net.sf.saxon.expr
Class PathExpression

java.lang.Object
  net.sf.saxon.expr.Expression
      net.sf.saxon.expr.SlashExpression
          net.sf.saxon.expr.PathExpression

All Implemented Interfaces:

Serializable, SourceLocator, LocationProvider, SaxonLocator, ContextMappingFunction, ContextSwitchingExpression, EvaluableItem, SequenceIterable, InstructionInfo, Locator

public final class PathExpression
extends SlashExpression
implements ContextMappingFunction

An expression that establishes a set of nodes by following relationships between nodes in the document. Specifically, it consists of a start expression which defines a set of nodes, and a step which defines a relationship to be followed from those nodes to create a new set of nodes.

This class inherits from SlashExpression; it is used in the common case where the SlashExpression is known to return nodes rather than atomic values.

This class is not responsible for sorting the results into document order or removing duplicates. That is done by a DocumentSorter expression which is wrapped around the path expression. However, this class does contain the logic for deciding statically whether the DocumentSorter is needed or not.

See Also:

Serialized Form

Field Summary

Fields inherited from class net.sf.saxon.expr.Expression

EVALUATE_METHOD, EVENT_FEED_METHOD, ITEM_FEED_METHOD, ITERATE_METHOD, locationId, PROCESS_METHOD, PUSH_SELECTION, staticProperties, WATCH_METHOD

Constructor Summary

PathExpression(Expression start, Expression step)
Constructor

Method Summary

int	computeSpecialProperties() Get the static properties of this expression (other than its type).
Expression	copy() Copy an expression.
void	explain(ExpressionPresenter destination) Diagnostic print of expression structure.
String	getExpressionName() Get a name identifying the kind of expression, in terms meaningful to a user.
Expression	getRemainingSteps() Get all steps after the first.
boolean	isAbsolute(TypeHierarchy th) Test whether a path expression is an absolute path - that is, a path whose first step selects a document node
boolean	isHybrid() Determine whether this expression is capable (as far as static analysis is concerned) of returning a mixture of nodes and atomic values.
SequenceIterator	iterate(XPathContext context) Iterate the path-expression in a given context
Expression	optimize(ExpressionVisitor visitor, ItemType contextItemType) Optimize the expression and perform type analysis
Expression	promote(PromotionOffer offer, Expression parent) Promote this expression if possible
Expression	simplify(ExpressionVisitor visitor) Simplify an expression
String	toString() The toString() method for an expression attempts to give a representation of the expression in an XPath-like form, but there is no guarantee that the syntax will actually be true XPath.
PathExpression	tryToMakeAbsolute(TypeHierarchy th) Test whether a path expression is an absolute path - that is, a path whose first step selects a document node; if not, see if it can be converted to an absolute path.
Expression	typeCheck(ExpressionVisitor visitor, ItemType contextItemType) Perform type analysis

Methods inherited from class net.sf.saxon.expr.SlashExpression

addToPathMap, computeCardinality, computeDependencies, equals, getControlledExpression, getControllingExpression, getFirstStep, getItemType, getLastStep, getLeadingSteps, hashCode, hasLoopingSubexpression, iterateSameFocusSubExpressions, iterateSubExpressions, map, promoteFocusIndependentSubexpressions, replaceSubExpression, setStartExpression, setStepExpression

Methods inherited from class net.sf.saxon.expr.Expression

adoptChildExpression, checkForUpdatingSubexpressions, checkPermittedContents, computeStaticProperties, doPromotion, dynamicError, effectiveBooleanValue, evaluateAsString, evaluateItem, evaluatePendingUpdates, explain, getCardinality, getColumnNumber, getColumnNumber, getConstructType, getContainer, getDependencies, getEvaluationMethod, getExecutable, getHostLanguage, getImplementationMethod, getIntrinsicDependencies, getLineNumber, getLineNumber, getLocationId, getLocationProvider, getObjectName, getProperties, getProperty, getPublicId, getSlotsUsed, getSpecialProperties, getSystemId, getSystemId, implementsStaticTypeCheck, isSubtreeExpression, isUpdatingExpression, isVacuousExpression, iterateEvents, markTailFunctionCalls, process, resetLocalStaticProperties, setContainer, setEvaluationMethod, setFiltered, setFlattened, setLocationId, staticTypeCheck, suppressValidation, typeError

Methods inherited from class java.lang.Object

clone, finalize, getClass, notify, notifyAll, wait, wait, wait

Methods inherited from interface net.sf.saxon.expr.ContextMappingFunction

map

Constructor Detail

PathExpression

public PathExpression(Expression start,
                      Expression step)

Constructor

Parameters:

start - A node-set expression denoting the absolute or relative set of nodes from which the navigation path should start.

step - The step to be followed from each node in the start expression to yield a new node-set

Method Detail

isHybrid

public boolean isHybrid()

Description copied from class: SlashExpression

Determine whether this expression is capable (as far as static analysis is concerned) of returning a mixture of nodes and atomic values. If so, this needs to be prevented at run time

Overrides:

isHybrid in class SlashExpression

Returns:

true if the static type allows both nodes and atomic values

simplify

public Expression simplify(ExpressionVisitor visitor)
                    throws XPathException

Simplify an expression

Overrides:

simplify in class SlashExpression

Parameters:

visitor - the expression visitor

Returns:

the simplified expression

Throws:

XPathException - if an error is discovered during expression rewriting

typeCheck

public Expression typeCheck(ExpressionVisitor visitor,
                            ItemType contextItemType)
                     throws XPathException

Perform type analysis

Overrides:

typeCheck in class SlashExpression

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)

optimize

public Expression optimize(ExpressionVisitor visitor,
                           ItemType contextItemType)
                    throws XPathException

Optimize the expression and perform type analysis

Overrides:

optimize in class SlashExpression

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 if appropriate to optimize execution

Throws:

XPathException - if an error is discovered during this phase (typically a type error)

promote

public Expression promote(PromotionOffer offer,
                          Expression parent)
                   throws XPathException

Promote this expression if possible

Overrides:

promote in class SlashExpression

Parameters:

offer - details of the offer, for example the offer to move expressions that don't depend on the context to an outer level in the containing expression

Returns:

if the offer is not accepted, return this expression unchanged. Otherwise return the result of rewriting the expression to promote this subexpression

Throws:

XPathException - if any error is detected

copy

public Expression copy()

Copy an expression. This makes a deep copy.

Overrides:

copy in class SlashExpression

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 SlashExpression

Returns:

the special properties, as a bit-significant integer

getRemainingSteps

public Expression getRemainingSteps()

Get all steps after the first. This is complicated by the fact that A/B/C is represented as ((A/B)/C; we are required to return B/C

Overrides:

getRemainingSteps in class SlashExpression

Returns:

a path expression containing all steps in this path expression other than the first, after expanding any nested path expressions

isAbsolute

public boolean isAbsolute(TypeHierarchy th)

Test whether a path expression is an absolute path - that is, a path whose first step selects a document node

Parameters:

th - the type hierarchy cache

Returns:

true if the first step in this path expression selects a document node

tryToMakeAbsolute

public PathExpression tryToMakeAbsolute(TypeHierarchy th)

Test whether a path expression is an absolute path - that is, a path whose first step selects a document node; if not, see if it can be converted to an absolute path. This is possible in cases where the path expression has the form a/b/c and it is known that the context item is a document node; in this case it is safe to change the path expression to /a/b/c

Parameters:

th - the type hierarchy cache

Returns:

the path expression if it is absolute; the converted path expression if it can be made absolute; or null if neither condition applies.

iterate

public SequenceIterator iterate(XPathContext context)
                         throws XPathException

Iterate the path-expression in a given context

Specified by:

iterate in interface SequenceIterable

Overrides:

iterate in class SlashExpression

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

toString

public String toString()

The toString() method for an expression attempts to give a representation of the expression in an XPath-like form, but there is no guarantee that the syntax will actually be true XPath. In the case of XSLT instructions, the toString() method gives an abstracted view of the syntax

Overrides:

toString in class SlashExpression

Returns:

a representation of the expression as a string

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.

explain

public void explain(ExpressionPresenter destination)

Diagnostic print of expression structure. The abstract expression tree is written to the supplied output destination.

Overrides:

explain in class SlashExpression

Parameters:

destination - the expression presenter used to display the structure