net.sf.saxon.value
Class SingletonClosure

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


Copyright (c) 2004-2011 Saxonica Limited. All rights reserved.