net.sf.saxon.expr
Class FirstItemExpression

java.lang.Object
  |
  +--net.sf.saxon.expr.ComputedExpression
        |
        +--net.sf.saxon.expr.UnaryExpression
              |
              +--net.sf.saxon.expr.FirstItemExpression
All Implemented Interfaces:
Container, Expression, InstructionInfoProvider, java.io.Serializable, javax.xml.transform.SourceLocator

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

Field Summary
 
Fields inherited from class net.sf.saxon.expr.UnaryExpression
operand
 
Fields inherited from class net.sf.saxon.expr.ComputedExpression
locationId, staticProperties
 
Fields inherited from interface net.sf.saxon.expr.Expression
EVALUATE_METHOD, ITERATE_METHOD, PROCESS_METHOD
 
Constructor Summary
FirstItemExpression(Expression base)
          Constructor
 
Method Summary
 Expression analyze(StaticContext env, ItemType contextItemType)
          Type-check the expression
 int computeCardinality()
          Get the static cardinality
 java.lang.String displayOperator(NamePool pool)
          Diagnostic print of expression structure
 Item evaluateItem(XPathContext context)
          Evaluate the expression
 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, simplify
 
Methods inherited from class net.sf.saxon.expr.ComputedExpression
adoptChildExpression, computeDependencies, computeStaticProperties, dynamicError, dynamicError, effectiveBooleanValue, evaluateAsString, getCardinality, getColumnNumber, getConstructType, getDependencies, getExecutable, getImplementationMethod, getInstructionInfo, getIntrinsicDependencies, getLineNumber, getLocationId, getParentExpression, getPublicId, getSpecialProperties, getSystemId, iterate, markTailFunctionCalls, process, resetStaticProperties, setLocationId, setParentExpression, typeError
 
Methods inherited from class java.lang.Object
clone, finalize, getClass, notify, notifyAll, toString, wait, wait, wait
 

Constructor Detail

FirstItemExpression

public FirstItemExpression(Expression base)
Constructor

Parameters:
base - A sequence expression denoting sequence whose first item is to be returned
Method Detail

analyze

public Expression analyze(StaticContext env,
                          ItemType contextItemType)
                   throws XPathException
Type-check the expression

Specified by:
analyze in interface Expression
Overrides:
analyze in class UnaryExpression
Parameters:
env - the static context of the expression
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
XPathException

promote

public Expression promote(PromotionOffer offer)
                   throws XPathException
Promote this expression if possible

Specified by:
promote in interface Expression
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

evaluateItem

public Item evaluateItem(XPathContext context)
                  throws XPathException
Evaluate the expression

Specified by:
evaluateItem in interface Expression
Overrides:
evaluateItem in class ComputedExpression
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

displayOperator

public java.lang.String displayOperator(NamePool pool)
Diagnostic print of expression structure

Specified by:
displayOperator in class UnaryExpression
Returns:
the operator, as a string