMap.PathMapNodeSet addToPathMap(PathMap pathMap, PathMap.PathMapNodeSet pathMapNodeSet)
          Add a representation of this expression to a PathMap.
 int computeCardinality()
          Determine the cardinality of the result of this expression
 int computeSpecialProperties()
          Get the static properties of this expression (other than its type).
 Expression copy()
          Copy an expression.
 boolean equals(Object other)
          Is this expression the same as another expression?
 void explain(ExpressionPresenter destination)
          Diagnostic print of expression structure.
 byte getAxis()
          Get the axis
 ItemType getContextItemType()
          Get the static type of the context item for this AxisExpression.
 String getExpressionName()
          Get a name identifying the kind of expression, in terms meaningful to a user.
 int getIntrinsicDependencies()
          Determine the intrinsic dependencies of an expression, that is, those which are not derived from the dependencies of its subexpressions.
 ItemType getItemType(TypeHierarchy th)
          Determine the data type of the items returned by this expression
 NodeTest getNodeTest()
          Get the NodeTest.
 int hashCode()
          get HashCode for comparing two expressions
 boolean isContextPossiblyUndefined()
          Ask whether there is a possibility that the context item will be undefined
 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.
 SequenceIterator iterate(Item origin)
          Iterate the axis from a given starting node, without regard to context
 AxisIterator<? extends NodeInfo> iterate(XPathContext context)
          Evaluate the path-expression in a given context to return a NodeSet
 Expression simplify(ExpressionVisitor visitor)
          Simplify an expression
 String toString()
          Represent the expression as a string for diagnostics
 String toString(NamePool pool)
          Represent the expression as a string for diagnostics
 Expression typeCheck(ExpressionVisitor visitor, ExpressionVisitor.ContextItemType contextItemType)
          Type-check the expression
 
Methods inherited from class net.sf.saxon.expr.Expression
adoptChildExpression, checkForUpdatingSubexpressions, checkPermittedContents, computeDependencies, computeStaticProperties, doPromotion, dynamicError, effectiveBooleanValue, evaluateAsString, evaluateItem, evaluatePendingUpdates, explain, getCardinality, getColumnNumber, getColumnNumber, getConstructType, getContainer, getDependencies, getEvaluationMethod, getExecutable, getHostLanguage, getImplementationMethod, getIntegerBounds, getLineNumber, getLineNumber, getLocationId, getLocationProvider, getObjectName, getProperties, getProperty, getPublicId, getSlotsUsed, getSpecialProperties, getSystemId, getSystemId, hasLoopingSubexpression, hasVariableBinding, implementsStaticTypeCheck, isUpdatingExpression, isVacuousExpression, iterateEvents, iterateSameFocusSubExpressions, iterateSubExpressions, markTailFunctionCalls, optimize, process, promote, replaceSubExpression, resetLocalStaticProperties, setContainer, setEvaluationMethod, setFiltered, setFlattened, setLocationId, staticTypeCheck, suppressValidation, typeError
 
Methods inherited from class java.lang.Object
clone, finalize, getClass, notify, notifyAll, wait, wait, wait
 

Constructor Detail

AxisExpression

public AxisExpression(byte axis,
                      NodeTest nodeTest)
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 Axis.
nodeTest - The conditions to be satisfied by selected nodes. May be null, indicating that any node on the axis is acceptable
See Also:
Axis
Method Detail

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.

simplify

public Expression simplify(ExpressionVisitor visitor)
                    throws XPathException
Simplify an expression

Overrides:
simplify in class Expression
Parameters:
visitor - an expression visitor
Returns:
the simplified expression
Throws:
XPathException - if an error is discovered during expression rewriting

typeCheck

public Expression typeCheck(ExpressionVisitor visitor,
                            ExpressionVisitor.ContextItemType contextItemType)
                     throws XPathException
Type-check the expression

Overrides:
typeCheck in class Expression
Parameters:
visitor - an expression visitor
contextItemType - 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 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)

getContextItemType

public ItemType getContextItemType()
Get the static type of the context item for this AxisExpression. May be null if not known.

Returns:
the statically-inferred type, or null if not known

equals

public boolean equals(Object other)
Is this expression the same as another expression?

Overrides:
equals in class Object

hashCode

public int hashCode()
get HashCode for comparing two expressions

Overrides:
hashCode in class Object

copy

public Expression copy()
Copy an expression. This makes a deep copy.

Specified by:
copy in class Expression
Returns:
the copy of the original expression

computeSpecialProperties

public 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 Expression
Returns:
the special properties, as a bit-significant integer

getItemType

public final ItemType getItemType(TypeHierarchy th)
Determine the data type of the items returned by this expression

Specified by:
getItemType in class Expression
Parameters:
th - the type hierarchy cache
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

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

public final int computeCardinality()
Determine the cardinality of the result 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

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 byte getAxis()
Get the axis

Returns:
the axis number, for example Axis.CHILD

getNodeTest

public NodeTest getNodeTest()
Get the NodeTest. Returns null if the AxisExpression can return any node.

Returns:
the node test, or null if all nodes are returned

addToPathMap

public PathMap.PathMapNodeSet addToPathMap(PathMap pathMap,
                                           PathMap.PathMapNodeSet pathMapNodeSet)
Add a representation of this expression to a PathMap. The PathMap captures a map of the nodes visited by an expression in a source tree.

Overrides:
addToPathMap in class Expression
Parameters:
pathMap - the PathMap to which the expression should be added
pathMapNodeSet -
Returns:
the pathMapNode representing the focus established by this expression, in the case where this expression is the first operand of a path expression or filter expression

isContextPossiblyUndefined

public boolean isContextPossiblyUndefined()
Ask whether there is a possibility that the context item will be undefined

Returns:
true if this is a possibility

iterate

public AxisIterator<? extends NodeInfo> 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(Item origin)
                         throws XPathException
Iterate the axis from a given starting node, without regard to context

Parameters:
origin - the starting node
Returns:
the iterator over the axis
Throws:
XPathException

explain

public void explain(ExpressionPresenter destination)
Diagnostic print of expression structure. The abstract expression tree is written to the supplied output destination.

Specified by:
explain in class Expression
Parameters:
destination - the expression presenter used to display the structure

toString

public String toString()
Represent the expression as a string for diagnostics

Overrides:
toString in class Expression
Returns:
a representation of the expression as a string

toString

public String toString(NamePool pool)
Represent the expression as a string for diagnostics

Parameters:
pool - the name pool, used for expanding names in the node test
Returns:
a string representation of the expression


Copyright (c) 2004-2011 Saxonica Limited. All rights reserved.