Package net.sf.saxon.expr
Class LazyExpression
- java.lang.Object
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- net.sf.saxon.expr.Expression
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- net.sf.saxon.expr.UnaryExpression
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- net.sf.saxon.expr.LazyExpression
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- All Implemented Interfaces:
java.io.Serializable
,javax.xml.transform.SourceLocator
,LocationProvider
,SaxonLocator
,EvaluableItem
,SequenceIterable
,InstructionInfo
,org.xml.sax.Locator
public class LazyExpression extends UnaryExpression
A LazyExpression is an expression that forces lazy evaluation: it must not be evaluated eagerly, because a failure must not be reported unless the value is actually referenced. This is used for an expression that has been moved out of a loop. If the loop iterates zero times, the expression will not be evaluated, and in particular, it will not cause a dynamic error.Note that the LazyExpression class does not itself implement any kind of delayed evaluation: calling its evaluateItem() and iterate() methods produces an immediate result. Instead, the existence of a LazyExpression on the expression tree acts as a signal to other classes that evaluation should be delayed, typically by holding the result of the iterate() method in a Closure object.
- 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.Expression
EVALUATE_METHOD, ITERATE_METHOD, locationId, PROCESS_METHOD, staticProperties
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Constructor Summary
Constructors Constructor Description LazyExpression(Expression operand)
Create a LazyExpression
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Method Summary
All Methods Static Methods Instance Methods Concrete Methods Modifier and Type Method Description Expression
copy()
Copy an expression.protected java.lang.String
displayExpressionName()
Return the element name to use in the expression treeItem
evaluateItem(XPathContext context)
Evaluate an expression as a single item.SequenceIterator
iterate(XPathContext context)
Return an Iterator to iterate over the values of a sequence.static Expression
makeLazyExpression(Expression operand)
Create a LazyExpression (factory method)Expression
optimize(ExpressionVisitor visitor, ItemType contextItemType)
Perform optimisation of an expression and its subexpressions.void
process(XPathContext context)
Process the instruction, without returning any tail callsExpression
typeCheck(ExpressionVisitor visitor, ItemType contextItemType)
The typeCheck method suppresses compile-time evaluation-
Methods inherited from class net.sf.saxon.expr.UnaryExpression
computeCardinality, computeSpecialProperties, displayOperator, equals, explain, getBaseExpression, getItemType, hashCode, iterateSubExpressions, promote, replaceSubExpression, simplify, toString
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Methods inherited from class net.sf.saxon.expr.Expression
addToPathMap, adoptChildExpression, checkForUpdatingSubexpressions, checkPermittedContents, computeDependencies, computeStaticProperties, display, doPromotion, dynamicError, effectiveBooleanValue, evaluateAsString, evaluatePendingUpdates, explain, findParentOf, getCardinality, getColumnNumber, getColumnNumber, getConstructType, getContainer, getDependencies, getExecutable, getHostLanguage, getImplementationMethod, getIntrinsicDependencies, getLineNumber, getLineNumber, getLocationId, getLocationProvider, getObjectName, getProperties, getProperty, getPublicId, getSlotsUsed, getSpecialProperties, getSystemId, getSystemId, hasLoopingSubexpression, implementsStaticTypeCheck, isUpdatingExpression, iterateEvents, markTailFunctionCalls, resetLocalStaticProperties, setContainer, setFiltered, setFlattened, setLocationId, staticTypeCheck, suppressValidation, typeError
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Constructor Detail
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LazyExpression
public LazyExpression(Expression operand)
Create a LazyExpression- Parameters:
operand
- the expression to be evaluated lazily
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Method Detail
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makeLazyExpression
public static Expression makeLazyExpression(Expression operand)
Create a LazyExpression (factory method)- Parameters:
operand
- the expression to be evaluated lazily- Returns:
- the LazyExpression
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typeCheck
public Expression typeCheck(ExpressionVisitor visitor, ItemType contextItemType) throws XPathException
The typeCheck method suppresses compile-time evaluation- Overrides:
typeCheck
in classUnaryExpression
- Parameters:
visitor
- an expression visitorcontextItemType
- the static type of the context item- Returns:
- the expression after typechecking
- Throws:
XPathException
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optimize
public Expression optimize(ExpressionVisitor visitor, ItemType contextItemType) throws XPathException
Perform optimisation of an expression and its subexpressions. This method suppresses compile-time evaluation- Overrides:
optimize
in classUnaryExpression
- Parameters:
visitor
- an expression visitorcontextItemType
- 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:
XPathException
- if an error is discovered during this phase (typically a type error)
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evaluateItem
public Item evaluateItem(XPathContext context) throws XPathException
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 interfaceEvaluableItem
- Overrides:
evaluateItem
in classExpression
- 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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iterate
public SequenceIterator iterate(XPathContext context) throws XPathException
Return an Iterator to iterate over the values of a sequence. The value of every expression can be regarded as a sequence, so this method is supported for all expressions. This default implementation handles iteration for expressions that return singleton values: for non-singleton expressions, the subclass must provide its own implementation.- Specified by:
iterate
in interfaceSequenceIterable
- Overrides:
iterate
in classExpression
- Parameters:
context
- supplies the context for evaluation- 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
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process
public void process(XPathContext context) throws XPathException
Process the instruction, without returning any tail calls- Overrides:
process
in classExpression
- Parameters:
context
- The dynamic context, giving access to the current node, the current variables, etc.- Throws:
XPathException
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copy
public Expression copy()
Copy an expression. This makes a deep copy.- Specified by:
copy
in classExpression
- Returns:
- the copy of the original expression
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displayExpressionName
protected java.lang.String displayExpressionName()
Description copied from class:UnaryExpression
Return the element name to use in the expression tree- Overrides:
displayExpressionName
in classUnaryExpression
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