net.sf.saxon.expr

Class LastItemExpression

java.lang.Object

net.sf.saxon.expr.Expression

net.sf.saxon.expr.UnaryExpression

net.sf.saxon.expr.LastItemExpression

All Implemented Interfaces:

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

public final class LastItemExpression
extends UnaryExpression

A LastItemExpression returns the last item in the sequence returned by a given base expression. The evaluation strategy is to read the input sequence with a one-item lookahead.

See Also:

Serialized Form

Field Summary

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

operand

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

Constructors

Constructor and Description
LastItemExpression(Expression base) Constructor

Method Summary

Modifier and Type	Method and Description
int	computeCardinality() Get the static cardinality
Expression	copy() Copy an expression.
Item	evaluateItem(XPathContext context) Evaluate the expression
String	getExpressionName() Get a name identifying the kind of expression, in terms meaningful to a user.
Expression	optimize(ExpressionVisitor visitor, ItemType contextItemType) Perform optimisation of an expression and its subexpressions.
Expression	promote(PromotionOffer offer, Expression parent) Promote this expression if possible
Expression	typeCheck(ExpressionVisitor visitor, ItemType contextItemType) Type-check the expression.

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

computeSpecialProperties, displayOperator, equals, explain, getBaseExpression, getItemType, hashCode, iterateSubExpressions, replaceSubExpression, simplify, toString

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

addToPathMap, adoptChildExpression, checkForUpdatingSubexpressions, checkPermittedContents, computeDependencies, computeStaticProperties, doPromotion, dynamicError, effectiveBooleanValue, evaluateAsString, 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, hasLoopingSubexpression, implementsStaticTypeCheck, isSubtreeExpression, isUpdatingExpression, isVacuousExpression, iterate, iterateEvents, iterateSameFocusSubExpressions, 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

Constructor Detail

LastItemExpression

public LastItemExpression(Expression base)

Constructor

Parameters:

base - A sequence expression denoting sequence whose first item is to be returned

Method Detail

typeCheck

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

Description copied from class: UnaryExpression

Type-check the expression. Default implementation for unary operators that accept any kind of operand

Overrides:

typeCheck in class UnaryExpression

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 UnaryExpression

Parameters:

visitor - an expresion 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 UnaryExpression

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

computeCardinality

public int computeCardinality()

Get the static cardinality

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

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

evaluateItem

public Item evaluateItem(XPathContext context)
                  throws XPathException

Evaluate the expression

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

getExpressionName

public String getExpressionName()

Description copied from class: Expression

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.