net.sf.saxon.expr
Class SingleNodeExpression

java.lang.Object
  net.sf.saxon.expr.Expression
      net.sf.saxon.expr.SingleNodeExpression

All Implemented Interfaces:

Serializable, SourceLocator, EvaluableItem, SequenceIterable, InstructionInfoProvider

Direct Known Subclasses:

ParentNodeExpression, RootExpression

public abstract class SingleNodeExpression
extends Expression

A node set expression that will always return zero or one nodes

See Also:

Serialized Form

Field Summary

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

EVALUATE_METHOD, ITERATE_METHOD, PROCESS_METHOD

Constructor Summary

SingleNodeExpression()

Method Summary

int	computeCardinality() Specify that the expression returns a singleton
int	computeSpecialProperties() Compute the special properties of this expression.
boolean	effectiveBooleanValue(XPathContext context) Get the effective boolean value of the expression.
Item	evaluateItem(XPathContext context) Evaluate an expression as a single item.
int	getIntrinsicDependencies() Determine which aspects of the context the expression depends on.
ItemType	getItemType(TypeHierarchy th) Determine the data type of the items returned by this expression
abstract NodeInfo	getNode(XPathContext context) Get the single node to which this expression refers.
SequenceIterator	iterate(XPathContext context) Evaluate the expression in a given context to return an iterator
Expression	optimize(ExpressionVisitor visitor, ItemType contextItemType) Perform optimisation of an expression and its subexpressions.
Expression	typeCheck(ExpressionVisitor visitor, ItemType contextItemType) Type-check the expression.

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

addToPathMap, adoptChildExpression, checkPermittedContents, computeDependencies, computeStaticProperties, copy, display, doPromotion, evaluateAsString, explain, explain, findParentOf, getCardinality, getColumnNumber, getContainer, getDependencies, getExecutable, getHostLanguage, getImplementationMethod, getInstructionInfo, getLineNumber, getLocationId, getLocationProvider, getPublicId, getSlotsUsed, getSpecialProperties, getSystemId, hasLoopingSubexpression, implementsStaticTypeCheck, iterateEvents, iterateSubExpressions, markTailFunctionCalls, process, promote, replaceSubExpression, setContainer, setFiltered, setFlattened, setLocationId, simplify, staticTypeCheck, suppressValidation, toString

Methods inherited from class java.lang.Object

equals, getClass, hashCode, notify, notifyAll, wait, wait, wait

Constructor Detail

SingleNodeExpression

public SingleNodeExpression()

Method Detail

typeCheck

public Expression typeCheck(ExpressionVisitor visitor,
                            ItemType 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)

optimize

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

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 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 if appropriate to optimize execution

Throws:

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

computeCardinality

public int computeCardinality()

Specify that the expression returns a singleton

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

getItemType

public 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

getIntrinsicDependencies

public int getIntrinsicDependencies()

Determine which aspects of the context the expression depends on. The result is a bitwise-or'ed value composed from constants such as StaticProperty.VARIABLES and StaticProperty.CURRENT_NODE

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

computeSpecialProperties

public int computeSpecialProperties()

Description copied from class: Expression

Compute the special properties of this expression. These properties are denoted by a bit-significant integer, possible values are in class StaticProperty. The "special" properties are properties other than cardinality and dependencies, and most of them relate to properties of node sequences, for example whether the nodes are in document order.

Returns:

the special properties, as a bit-significant integer

getNode

public abstract NodeInfo getNode(XPathContext context)
                          throws XPathException

Get the single node to which this expression refers. Returns null if the node-set is empty

Throws:

XPathException

iterate

public SequenceIterator iterate(XPathContext context)
                         throws XPathException

Evaluate the expression in a given context to return an iterator

Specified by:

iterate in interface SequenceIterable

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

evaluateItem

public 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.

Specified by:

evaluateItem in interface EvaluableItem

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

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