net.sf.saxon.value

Class SingletonClosure

java.lang.Object

net.sf.saxon.value.Closure

net.sf.saxon.value.SingletonClosure

All Implemented Interfaces:

ContextOriginator, Sequence

public class SingletonClosure
extends Closure

A SingletonClosure represents a value that has not yet been evaluated: the value is represented by an expression, together with saved values of all the context variables that the expression depends on. The value of a SingletonClosure is always either a single item or an empty sequence.

The expression may depend on local variables and on the context item; these values are held in the saved XPathContext object that is kept as part of the Closure, and they will always be read from that object. The expression may also depend on global variables; these are unchanging, so they can be read from the Bindery in the normal way. Expressions that depend on other contextual information, for example the values of position(), last(), current(), current-group(), should not be evaluated using this mechanism: they should always be evaluated eagerly. This means that the Closure does not need to keep a copy of these context variables.

Field Summary

Fields inherited from class net.sf.saxon.value.Closure

depth, expression, inputIterator, savedXPathContext

Constructor Summary

Constructors

Constructor and Description
SingletonClosure(Expression exp, XPathContext context) Constructor should not be called directly, instances should be made using the make() method.

Method Summary

Methods

Modifier and Type	Method and Description
Item	asItem() Return the value in the form of an Item
int	getLength() Get the length of the sequence
Item	itemAt(int n) Get the n'th item in the sequence (starting from 0).
SequenceIterator	iterate() Evaluate the expression in a given context to return an iterator over a sequence
GroundedValue	materialize() Return a value containing all the items in the sequence returned by this SequenceIterator
void	process(XPathContext context) Process the expression by writing the value to the current Receiver

Methods inherited from class net.sf.saxon.value.Closure

getExpression, getSavedXPathContext, head, make, reduce, saveContext, setExpression, setSavedXPathContext

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

SingletonClosure

public SingletonClosure(Expression exp,
                XPathContext context)
                 throws XPathException

Constructor should not be called directly, instances should be made using the make() method.

Parameters:

exp - the expression to be lazily evaluated

context - the context in which the expression should be evaluated

Throws:

XPathException - if an error occurs saving the dynamic context

Method Detail

iterate

public SequenceIterator iterate()
                         throws XPathException

Evaluate the expression in a given context to return an iterator over a sequence

Specified by:

iterate in interface Sequence

Overrides:

iterate in class Closure

Returns:

an iterator over all the items

Throws:

XPathException - in the situation where the sequence is evaluated lazily, and constructing an iterator over the items causes a dynamic error.

process

public void process(XPathContext context)
             throws XPathException

Process the expression by writing the value to the current Receiver

Overrides:

process in class Closure

Parameters:

context - The dynamic context, giving access to the current node, the current variables, etc.

Throws:

XPathException - if an error occurs evaluating the input expression

asItem

public Item asItem()
            throws XPathException

Return the value in the form of an Item

Returns:

the value in the form of an Item

Throws:

XPathException

itemAt

public Item itemAt(int n)
            throws XPathException

Get the n'th item in the sequence (starting from 0). This is defined for all SequenceValues, but its real benefits come for a SequenceValue stored extensionally

Throws:

XPathException

getLength

public int getLength()
              throws XPathException

Get the length of the sequence

Throws:

XPathException

materialize

public GroundedValue materialize()
                          throws XPathException

Return a value containing all the items in the sequence returned by this SequenceIterator

Returns:

the corresponding value

Throws:

XPathException