net.sf.saxon.value
Class SingletonClosure

java.lang.Object
  net.sf.saxon.value.Value<T>
      net.sf.saxon.value.Closure
          net.sf.saxon.value.SingletonClosure

All Implemented Interfaces:

ValueRepresentation

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

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

INDETERMINATE_ORDERING

Fields inherited from interface net.sf.saxon.om.ValueRepresentation

EMPTY_VALUE_ARRAY

Constructor Summary

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

Method Summary

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
Value	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

getCardinality, getExpression, getItemType, getSavedXPathContext, make, makeIteratorClosure, reduce, saveContext, setExpression, setSavedXPathContext

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

asItem, asIterator, asValue, checkPermittedContents, convertToJava, effectiveBooleanValue, equals, fromItem, getCanonicalLexicalRepresentation, getIterator, getSchemaComparable, getSequenceLength, getStringValue, getStringValueCS, hashCode, isIdentical

Methods inherited from class java.lang.Object

clone, finalize, getClass, 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

Method Detail

iterate

public SequenceIterator iterate()
                         throws XPathException

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

Overrides:

iterate in class Closure

Returns:

an iterator over the sequence of items

Throws:

XPathException - if a dynamic error occurs. This is possible only in the case of values that are materialized lazily, that is, where the iterate() method leads to computation of an expression that delivers the values.

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 (for example if the value is a closure that needs to be evaluated)

asItem

public Item asItem()
            throws XPathException

Return the value in the form of an Item

Overrides:

asItem in class Value

Returns:

the value in the form of an Item

Throws:

XPathException - if an error occurs, notably if this value is a sequence containing more than one item

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

Overrides:

itemAt in class Value

Parameters:

n - position of the required item, counting from zero.

Returns:

the n'th item in the sequence, where the first item in the sequence is numbered zero. If n is negative or >= the length of the sequence, returns null.

Throws:

XPathException - if an error occurs (for example if the value is a Closure that needs to be evaluated to find the Nth item)

getLength

public int getLength()
              throws XPathException

Get the length of the sequence

Overrides:

getLength in class Value

Returns:

the number of items in the sequence

Throws:

XPathException - if an error occurs (for example if the value is a closure that needs to be read to determine its length)

materialize

public Value materialize()
                  throws XPathException

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

Returns:

the corresponding value

Throws:

XPathException