Package net.sf.saxon.expr
Class FirstItemExpression
- java.lang.Object
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- net.sf.saxon.expr.ComputedExpression
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- net.sf.saxon.expr.UnaryExpression
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- net.sf.saxon.expr.FirstItemExpression
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- All Implemented Interfaces:
java.io.Serializable
,javax.xml.transform.SourceLocator
,Container
,Expression
,InstructionInfoProvider
public final class FirstItemExpression extends UnaryExpression
A FirstItemExpression returns the first item in the sequence returned by a given base expression- See Also:
- Serialized Form
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Field Summary
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Fields inherited from class net.sf.saxon.expr.UnaryExpression
operand
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Fields inherited from class net.sf.saxon.expr.ComputedExpression
locationId, staticProperties
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Fields inherited from interface net.sf.saxon.expr.Expression
EVALUATE_METHOD, ITERATE_METHOD, PROCESS_METHOD
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Constructor Summary
Constructors Constructor Description FirstItemExpression(Expression base)
Constructor
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Method Summary
All Methods Instance Methods Concrete Methods Modifier and Type Method Description int
computeCardinality()
Get the static cardinalityjava.lang.String
displayOperator(Configuration config)
Diagnostic print of expression structureItem
evaluateItem(XPathContext context)
Evaluate the expressionExpression
optimize(Optimizer opt, StaticContext env, ItemType contextItemType)
Perform optimisation of an expression and its subexpressions.Expression
promote(PromotionOffer offer)
Promote this expression if possible-
Methods inherited from class net.sf.saxon.expr.UnaryExpression
computeSpecialProperties, display, equals, getBaseExpression, getItemType, hashCode, iterateSubExpressions, replaceSubExpression, simplify, typeCheck
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Methods inherited from class net.sf.saxon.expr.ComputedExpression
adoptChildExpression, checkPermittedContents, computeDependencies, computeStaticProperties, doPromotion, dynamicError, effectiveBooleanValue, evaluateAsString, getCardinality, getColumnNumber, getConstructType, getDependencies, getExecutable, getHostLanguage, getImplementationMethod, getInstructionInfo, getIntrinsicDependencies, getLineNumber, getLocationId, getLocationProvider, getParentExpression, getPublicId, getSlotsUsed, getSpecialProperties, getSystemId, hasBadParentPointer, iterate, markTailFunctionCalls, process, resetStaticProperties, setLocationId, setParentExpression, setParentExpression, suppressValidation, typeError
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Constructor Detail
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FirstItemExpression
public FirstItemExpression(Expression base)
Constructor- Parameters:
base
- A sequence expression denoting sequence whose first item is to be returned
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Method Detail
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optimize
public Expression optimize(Optimizer opt, StaticContext env, 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.
- Specified by:
optimize
in interfaceExpression
- Overrides:
optimize
in classUnaryExpression
- Parameters:
opt
- the optimizer in use. This provides access to supporting functions; it also allows different optimization strategies to be used in different circumstances.env
- the static context of the expressioncontextItemType
- 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 toType.ITEM_TYPE
- Returns:
- the original expression, rewritten if appropriate to optimize execution
- Throws:
StaticError
- if an error is discovered during this phase (typically a type error)XPathException
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promote
public Expression promote(PromotionOffer offer) throws XPathException
Promote this expression if possible- Specified by:
promote
in interfaceExpression
- Overrides:
promote
in classUnaryExpression
- 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
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computeCardinality
public int computeCardinality()
Get the static cardinality- Overrides:
computeCardinality
in classUnaryExpression
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evaluateItem
public Item evaluateItem(XPathContext context) throws XPathException
Evaluate the expression- Specified by:
evaluateItem
in interfaceExpression
- Overrides:
evaluateItem
in classComputedExpression
- 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
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displayOperator
public java.lang.String displayOperator(Configuration config)
Diagnostic print of expression structure- Specified by:
displayOperator
in classUnaryExpression
- Parameters:
config
-- Returns:
- the operator, as a string
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